SEARCH

SEARCH BY CITATION

Abstract

  1. Top of page
  2. Abstract
  3. Historic context of disturbance and shelf biodiversity
  4. Ice mediated disturbance
  5. Sedimentation
  6. Other disturbances
  7. Confidence of predictions in a rapidly changing world
  8. Conclusions
  9. Acknowledgements
  10. References

Disturbance has always shaped the evolution and ecology of organisms and nowhere is this more apparent that on the iceberg gouged continental shelves of the Antarctic Peninsula (AP). The vast majority of currently described polar biodiversity occurs on the Southern Ocean shelf but current and projected climate change is rapidly altering disturbance intensities in some regions. The AP is now amongst the fastest warming and changing regions on earth. Seasonal sea ice has decreased in time and extent, most glaciers in the region have retreated, a number of ice shelves have collapsed, and the surface waters of the seas west of the AP have warmed. Here, we review the influences of disturbance from ice, sedimentation, freshening events, wave action and humans on shallow water benthic assemblages, and suggest how disturbance pressures will change during the 21st century in the West Antarctic Peninsula (WAP) and Scotia Arc region. We suggest that the intensity of ice scouring will increase in the region over the next few decades as a result of decreased winter sea ice periods and increased ice loading into coastal waters. Thus, the most frequently disturbed environment on earth will become more so, which will lead to considerable changes in community structure and species distributions. However, as ice fronts retreat past their respective grounding lines, sedimentation and freshening events will become relatively more important. Human presence in the region is increasing, through research, tourism, and resource exploitation, which represents a considerable threat to polar biodiversity over the next century. Adapting to or tolerating multiple, changing environmental stressors will be difficult for a fauna with typically slow generation turnovers that has evolved largely in isolation. We suggest that intensifying acute and chronic disturbances are likely to cause significant changes in ecosystem structure, and probably a considerable loss of polar marine biodiversity, over relatively short timescales.

The West Antarctic Peninsula (WAP) region has experienced rapid atmospheric warming in recent years, with some locations having warmed by ca 3°C in the last 50 yr (Vaughan et al. 2003). The region is one of three areas of the world experiencing recent rapid regional warming (Hansen et al. 1999, Solomon et al. 2007). Surface air temperatures have increased at the rate of 3.7±1.6°C per century (Vaughan et al. 2003), compared with a global average of 0.74±0.18°C during the 20th century, as calculated by the 4th assessment of the IPCC (Solomon et al. 2007). As a result of increasing air temperature, the surface waters to the west of the Antarctic Peninsula have also warmed. Meredith and King (2005) reported that the summer surface water temperature of the Bellingshausen Sea has risen by ca 1°C since the 1950s, whilst Ducklow et al. (2007) showed that the oceanic warming can be detected to depths of up to 300 m. In addition, there are now data to suggest that the deep waters of the both the Antarctic Circumpolar Current (Gille 2002) and the Weddell Sea (Robertson et al. 2002) have warmed during the past half century.

Environmental changes associated with the rapid warming have recently been detected, both on land and in sea, and these changes will undoubtedly have a strong influence on the structure and function of marine communities in the region. For example, the majority of the glaciers in the region have retreated in last 50 yr (Cook et al. 2005), and increasing air temperatures have also destabilised a number of ice sheets in the region. In all, seven ice shelves on the Antarctic Peninsula have retreated in the last 50 yr, including the noteworthy collapses of the Wordie Ice Shelf (1989) and the Larsen Ice Shelf, which has disintegrated over a number of discrete events since the mid 1990s (Doake and Vaughan 1991, Vaughan and Doake 1996, Luchitta and Rosanova 1998). Other ice shelves on the WAP, such as the Wilkins Ice Shelf, have significantly retreated and may soon completely collapse (Scambos et al. 2000). Retreating maritime glaciers and ice shelves may expose areas of seabed, thereby presenting opportunities for benthic colonisation, and alter physical processes such as sedimentation and ice disturbance.

There are two important points to consider in the context of this paper, the first relating to space and the second to time. First, the concepts discussed here relate to the WAP region, which has warmed significantly in recent years, and not to Antarctica as a whole, which has demonstrated considerable spatial variation in climatic change (Vaughan et al. 2003). As such, the paper will focus on the role of disturbance on benthic marine communities inhabiting the coastlines of the WAP coastline and the islands of the Scotia Arc, a chain of islands and ridges linking the northern Antarctic Peninsula with South America (Fig. 1). However, the content will not be entirely restricted to this region, and we will use evidence from other polar locations. Another factor that restricts the size of the area likely to experience rapid environmental change is depth. The Southern Ocean is deep and most of the seabed is characterised by stable environmental conditions (Gage and Tyler 1992), so changing conditions at the sea surface are likely to take many years to influence benthos at great depths. Also, one of the key mechanisms of disturbance in the region, ice scouring, influences benthos to depths of ca 600 m, which is the maximum keel draught of modern Antarctic icebergs (Dowdeswell and Bamber 2007). If changes in the frequency of ice scouring do occur, they will therefore not affect benthic communities in deep waters. For these reasons, increasing marine disturbance, as a consequence of the recent rapid warming of the WAP, is likely to have the greatest effect on benthic communities in coastal waters. Thus, the concepts discussed here broadly relate to the ecology of continental shelf to the west of the Antarctic Peninsula and that surrounding the islands of the Scotia Arc (Fig. 1), which represents an area of ca 421 000 km2 (or approximately the same areal coverage as the Black Sea). However, the intensity of physical disturbance is greatest in the shallows, so it is likely that nearshore benthic fauna will be most affected by change. Even so, the length of coastline in the region is ca 17 000 km (mapped at 1:1 000 000 scale), which is about one-third greater than that of the United Kingdom.

image

Figure 1. Map of the Antarctic Peninsula and Scotia Arc region, indicating the extent of the continental shelf (defined as depths<1000 m). Also shown are glaciers that have retreated in the last half century and ice shelves that have collapsed, or significantly retreated, in the last 20 yr.

Download figure to PowerPoint

Second, historically the Antarctic environment has not been stable and environmental conditions, such as the extent of the Antarctic icecap, have varied dramatically through time. Even during the last 1000–10 000 yr the thickness and extent of ice cover has been considerably less than the present time (Clapperton and Sugden 1982, Lorius et al. 1985, Pudsey and Evans 2001) whilst during the past 2 million yr the ice sheet has advanced and retreated in cycles (Williams et al. 1998). Thus, over evolutionary timescales (hundreds of thousands to millions of years) the marine fauna of Antarctica has been shaped by various agents and intensities of disturbance. However, benthic communities are likely to initially respond to the recent warming of the WAP, and its associated effects, over ecological timescales (seasons to hundreds of years), which shall be the focus of this paper.

In the last decade there has been considerable progress in the field of disturbance ecology in polar regions. Here, we update the reviews concerning the role of ice disturbance given by Barnes (1999) and Gutt (2001) and, crucially, use recent evidence to discuss the likely consequences of changing environmental pressures on benthic communities in the WAP region. This review has three main aims: 1) to describe the main mechanisms of physical disturbance, and their influence on nearshore benthic communities, in the region. 2) To assess how the intensity of these disturbance mechanisms may change with continued climate warming. 3) To suggest how shallow water communities in the region will respond to changing physical disturbance.

Historic context of disturbance and shelf biodiversity

  1. Top of page
  2. Abstract
  3. Historic context of disturbance and shelf biodiversity
  4. Ice mediated disturbance
  5. Sedimentation
  6. Other disturbances
  7. Confidence of predictions in a rapidly changing world
  8. Conclusions
  9. Acknowledgements
  10. References

Compared to other coasts around the planet, a cursory view of the Antarctic shelf today reveals a cold, constant and isolated place. A glance at life there might further suggest a rich, mainly endemic, slow growing and moving benthos. Such brief considerations would be very misleading. The Antarctic coastal marine environment, even when considering only the WAP region, is complex in both time and space. Considering these in turn, the shelf environment has changed to differing degrees across time scales from millions of years to seasons. At larger, evolutionary, time scales Antarctica has cooled fairly gradually to reach current temperatures about 4 ma (Zachos et al. 2001). The massive ice sheet that formed as a result depressed the continental shelf considerably (to as deep as 1000 m in places) with strong implications for disturbance patterns and consequences for benthic life there (Clarke et al. 2004a). Within the recent cool period there has been regular glacial cyclicity at ca 100 ka (following solar activity [Milankovitch] cycles) frequencies. Thus, following each short warm period (interglacials) huge ice shelves have grown outwards from the continental landmass to cover much or most of the shelf, scraping the seabed hundreds of meters down. Although ice shelves probably advanced slowly thousands of years this could be argued as massive disturbance on evolutionary time scales and probably had profound influences on speciation and species survival (Poulin et al. 2002, Thatje et al. 2005). The physical environment has varied with many cycles shorter than this (e.g. “short” Milankovitch cycles of 41 ka) but longer than the better studied ecological time scales such as ENSO, annual and seasonal variability (Leventer et al. 1996). Movements of oceanic current systems, such as the Polar Front in both ecological and evolutionary time (Moore et al. 1999) will undoubtedly have brought considerable change to some areas of the shelf. However, as there is no evidence that these movements were close to the WAP shelf, we do not consider it further here. Other than warming pulses there have at times been other brief but major historic disturbance events on ecological time scales, such as the Eltanin meteorite impact 2.5 ma (Gersonde et al. 1997). This undoubtedly had a massive influence on the WAP and South American shelves at the time but subsequent glacial activity has probably removed most trace of this, except in the deep sea. There have also been a number of recent volcanism events in the WAP region, including those on Deception Island (WAP), which have provided a rare opportunity to study benthic responses to intense physical disturbance. Thus, present ice scouring (and other physical disturbances) is superimposed on a benthos that has had to adapt to massive and widespread fluctuations in environmental conditions on the shelf over greater time scales.

Until the advent of modern trawling, the benthic environment around Antarctica would have been both the most, grading to the least disturbed continental shelves on the planet. The shallows are seasonally pounded by the largest mean wave heights and frequently scraped by icebergs, estimated to be one of the most intense natural forces (Gutt and Starmans 2001). This frequency of catastrophic disturbance is even more severe when the slow development and growth rates of Antarctic ectotherms are taken into consideration (Peck 2002, Bowden et al. 2006, Barnes et al. 2007). Towards the deepest parts of the shelf, icebergs probably never penetrate and physical conditions are amongst the most constant of anywhere on the planet. Across time scales there has been a strong gradient of disturbance of various types from the shallows to the deeper parts of the shelf. Disturbance also varies considerably with geography, as well as bathymetry around the WAP. Although disturbance by iceberg groundings is to some extent stochastic, frequencies are likely to be much higher adjacent to glaciers terminating in the shallows. Similarly, disturbance through freshwater inundation and sedimentation are focussed around glaciers, which have retreated past the grounding line (that is they are now inland). Elsewhere around the globe the higher frequency and more intense nature of acute impacts such as warming events are currently the focus of much scientific and popular attention. Around Antarctica acute events are very restricted spatially. Seasonally high UV or warming are concentrated in the small area of the shallow subtidal zone, pollution is restricted to permanent stations or areas of tourism, and volcanism occurs just at Deception Island on the WAP and the South Sandwich Islands in the Scotia Arc. Even some chronic changes, such as sea level rise, will probably have little impact in Antarctica compared to elsewhere because the shelf is so deep and so little of its biodiversity is in the shallows (in direct contrast to for example the tropics).

Fossil evidence from locations such as Seymour Island shows that Antarctic benthic communities in the Eocene were similar to those north of the Southern Ocean (Aronson and Blake 2001). Since then, coincident with a long phase of cooling, a number of taxa disappeared or greatly reduced in diversity and abundance, particularly the fast moving durophagus predators, such as fish and brachyuran crabs. Slow movers, such as various worm phyla, sea spiders (pycnogona), echinoderms and molluscs are now the dominant benthic predators, and sessile suspension feeders (e.g. sponges, soft corals, bryozoans) are their main prey and the major space occupiers of the upper shelf. Long-term geographic and oceanographic isolation has also resulted in generation of many genera and high species level endemism (Arntz et al. 1997). The Antarctic benthos seems to have strongly responded to the more recent, drastic glacial extensions and retreats in the alternation of ice ages. For example on the longer time scales of 100s ka echinoid species with planktotrophic larvae have waned whilst brooders have radiated (Poulin et al. 2002). However, in the brief interglacials, such as now, fauna with planktonic larvae seem to have more rapidly colonised the shallows. Thus, the ability of shallow shelf organisms to recolonise disturbed patches is greater than the wider shelf fauna because they are mainly pioneers, many being broadcast spawners. Another unusual feature of the Antarctic benthos and an adaptation that may be linked to alternating periods of ice shelves overlying the shelf to hundreds of meters depth is a typical high level of eurybathy (Brey et al. 1996). If during glacial maxima grounded ice shelves did cover most of the continental shelf around Antarctica to depths of 500 m or more as some scientists think (Thatje et al. 2005), such eurybathy would have been crucial to the survival of many species. High eurybathy may also be strongly linked to the unusual lack of physical variability (such as temperature or salinity) with bathymetry in the Southern Ocean. However, there is now some evidence to suggest that species with broad bathymetric and latitudinal ranges are actually species complexes, rather than eurybathic species (Held and Wagele 2005), but further investigation is needed to determine the generality of such observations.

Examination of shelf faunal composition at any given point during an interglacial would arguably simply reflect the time the fauna has had to recolonise back up-shelf or whether the area had been bulldozed by an ice shelf or not. Within this large-scale view, it seems that at a local scale the faunal composition also reflects recovery from past disturbance, such as ice-scouring (Conlan et al. 1998, Gutt and Piepenburg 2003). So the Antarctic shelf benthos is unusual in many ways and in any one place could be recovering from disturbance on multiple scales in time and space. The ability of populations, species and communities to respond to disturbance at the current time is set by not only life history characteristics, but by timing of disturbance events relative to larval release and the phytoplankton bloom, and by growth rates and many stochastic events.

Ice mediated disturbance

  1. Top of page
  2. Abstract
  3. Historic context of disturbance and shelf biodiversity
  4. Ice mediated disturbance
  5. Sedimentation
  6. Other disturbances
  7. Confidence of predictions in a rapidly changing world
  8. Conclusions
  9. Acknowledgements
  10. References

Current patterns and processes

Ice scour, the action of floating ice colliding with the seabed, is a major structuring force acting on polar benthic communities (Sahade et al. 1998, Peck et al. 1999, Gutt and Starmans 2001, Barnes and Brockington 2003, Gutt and Piepenburg 2003, Conlan and Kvitek 2005, Smale 2007, Laudien et al. 2007, Smale et al. 2007b). Floating ice can be both sea (seasonal sea ice) and land (ice bergs) derived and, once in the sea, is moved by the action of winds and oceanic currents. If the keel depth of the ice exceeds water depth, the contact between ice and substratum can cause considerable changes to the physical environment, including altered topography, sediment structure and current flow (Lien et al. 1989, Rearic et al. 1990, Woodworth-Lynas et al. 1991). Furthermore, ice scour occurs across a considerable geographic area, as icebergs have been recorded between 75 to 26° (but more typically ca 40°) in the southern hemisphere and between 80 to 44° in the north (Wadhams 2000). Peck et al. (1999) suggested that ice scouring is extensive as it “affects one-third of the world's coastlines”. Gutt and Starmans (2001) later calculated that “iceberg scouring is amongst the five most significant disturbances that any large ecosystem on earth experiences”.

The frequency of ice collisions with the seabed varies considerably in space between poles, regions, sites and depths, and in time from longer term Milankovitch cycles to years and seasons. Broad scale estimates of scour frequency have been proposed for both the Arctic and the Antarctic. Reimnitz et al. (1977) suggested that in some Arctic regions, shallow shelf sediments are completely reworked by ice in 50 yr, which was later supported by Gutt et al. (1996) who calculated that each square metre of their shallow Arctic study area (east Greenland Sea) was impacted once every 53 yr, on average. Ice scouring may occur to depths of up to 600 m in the Antarctic (Dowdeswell and Bamber 2007), which is likely to be considerably deeper compared with the Arctic, where icebergs are generally smaller. Calculations primarily from the Weddell Sea would suggest that each square metre of the entire Antarctic shelf is disturbed by ice on average once every 340 yr, although variability between regions is undoubtedly high (Gutt et al. 1996, Gutt and Starmans 2001).

These estimates of scouring frequency were based on observations of relict scour marks, rather than measures of actual disturbance. There have, however, been two attempts to estimate present-day scouring frequency, both conducted at Adelaide Island on the WAP. In the first study of its kind, Brown et al. (2004) deployed concrete markers in the shallows of two contrasting sites, and recorded how often these markers were damaged by iceberg impacts over a two year period. They observed significant differences in scouring frequencies between sites and found that some areas of seabed at ca 10 m depth were disturbed at least three times during the two-year study. Smale et al. (2007b) expanded this study to determine the effects of depth, site and season on the frequency of ice disturbance. Their study showed that the frequency of iceberg disturbance significantly decreased with depth in the shallow waters of Adelaide Island, and that disturbance varied between sites, seasons and years. Temporal variability in ice scouring frequency is largely driven by the formation and duration of seasonal fast ice. When the sea surface freezes during the winter months to form fast ice, icebergs are trapped within the ice and locked into position, which may restrict their movements and reduce the frequency of ice scouring (Smale et al. 2007b).

Ice scouring has a wide range of effects on marine benthos, which can be considered on three spatial scales: 1) the localised effect of a discrete iceberg grounding, in terms of both the immediate alteration to community structure and the subsequent recovery processes, 2) at the scale of habitat and how communities respond to continuous disturbance and 3) the influence of ice scouring at the regional scale of the WAP and Scotia Arc. The smallest scale, that of the actual collision between ice and sea bed, can range in area from less than a square metre in the shallows to scour marks of up to 350 m width and 15 km in length in deeper waters (Hotzel and Miller 1983). Iceberg impacts are highly energetic, even in the shallows (Smale et al. 2007b), and cause severe damage and high mortality to benthos (Fig. 2). For example, in shallow waters Peck et al. (1999) observed a 99.5% decrease in macrofaunal abundance in newly formed scours compared with unscoured zones in the South Orkney Islands whilst similarly, Smale et al. (2007a) recorded a 94.9% average decrease in abundance on the Antarctic Peninsula. Significantly lower abundance, biomass and richness values in scours compared with unscoured areas have also been recorded in deeper waters in the Weddell Sea (Gerdes et al. 2003), and in coastal waters of McMurdo Sound in the Ross Sea (Lenihan and Oliver 1995) and the Canadian Arctic (Conlan et al. 1998) (Fig. 2). However, iceberg impacts also resuspend benthic material, including food (Peck et al. 2005) and larvae (Dayton 1989).

image

Figure 2. Total faunal abundance and species richness of macrofauna (i.e. ≥0.5 mm size) found in scours compared with adjacent undisturbed zones. The Signy Island study sampled a single scour at 9 m depth, 1 d after formation (Peck et al. 1999). The Adelaide Island data are means (±SE) from 12 scours of known age (1–20 d old) across 3 sites at 10–17 m depth (Smale et al. 2007a). Data from the Weddell Sea are means from 4 scours, thought to be recent, occurring at 225–360 m depth. The species richness data are for polychaetes only (Gerdes et al. 2003). Data from McMurdo Sound are means of 3 scours of unknown age (but thought to be recent) from 3 sites, at 25–40 m depth (Lenihan and Oliver 1995). Finally, the study conducted in Canada sampled 4 scours, also thought to be recent, across 3 sites at 6–15 m depth (Conlan et al. 1998).

Download figure to PowerPoint

Following the retreat of the ice from the impacted site benthic recolonisation and succession processes commence, although it is currently unclear to what extent these processes are predictable. For example, at some shallow polar locations the pioneer assemblage and early patterns of development are relatively constant across scours (Conlan and Kvitek 2005, Smale et al. in press), whilst at deeper shelf sites pioneer species may vary considerably between disturbed areas (Gutt et al. 1996). Even so, a few general points regarding post-scour recovery can be made. Following disturbance, a diverse group of motile scavengers, usually dominated by amphipods, echinoderms and nemerteans, move into scoured zones to feed on the damaged benthos (Richardson and Hedgepeth 1977). Subsequently the scour depression begins to infill with sediment and biogenic matter such as algal detritus, whilst motile deposit feeders and other scavengers move through/ across the substrata and into the scour (Peck et al. 1999, Conlan and Kvitek 2005). The scoured area, now exposed to new recruits, is soon recolonised by early pioneers. For example, on hard substrata the bryozoan Fenestrulina rugula and spirorbid polychaete worms (Brown et al. 2004, Bowden et al. 2006) are often conspicuous members of early post-disturbance assemblages and in soft sediments the polychaetes Capitella sp., Leitoscoloploskerguelensis and Kefersteineni fauveli are of comparable importance (Lenihan and Oliver 1995). Finally, small fauna, such as bivalves and ostracods, may be advected into scour depressions by strong currents or water movements induced by storms or nearby iceberg groundings (Peck et al. 1999).

These recolonisation processes act at differing timescales and, as such, each stage of benthic recovery may be characterised by a different community. Therefore, even within a single site there may be hundreds of iceberg scours of differing ages, which have different physical properties and support, to some degree, a distinct assemblage. However, at intensely disturbed locations assemblages can be constantly held at early successional stages, as the period between disturbance events is too short to allow any prolonged assemblage development. This is particularly evident in benthic assemblages inhabiting the intertidal and very shallow subtidal zones near glaciers or ice cliffs (Pugh and Davenport 1997, Brown et al. 2004). Also at the scale of habitat, ice disturbance almost certainly drives the patterns of community change along a depth gradient in the shallow waters along the WAP and elsewhere. Most floating ice is small in size, as large icebergs and sea ice floes ultimately break up into small pieces of “brash” ice. These small fragments of ice ground out in shallow waters, thus the shallows experience greatest frequency of disturbance (Smale et al. 2007b) and generally support assemblages with low species richness and complexity, and a high abundance of pioneers motile scavengers or deposit feeders. Thus, the shallows represent highly disturbed habitats and support few species, as could be predicted by environmental stress models (see Scrosati and Heaven 2007 for recent discussion). Conversely, the frequency of the disturbance in deeper waters (ca 20 m depth) is significantly reduced (Smale et al. 2007b) and as a result assemblages are considerably more diverse, abundant and massive compared with those in shallower waters. This community change occurs continuously along a depth gradient in polar coastal waters and patterns can be remarkably consistent between contrasting sites (Gambi et al. 2000, Smale 2008). However, ice scouring promotes high patchiness across the depth-gradient (horizontal variability), which in some cases may be as important as variability along the depth gradient (vertical variability).

Although catastrophic at small scales, at the regional spatial scale ice disturbance may play a key role in promoting and maintaining species richness. This is because in undisturbed communities, that is those which have reached a state of near-equilibrium but are probably still in recovery from the last glacial maxima, a number of species are out-competed and displaced (Barnes 2002, Gutt and Piepenburg 2003). However, such species may thrive in early successional stages and as the Antarctic shelf (to ca 300 m depth) is effectively a patchy mosaic of co-existing successional stages following ice disturbance, beta diversity is elevated and maintained. Increased habitat heterogeneity and niche separation, as a direct consequence of ice scouring, may explain (in part) the high biodiversity of Antarctic shelf fauna. In addition, the co-existence of different basal species is likely to provide a range of different microhabitats, which in turn can support a range of epifaunal and infaunal species (i.e. facilitation, Bruno et al. 2003).

It has been assumed for many years that post-disturbance recovery in the Southern Ocean is slow compared with lower latitudes, primarily because of slower growth rates and community development in the Antarctic, but only recently has sufficient data been collected to test this longstanding paradigm. It seems that growth rates are in fact considerably slower compared with lower latitudes (Clarke et al. 2004b, Heilmayer et al. 2004, Barnes et al. 2007) and that, at least in some locations, the rate of community development on hard substrata is an order of magnitude slower in the Southern Ocean, at least at equivalent depths (Stanwell-Smith and Barnes 1997, Bowden et al. 2006). So, community recovery in iceberg scours may take a long time, and early and intermediate successional stages may persist for hundreds of years and in doing so, promote regional diversity. It should be noted, however, that the time required for recovery is determined by the nature of the “undisturbed” community adjacent to the disturbance event. For example, in the highly disturbed shallows scour assemblages regain similarity to those outside the scour within decades (Peck et al. 1999, Conlan and Kvitek 2005, Smale et al. in press). Conversely, in deeper waters that are infrequently disturbed, scours may take hundreds of years to recover as background assemblages are dominated by slow-growing sessile species such as sponges or bryozoans, or structured by dense sponge spicule mats (Dayton 1990, Gutt 2001). Benthic community structure can also be affected over longer timescales, as relict ice scour marks (with altered sediment characteristics and topographies) may persist for thousands of years (Duncan and Goff 2001).

Other forms of ice disturbance that structure Antarctic benthic communities are the action of anchor ice and the seasonal presence of an ice foot. Anchor ice, the formation of ice platelets on the sea bed or organisms, is an important disturbance pressure acting on communities at high latitudes and is particularly evident at East Antarctic sites and the Ross Sea (Dayton et al. 1969). However, there are very few reports of its formation from the WAP and Scotia Arc region, and it is almost certainly very rare here (probably because of oceanographic conditions and the relative lack of large floating ice shelves compared with elsewhere) and it therefore falls outside the scope of this paper. The winter ice foot, on the other hand, is a prominent feature of the region and has a profound, but somewhat spatially confined, effect on its shallow water ecology. The ice foot is a “narrow fringe of ice attached to the coast” (World Meteorological Organisation definition 1970) and is formed during the winter as seawater meets the cold rocky shore and freezes. This layer of ice extends seaward and covers the intertidal and shallow subtidal zone to several metres depth, and persists for more of the year than seasonal fast ice. The encasement of the biota by the ice foot restricts the exchange of gases and water, affects substratum temperature and limits colonisation by new settlers to just a few months of the year (Barnes 1995b, 1999, Waller et al. 2006). Furthermore, the ice foot itself exerts a powerful crushing force on the underlying surface (Scrosati and Heaven 2006). At Adelaide Island, for example, the ice foot persists for up to 9 months of the year and assemblages on the surface of the intertidal and shallow subtidal zones are very species poor and consist only of transient summer grazers and no sessile species. Along some coastlines, however, the protection offered by boulders and cobbles allows the development of more complex subsurface assemblages, which may include over-wintering sessile species and their associated predators (Waller et al. 2006).

Future scenarios and likely benthic response

Currently we are in an interglacial and as such expect that a degree of the glacier and ice shelf retreat should be an expected (normal) response following emergence from a glacial maximum. Thus to evaluate changes in ocean ice-loading, the major form of disturbance to the shelf, we must consider current rapid change in the historical context outlined above. However, following the recent rapid warming of the WAP and Scotia Arc region, glacier retreat and ice shelf collapse have almost certainly occurred at a greater rate than would be expected during natural cycles. To expand, 87% of tidewater glaciers on the Antarctic Peninsula have retreated since the 1940s (Cook et al. 2005), and the rate of ice flow from land to sea along these glaciers has increased by ca 12% since the early 1990s (Pritchard and Vaughan 2007). The vast majority of these retreating maritime glaciers have not yet retreated past their grounding lines and therefore calving ice will, at least over short timescales, generate floating icebergs. Furthermore, the past two decades have seen the retreat of more than ten ice shelves on the Antarctic Peninsula (Vaughan and Doake 1996, Vaughan et al. 2003). To the west of the Antarctic Peninsula, the collapses of the Wordie and Wilkins ice shelves, and the recent retreat of the George VI Sound ice shelf, have almost certainly resulted in an increased ice loading into nearshore waters, again over short timescales. Thus, intuitively it seems that the number of icebergs calving into coastal waters in the WAP and Scotia Arc regions has increased, and will continue to do so until glaciers and ice fronts retreat past their respective grounding lines. It should be noted, however, that long-term iceberg population data are very scarce, and there is currently some debate as to whether the populations of icebergs around the Antarctic Peninsula, and indeed the Southern Ocean as a whole, have increased (Bindschadler and Rignot 2001, Ballantyne and Long 2002, Long et al. 2002).

A second environmental factor, which is also changing rapidly in the region and is linked to ice scouring frequencies, is the duration and extent of sea ice. Sea ice coverage of the Southern Ocean as a whole is highly variable between years and regions, and the region to the west of the Antarctic Peninsula is the only sector to show a significant decrease in sea ice extent. Twenty years of sea ice and air temperature data have been analysed by the Long Term Ecological Research programme at Palmer Station (Smith and Stammerjohn 2001). The study defined an area of ca 2×105 km2 to the west of the Antarctic Peninsula and used satellite imagery to detect a significant decrease in sea ice coverage in the region over the past two decades, which was mainly due to a reduction in the duration of winter sea ice (Smith and Stammerjohn 2001). Other data sets from the Bellingshausen Sea confirm this recent decrease in the extent of sea ice (Jacobs and Comiso 1993, Zwally et al. 2002). In nearshore environments along the WAP, the frequency of collisions between icebergs and the seabed is significantly reduced in winter compared with summer (Smale et al. 2007b). This is because during the winter months icebergs are “locked-in” by the surrounding sea ice, which restricts their movements and hence their potential to cause disturbance (Fig. 3). Intuitively, if icebergs are surrounded by restrictive sea ice for a shorter period of the year, annual ice scouring frequencies in shallow waters will increase.

image

Figure 3. Left: an aerial photograph showing icebergs “locked-in” by fast sea ice during winter, which restricts their movements. White areas are winter sea ice, whilst dark areas are patches of open water. Image reproduced from Peck et al. (2005). Right: actual frequencies of iceberg scouring at two contrasting sites at Adelaide Island, with grey boxes indicating periods of fast ice formation. Adapted from Smale et al. (2007b).

Download figure to PowerPoint

Over the century, it seems very likely that the frequency of iceberg disturbances to shelf benthos along the WAP will increase, as a consequence of greater ice loading and decreased extent and duration of sea ice. At small spatial scales (i.e. sites), increased ice scouring intensity is likely to have important implications for benthic community structure. Recent studies from the Arctic suggest that ice scouring has a positive effect on benthic richness, as sites that are occasionally disturbed by ice support a greater diversity of species than those that are disturbed rarely or never (Conlan and Kvitek 2005, Laudien et al. 2007). However, icebergs are far less numerous and considerably smaller in the Arctic compared with the Antarctic (Wadhams 2000) and the vast majority of nearshore habitats in the WAP and Scotia Arc regions are disturbed by ice within ecological time frames (Barnes 1995a, Gutt et al. 1996, Sahade et al. 1998, Brown et al. 2004). Thus, any increase in disturbance frequency in the region will more likely force assemblages at moderately disturbed sites to become increasingly similar to those at intensely disturbed sites. So, how do nearshore benthic communities shaped by frequent ice scouring differ from those shaped by a moderate frequency of disturbance? The only studies that have linked empirical disturbance frequencies with parameters of community structure have been conducted at Adelaide Island. Brown et al. (2004) quantified ice disturbance frequency at two adjacent sites and made novel and important links between ice disturbance and benthic assemblages. They recorded twice as many iceberg impacts at one site, and the bryozoan communities at this site had half the number of species, two-thirds the space occupation and twice the mortality level compared with those at the less frequently disturbed site (Brown et al. 2004). Similarly, Smale (2007) linked empirical disturbance data with megafaunal, macrofaunal and lithophylic assemblage composition. The study showed that increased ice disturbance was significantly related to decreased species richness, biomass and space coverage. Furthermore, the relative abundance of sessile macrofauna and megafauna was negatively correlated with ice scouring intensity (Smale 2007). These sessile fauna, such as sponges and erect bryozoans, provide habitats for a wide range of epifaunal species on the Antarctic shelf (Gutt and Schickan 1998) and as such play a key ecological role in structuring these communities (i.e. facilitation, Bruno et al. 2003). In summary, the experimental (rather than observational) studies suggest that increased ice disturbance in nearshore habitats along the WAP would lead to reduced species richness, benthic biomass, space coverage of hard substrata, and abundance of ecologically important large sessile taxa.

Despite a relative paucity of experimental work concerning the effects of ice scouring, there have been a number of observational studies on the influence of ice scouring on benthic communities. It seems that ice scouring may select for certain fauna, such as nematodes (Lee et al. 2001), opportunistic and mobile polychaetes and amphipods (Richardson and Hedgepeth 1977, Lenihan and Oliver 1995), pioneering bryozoans (Gutt and Starmans 2001, Brown et al. 2004), fish species (Brenner et al. 2001) and perhaps for small individuals (Smale et al. 2007a). Furthermore, habitats that are frequently impacted by icebergs may support an elevated relative abundance of secondary consumers, as ice impacts damage benthos and promote feeding opportunities for scavenging species (Richardson and Hedgepeth 1977, Conlan et al. 1998). Intensely disturbed locations are also characterised by the persistence of early successional stages (McCook and Chapman 1993, Pugh and Davenport 1997), skewed population structures (Peck and Bullough 1993, Brown et al. 2004), high levels of sublethal damage (Barnes and Arnold 2001, Clark et al. 2007), and a paucity of long-lived, large, sessile species (Sahade et al. 1998, Smale 2008). In the northern hemisphere, Scrosati and Heaven (2007) recently showed that species richness on rocky shores was negatively related to ice scour stress. Thus, if ice scouring to nearshore benthos intensifies over ecological time scales, it seems likely that communities at some locations will become dominated by opportunistic pioneers and highly motile scavengers, whilst species richness and the abundance of large sessile fauna will decline.

The frequency of ice scouring is broadly related to depth (Gutt 2000, Smale et al. 2007b); most icebergs are small and ground in relatively shallow water (i.e. 0–100 m depth) whilst only a few are large enough to impact the seabed to depths of up to 600 m (Dowdeswell and Bamber 2007). Thus, benthic assemblages inhabiting the upper shelf zone are, broadly speaking, more influenced by disturbance than those in deeper water. However, in a historical context, much shelf fauna has been repeatedly bulldozed down to the deep continental margin by the highly dynamic ice sheet, which has cyclically advanced and retreated throughout the last 2 ma. During interglacial periods, the largely defaunated shallow shelf is recolonised by fauna inhabiting either the deep continental margins or isolated ice-free refuges on the shelf itself (Thatje et al. 2005). This is evidenced by the wide bathymetric ranges of many Antarctic taxa compared with elsewhere (Brey et al. 1996), and high degree of similarity between shelf and slope fauna recorded at some Antarctic locations (Galeron et al. 1992, Cattaneo-Vietti et al. 2000). For example, a recent study of cryptobenthos at Shag Rocks (Scotia Arc) found that most continental slope species were already known from the shelf and that 6 species from 1500 m have even been found in the intertidal zone (Barnes in press). Thus, if ice disturbance to the upper shelf zone intensifies, the most likely response of typical Antarctic species would be a bathymetric shift in distribution. Such a shift might take the form of effectively a contraction of species bathymetric ranges to the lower depths, and thus might also involve changes in the abundance of secondary consumers due to altering food availability. It could be the case that an increased frequency of ice scouring along the WAP would select for species with planktonic larvae, as these species have a higher dispersal capability than brooding species and can rapidly recolonise disturbed areas or newly available space (Poulin et al. 2002). However, predictions based on developmental modes alone should be made with caution, as the predicted dispersal potential (based on reproductive strategies and larval dispersal) does not always equal the realised dispersal ability for a given species.

Benthic macroalgae, although somewhat spatially restricted and lower in species richness and biomass in Antarctica compared with elsewhere, contribute significantly to the Antarctic coastal food web, both directly and as detritus (Heywood and Whitaker 1984, Wiencke 1996). Species richness decreases with increasing latitude in the Southern Ocean, probably as a result of disturbance intensity and temperature (Heywood and Whitaker 1984). Even within a single site, intense ice disturbance in the shallows may select for certain species, which are often fast growing, poor-competitors (Quartino et al. 2001). Thus, any change in the frequency of ice scouring is likely to affect both species richness and biomass values of macroalgae assemblages in nearshore Antarctic habitats. However, the is a severe paucity of investigations on macroalgae form the Antarctic Peninsula, so it is difficult to make any detailed predictions with an acceptable degree of confidence.

In contrast to the shelf system along the WAP generally, many of the intertidal and immediate subtidal habitats in the region are likely to experience a decrease in ice disturbance intensity over the next century. This may be particularly the case in the northern part of the Antarctic Peninsula and towards the latter half of the century. It is likely that the winter icefoot, which presently covers the hard substrata of the intertidal and shallow subtidal zone to depths of up to 5 m, will decrease in its extent and duration as a consequence of regional warming. The recent warming has been relatively more pronounced during the winter months compared with summer (Turner et al. 2005), and the persistence of coastal sea ice is likely to continue to decrease over the next few decades. The winter ice foot is, on small spatial scales, a dominant structuring force and holds benthic communities at very early successional stages (Barnes 1995b, Waller et al. 2006). Thus, any reduction in its extent or duration would decrease the severity of these habitats and perhaps allow the development of more complex assemblages, particularly at sheltered sites that are protected from summer ice scour. Long-standing paradigms in intertidal polar ecology have had to be revised recently, as new evidence suggests that some species can “over-winter” in the intertidal zone beneath the ice foot and, in some respects, these habitats are not as barren as traditionally perceived (Waller et al. 2006). It is possible that a decreasing ice foot presence will permit permanent (but simple) year-round communities to establish in the very shallow waters along the WAP. Conversely, the icefoot provides some thermal insulation to intertidal communities from freezing winter temperatures (Waller et al. 2006, Scrosati and Eckersley 2007) and may protect intertidal assemblages from wave action for much of the year, so a reduction in the extent and duration of the ice foot could also have some negative impacts on community development.

Predicting the intensity of ice disturbance over timescales greater than a few decades is, of course, very uncertain, but a few broad generalisations can be made nonetheless. Currently, ca 55% of the seaward margin of the Antarctic Ice Sheet is floating, but this proportion will be considerably reduced as the ice retreats in response to sustained and increased warming. Once maritime glaciers and ice fronts retreat onto land and past their grounding lines, calving ice will remain landlocked rather than being deposited into coastal waters where it can disturb marine benthos. Similarly, although the disintegration of an ice shelf can lead to rapid coastal ice loading by accelerating ice flows (Scambos et al. 2004, Rignot et al. 2005), this can only be sustained for a finite period (though this may be centuries along the margins of Ellsworth and Marie Byrd Land). So, whilst the major Antarctic ice streams will continue to transfer ice from land to sea over long time scales, the rate of ice deposition by small maritime glaciers and ice shelves will almost certainly reduce over time. The maximum depth at which ice scouring can occur is also likely to decrease due to the thinning of ice shelves and glaciers in the region, which has recently been observed and is set to continue (Rignot and Thomas 2002, Thomas et al. 2004). Intuitively, icebergs with smaller draughts will calve from thinner ice shelves and glaciers, with a decreased frequency of ice disturbance to shelf benthos a likely consequence. So, whilst the current maximum draught of an Antarctic tabular iceberg is ca 600 m (Dowdeswell and Bamber 2007), icebergs calving in the WAP region will have considerably smaller draughts if ice thinning persists. Marine benthos inhabiting the deeper waters of the shelf (i.e. >600 m depth) will be unaffected by ice scouring in the absence of icebergs with large draughts, which could have severe implications for habitat heterogeneity and regional biodiversity. Johst et al. (2006) recently modelled the Weddell Sea system and showed that any decrease in ice scouring intensity across the shelf as a whole would be more detrimental to benthic biodiversity than an increase, due to an unevenness in the longevity of late and early successional stages.

Sedimentation

  1. Top of page
  2. Abstract
  3. Historic context of disturbance and shelf biodiversity
  4. Ice mediated disturbance
  5. Sedimentation
  6. Other disturbances
  7. Confidence of predictions in a rapidly changing world
  8. Conclusions
  9. Acknowledgements
  10. References

Current patterns and processes

Most of the maritime glaciers along the WAP have dramatically retreated in the last few decades (Cook et al. 2005), and the flow of ice into coastal waters in the region is currently accelerating (Pritchard and Vaughan 2007). The increased sedimentation associated with these glacial retreats is likely to have a considerable, but localised, effect on benthic communities adjacent to glacial termini. For example, a retreating Alaskan glacier may deposit up to 14 cm of sediment annually at its terminus (Cowan et al. 2006), whilst acute ice calving events considerably increase water column turbidity and rapid glacier surges can lead to a 30-fold increase in seabed sedimentation for 4 km from the ice front (Gilbert et al. 2002). Directly beneath retreating ice fronts, where sedimentation rates are greatest, benthic fauna are completely smothered and the seabed is largely uninhabitable to mega and macrobenthos. Studies on the effects of sedimentation on polar coastal benthos have been conducted almost exclusively at Spitsbergen in the Arctic, but the physical processes driving the observed patterns are likely to be consistent at both poles, despite considerable differences in the assemblages concerned. It should first be noted that whilst fjord systems are excellent “natural laboratories” for studying the response of benthic assemblages to sedimentation, they are complex systems strongly influenced by other physical factors, such as ice scour, post-disturbance recovery times, freshening and sediment load. Hence, although fjords do represent gradients of sedimentation, this process cannot be considered in isolation and conclusions should be drawn tentatively.

In the most comprehensive study of its kind, Syvitski et al. (1989) sampled the benthos inhabiting ten Arctic fjords influenced by glaciers at differing stages of retreat. They proposed a general model of benthic community change during glacier retreat, which was principally driven by sedimentation rates. The seabed proximal to a retreating glacier is characterised by exceptionally high sedimentation and supports a pioneer assemblage of very few species (perhaps just one) of macrobenthic deposit feeders. This simple assemblage is likely to persist until the glacier front has retreated onto land, at which point sedimentation decreases to a moderate intensity and a more complex assemblage, still largely devoid of suspension feeders, can develop. The final stage in the faunal succession can occur once a glacier has retreated across land to expose an extensive valley floor, which filters sediment discharge and restricts the transport of sediment to the sea. The reduced sedimentation allows greater light penetration and causes minimal smothering, so that the seabed supports a diverse community of plants and animals, including suspension feeders and predators. There is some evidence that this model is applicable to benthic communities adjacent to Antarctic glaciers (Hyland et al. 1994), but there are currently very few field observations relating to sedimentation effects and no field manipulations of sedimentation rates.

Apart from the direct input of sediment into coastal waters near glacier termini, there are two other sedimentation processes which are known to disturb the present shelf fauna around Antarctica. The first, “slumping”, occurs when unstable sediments slide down steep sections of sea floor, analogous to landslides on mountain slopes. Slumping events smother benthos and can cause high mortality in populations of susceptible taxa, such as soft corals (Slattery and Bockus 1997). Although the only formal reports of slumping come from the Ross Sea region (Slattery and Bockus 1997, Gambi and Bussotti 1999), this disturbance mechanism also occurs along the WAP (Smale unpubl. from Adelaide Island, 2005), where the shallow shelf is steep and icebergs frequently impact unstable sediments. The second noteworthy mechanism of disturbance relates to the deposition of large dropstones, which are land-derived pieces of rock that freeze into glacial ice and eventually reach the sea on the underside of icebergs. As the icebergs decay the dropstones are released and deposited on the sea bed, which may occur many kilometres away from the source glacier. At small scales smothering by dropstones is a severe disturbance that damages sessile benthos and is hence detrimental, but at large scales dropstones represent an important source of hard substrata for colonisation and may aid the long distance dispersal of some species (Dayton 1990).

Future scenarios and likely benthic response

Compared with elsewhere, including even the Arctic, nearshore communities around Antarctica are strikingly less influenced by terrestrial inputs such as sediment deposition and freshwater run-off. However, it is very likely that sedimentation rates will increase in the coastal waters of the WAP region as glaciers, many of which are considerable distances from their respective grounding lines, continue to retreat. Also, ice on land is melting rapidly and the amount of exposed rock along the coastline has increased in recent years, with increased deposition of wind-derived inorganic material into coastal waters a likely consequence. Predicting the effects of increased sedimentation on benthic communities along the WAP is difficult as there have been very few direct studies conducted thus far, and making inferences based on observations made outside Antarctica would perhaps be misleading for two reasons. First, increased coastal sedimentation at lower latitudes, and to a lesser extent the Arctic, generally results in greater deposition of organic land-derived detritus and DOM into nearshore waters. The most likely response of the benthic community to this elevated organic input is an increased relative abundance of detritivores and fewer primary producers (Thrush et al. 2004). However, in Antarctica (at least for the next few hundred years) increased sedimentation following ice retreat will only increase the deposition of inorganic matter, as the potential input of land-derived organic matter is negligible. Therefore, observations from temperate, and even many Arctic, sites are not applicable to the Southern Ocean system. Second, the shelf fauna of the Southern Ocean has evolved in isolation for millions of years and, at least since the last glacial minima, has not been shaped by high sedimentation rates. This fauna is characterised by a high diversity and abundance of sessile suspension feeders (Arntz et al. 1994, Clarke et al. 2004a), which are relatively more prominent in the Southern Ocean than elsewhere, but may be particularly susceptible to smothering by sediments (Slattery and Bockus 1997). In summary, increased sedimentation is likely to have a very different, and perhaps even more detrimental, effect on nearshore benthos in Antarctica compared with elsewhere.

Even so, some predictions concerning the response of benthos to changing sedimentation rates and disturbance frequencies at retreating glacier termini can be tentatively made. Assemblages proximal to retreating ice fronts are very likely to be species poor and characterised by short-lived mobile taxa as a result of high sediment deposition, freshwater inflow and ice disturbance intensity (Syvitski et al. 1989, Hyland et al. 1994). As glaciers retreat, sediment deposition within a few kilometres of the ice front remains high, and the benthos is held at a relatively early successional stage. However, after a few decades of retreat it is likely that the environmental conditions structuring the benthos at the original glacier terminus become less severe, in terms of both sedimentation and disturbance frequency. At this point benthic assemblages generally become more diverse, abundant and massive. For example, at Anvers Island on the Antarctic Peninsula, a large glacier retreated by 250 m in 18 yr and the number of species at the original glacier terminus doubled, whilst abundance increased by a factor of 5.5 (Hyland et al. 1994). It seems that the general model proposed by Syvitski et al. (1989) is, to some degree, applicable to the Antarctic system, and significant benthic change is associated with glacial retreat because of changing environmental stressors, principally sedimentation and disturbance. However, without direct field manipulations of sedimentation rates (of which there are none from the Antarctic) it is difficult to predict in any detail the likely response of benthic assemblages to this factor. It is very likely that increased sediment deposition in shallow waters will promote sediment instability and increase the likelihood of slumping events, particularly on steep submarine slopes (of which there are many along the Antarctic Peninsula). An increased frequency of sediment slumping would be detrimental to benthos, as a number of common taxa are highly susceptible to smothering by fine sediment (Slattery and Bockus 1997). Thus, increased sediment deposition around retreating ice fronts is likely to affect a greater area of benthos than simply that which is initially smothered.

Finally, the deposition of biogenic material from the water column to the benthos is very likely to be affected by climatic change. Variation in oceanic primary production, as a result of climate-driven changes in water column recycling and sea ice cover, will certainly influence benthic biomass and community structure (Smith et al. 2006). However, due to a lack of multi-year studies of bentho-pelagic coupling on the Antarctic shelf, the nature of these changes is currently very difficult to predict (Smith et al. 2006).

Other disturbances

  1. Top of page
  2. Abstract
  3. Historic context of disturbance and shelf biodiversity
  4. Ice mediated disturbance
  5. Sedimentation
  6. Other disturbances
  7. Confidence of predictions in a rapidly changing world
  8. Conclusions
  9. Acknowledgements
  10. References

Wave action: current patterns and likely changes

For many years, marine ecologists have realised that exposure to wave action is a critical factor in determining benthic community structure in intertidal and shallow subtidal habitats. Considerable turbulence caused by rolling and breaking waves can be detected to depths of ca 20 m, and is an important disturbance mechanism to communities on both hard and soft substrata around the world (Dexter 1992, Ricciardi and Bourget 1999). The coast along the WAP (particularly to the north and the Scotia Arc) is no exception as some of the strongest winds and greatest wave heights have been recorded from the Southern Ocean (Sterl and Sofia 2005). The Southern Ocean is unique in that there are no continental barriers intersecting the ca 55–60° latitudinal belt, which permits the continuous flow of the Antarctic Circumpolar Current (ACC) and the West Wind Drift. As a result, some oceanic regions (e.g. Drake's Passage) have well-deserved reputations for being amongst the roughest seas in the world (Fig. 4). In the context of benthic communities, those inhabiting the shallows of the Scotia Arc islands and the northern tip of the Peninsula are structured, to some degree, by wave action in all but the most sheltered of sites. At Signy Island, Barnes (1995a) suggested that water movement caused by waves and currents, in conjunction with substrata characteristics were the key factors influencing the development of shallow subtidal assemblages. Wave action has a pronounced effect on assemblages encrusting semi-stable substrata (i.e. cobbles and rocks) in the region by increasing turnover (Barnes et al. 1996, Barnes and Lehane 2001), although frequent disturbance by waves, currents and ice probably prevent space monopolisation by highly dominant competitors (Barnes 2002). Crucially, the relative importance of wave action as a community-structuring disturbance decreases with increasing latitude along the WAP, for two reasons. First, wind speeds (and hence wave heights) to the south of the West Wind Drift (i.e. south of ca 60°) are considerably reduced (Fig. 4). Second, shorelines at high latitudes on the WAP are protected from wave action by seasonal sea ice for much of the year, which dissipates energy from oceanic swells before it can impact on shallow subtidal assemblages.

image

Figure 4. Top: mean annual wind speed and wave height in ocean basins with latitudes. Modified from Barnes (2002). Bottom: schematic of mean wind direction and speed influencing the West Antarctic Peninsula and Scotia Arc region. The sizes of the arrows indicate relative mean wind speed (adapted from various sources).

Download figure to PowerPoint

The WAP region has experienced considerable climatic changes over the past few decades. In addition to the well-documented atmospheric warming and ozone depletion, the strength of the westerly winds blowing across the northern Peninsula has increased (Thompson and Solomon 2002, Marshall et al. 2006). Intuitively, if the frequency of strong westerly winds continues to increase in the northern WAP area then the intensity of wave action influencing shallow water benthic structure is also likely to increase. Increased wave action is likely to influence the distribution of habitat structuring macroalgae, affect dispersal and recruitment dynamics and induce alterations in assemblage composition on semi-stable substrata as a consequence of increased turnover rates. It is important to note that whilst significant increases in wind speed and wave height have not been detected in the region at latitudes> ca 60°, a reduction in the extent and duration of seasonal sea ice, which protects shallow subtidal assemblages from wave action for most of the year, could lead to a relative increase in the importance of wave action as an agent of disturbance along the WAP coastline.

Freshening events: current patterns and likely changes

Compared with the Arctic (and lower latitudes) the influence of freshwater on marine benthos in Antarctica is minimal. Even so during Antarctic summers, melt water dilutes surface layers (Dierssen et al. 2002, Clarke et al. in press) and is an important stressor acting on shallow water benthos. The coastal waters of some regions, such as the WAP, are likely to experience a considerable increase in freshwater input over the next hundred years as a result of glacial and ice shelf melting (Dierssen et al. 2002, Jacobs 2006). Although such freshening will only influence benthos in very shallow waters, this fauna is not, historically, adapted to low salinity stress and no “estuarine” communities exist as at lower latitudes. Freshening events could, therefore, strongly affect benthos and perhaps be synergistic with other changing disturbance pressures in the shallows. Stockton (1984) observed mass mortality in an epifaunal bivalve population following the summer formation of a hyposaline lens of seawater at McMurdo Sound, but this is the only field report concerning this process. Conversely, in the laboratory some Antarctic algae are unaffected by hyposaline conditions but are severely impaired by increased salinity (Wiencke 1996). The intensity and frequency of biologically important freshening events may increase in some regions as a result of climate warming. For example, Jacobs et al. (2002) showed that the salinity of the surface waters of the Ross Sea decreased during the late 20th century, probably as a result of increased precipitation, reduced sea ice formation and continued melting of the West Antarctic ice sheet. Surface freshening on this scale can have a wide range of effects on both the water column and the seabed below it, including increased stratification and therefore reduced penetration of light and oxygen through the water column.

Anthropogenic impacts: current patterns and likely changes

There are a wide variety of influences on Antarctic biodiversity from human activity around the Scotia Arc and Antarctic Peninsula, such as harvesting food. In the last hundred years harvesting has lead to considerable shifts in the balance of foodwebs because of the over-exploitation of certain key guilds, such as some whales, seals, fish, squid and krill. However, trawling in the WAP region was banned by CCAMLR in 1998. In part this is because of concerns about damage to rich and sensitive coral/sponge communities but also because Antarctic growth rates are so slow (Clarke et al. 2004b, Heilmayer et al. 2004, Barnes and Conlan 2007), suggesting that community recovery would also be slow. However various exploratory commercial trawling has taken place around a number of the “sub Antarctic” islands north and south of the Polar Front. The collapse of low latitude fisheries and ever-increasing demand for food resources will translate to increased pressure to harvest living resources around the Southern Ocean. As a result, we can expect to see many similarities with how ecosystems have fared north of the Southern Ocean, such as serial collapse of populations of larger species, dominance of pioneers, decreasing size at first breeding, reduction of trophic complexity and increase in fragility to natural cyclical disturbance (Jackson et al. 2001).

Another considerable acute and direct anthropogenic impact considered here is the transportation (deliberate or accidental) and establishment of non-indigenous species (NIS). A wide variety of terrestrial NIS have been transported to the sub Antarctic islands during the last century (Frenot et al. 2005), on food, timber, clothing and even on other larger NIS (such as mites on mammals). However, the Southern Ocean is the only large-scale marine environment from which no animal NIS have yet been reported as established. However, in the last three decades a number of marine NIS have been confirmed as surviving transport into the Southern Ocean, most recently the highly invasive mussel (Mytilus sp.) on a scientific ship (Lee and Chown 2007). It is almost certain that one of the many marine faunal NIS on ship hulls, in ballast water or floating on plastic debris will establish at a Southern Ocean site, if it has not already happened. Several species of algal NIS have already done so inside the caldera of Deception Island (C. Wienke pers. comm.). We suggest that various localities in the South Shetland Islands and South Georgia are the most likely first places for marine NIS to establish as these have a high volume of ship traffic directly from major ports (hotspots of marine NIS) and they have amongst the warmest summer sea temperatures in the shallows of anywhere around Antarctica (Barnes et al. 2006).

On longer time scales there are also a wide variety of chronic and indirect anthropogenic impacts. Within the Southern Ocean, a “climate change” related warming signal has only been detected in the surface waters of the Bellingshausen Sea (Meredith and King 2005). Antarctic surface water (AASW) only varies seasonally by a few degrees C, so increase by a degree in 50 yr is likely to be ecologically important. Warming has also been detected in Weddell Deep Water (WDW) but only by thousandths of a degree (Robertson et al. 2002), and has not clearly been linked to regional warming. It does, however, seem likely that the geographic and bathymetric range of warming will increase given projected climate models. Evidence for how benthos might respond to medium term temperature rises is currently by short-term experimentation. These have suggested that Antarctic benthos is typically stenothermal and thus might be highly sensitive to predicted climate change (Peck 2005). Fish and most invertebrates tested to date, which are Antarctic endemics, are killed by temperatures raised to ca 10°C or even less (Somero and DeVries 1967, Portner et al. 1999, Peck 2005). Although this level of warming is not expected within the next century, short-term rises can restrict populations considerably before lethal temperatures. Laboratory work on molluscs such as Laternulaelliptica and Nacella concinna have shown that warming to just a few degrees above their normally experienced maximum severely impairs their ability to perform work, such as reburying or turning back over (Peck et al. 2004).

Current physiological evidence would suggest that many Antarctic species might have little scope to respond to even a few degrees of warming, which is a realistic possibility. The applicability of these data to real scenarios in a changing marine environment depends on a number of assumptions. These include that 1) species behave similarly to much longer term rises in sea temperature, 2) seasonally varying (in contrast to fixed experimental) sea temperatures do not alter vulnerability, 3) model organisms used are representative of their taxa and 4) that current distributions of many species which include populations at sites or depths encompassing a much greater temperature range than “typical Antarctic conditions” have little gene flow over time. At South Georgia, where sea temperatures are typically 3°C warmer than some AP sites in summer (Barnes et al. 2006) there are populations of many otherwise typical Antarctic species. Whether this is evidence that species can survive wider ranges than experiments predict, or they are edge of range populations surviving at their limit or even that they have passed the limit and survive only by continuous supply of recruits from elsewhere is unknown. We suggest that there is probably a wide range of responses to chronically raised temperatures, not least that non-endemic species are potentially more flexible than endemics given that populations of the former currently live in considerably warmer waters. Also, effects of temperature stress may well be synergistic with other changing stressors, such as sedimentation or acidification, so that the total selection pressure acting on any given species is much greater than simply the sum of the individual parts (Harley et al. 2006).

Other chronic anthropogenic impacts include effects of warming and causes of warming. For example, one of the many effects of warming is sea level rise, through both thermal expansion of water and melt of land based ice. Unlike elsewhere even considerable sea level rise will probably have minor influence on Antarctic marine biodiversity for several reasons. Coastal topography in most localities is very steep so rises of tens of meters changes the coastline little and will probably add or remove few habitats. There is no specialist intertidal or subtidal fauna (Waller et al. 2006) and species have unusually high bathymetric ranges (Brey et al. 1996). In the tropics past rapid rises in sea level have been associated with drowning low lying islands and coral reefs but there are no equivalent habitats in the polar regions. In contrast, a cause of region warming, increases of aerial CO2, drives a direct and major chronic disturbance, acidification. The levels and rates of increase of atmospheric concentrations of CO2, which are unprecedented for millions of years, have resulted in similarly unprecedented changes in ocean chemistry. Approximately half of anthropogenically emitted CO2 has been absorbed by the global ocean (Sabine et al. 2004), dissolved and dissociated into ions. The increased concentration of H+ ions has shifted oceanic pH by 0.1, equivalent to a 30% increase in acidity. Projections of acidity in the global ocean by Orr et al. (2005) suggest sustained and accelerated increases over the next few centuries, with lag phases increasing with increased bathymetry. Such models suggest ocean acidification, as an agent of disturbance, will increase for hundreds of years after all fossil fuels have been depleted. The effect, that organisms will find it harder to secrete and maintain CaCO3 (e.g. for skeletons), will not be evenly distributed. It will be most severe at the surface waters of the Southern Ocean, because of lower CaCO3 saturation levels and a shallower compensation depth (CCD) compared with elsewhere. Most animals with shells and hard skeletal components use CaCO3, and those which use the aragonite form will be affected first as the saturation levels of this are lower than the calcite form (of CaCO3). Orr et al. (2005) suggested that the problem for Southern Ocean benthos, such as some molluscs and brachiopods that have thin aragonite shells, could be critical within a century. This may represent less than ten generations for some of the deeper animals.

Confidence of predictions in a rapidly changing world

  1. Top of page
  2. Abstract
  3. Historic context of disturbance and shelf biodiversity
  4. Ice mediated disturbance
  5. Sedimentation
  6. Other disturbances
  7. Confidence of predictions in a rapidly changing world
  8. Conclusions
  9. Acknowledgements
  10. References

In this review we attempt to predict the most likely nature of disturbance based on current climate projections. It is important to assess the level of confidence with which we have presented likely patterns of future environmental and ecological change in the WAP and Scotia Arc region. There are two main complexities that promote uncertainty, the first relates to the confidence associated with climate projections and the second involves the stochastic nature of catastrophic disturbance events. Our predictions of environmental change, which will drive ecological change, rely on the applicability of the current generation of climate models. Whilst warming at the global scale is set to continue and even accelerate (in its 4th assessment report the Intergovernmental Panel on Climate Change projected that global mean surface temperature will rise between 1.3 and 1.8°C by the mid 21st century), climate projections at the regional scale are highly complex. Climate projections for Antarctica are even more so, as long-term meteorological records are spatially restricted compared with elsewhere, which impedes climate modelling. It is almost certain that the rapid warming of the Antarctic Peninsula observed over the last 50 yr will be sustained and probably increase in both intensity and geographic and bathymetric coverage. However it is important to realise that the magnitude and spatial distribution of warming is currently unknown, as recent climate models cannot yet reproduce the observed recent warming in a realistic way (Vaughan et al. 2003). For example, despite intense investigation over recent years, the extent of loss of Arctic sea ice in summer 2007 was not projected by current models and completely surprised most scientists. The second complexity relates to the stochastic nature of catastrophic events. The scenarios we have presented involve “normal” disturbance pressures which are clearly evident today. However, a large meteorite impact or volcanic eruption, which have had widespread effects on the ecology of the region previously (Gersonde et al. 1997, Lovell and Trego 2003), could be relatively more influential than the disturbance factors outlined above. Clearly, such catastrophes would have widespread consequences and could drastically alter some of our predictions.

Conclusions

  1. Top of page
  2. Abstract
  3. Historic context of disturbance and shelf biodiversity
  4. Ice mediated disturbance
  5. Sedimentation
  6. Other disturbances
  7. Confidence of predictions in a rapidly changing world
  8. Conclusions
  9. Acknowledgements
  10. References

Marine benthic communities inhabiting the shallow continental shelf around Antarctica are, and have been, hugely shaped by physical disturbance pressures – perhaps more so than anywhere else on the planet. Historically, the intensity and relative importance of these disturbance pressures has changed over evolutionary and ecological timescales, but the rate of change we can expect to occur over the next few centuries is unprecedented in millions of years. Over the next 100 yr, the relative importance of the major agents of disturbance in determining community structure and species distributions is likely to shift. In the short-term (i.e. decades) it seems almost certain that the frequency and intensity of ice scouring will be the dominant physical force structuring assemblages (Fig. 5), as ice loading into coastal waters continues to accelerate and the extent and duration of seasonal sea ice decreases. Sedimentation, freshening and direct human impacts will be important, but fairly localised, factors but their relative influence is likely to increase with time as ice fronts retreat, ice coverage decreases and man's activities intensify in the region (Fig. 5). It is, of course, difficult to predict the response of the benthic system to such changes with any precision, but we have attempted to present the most likely outcomes.

image

Figure 5. Schematic showing the relative importance of the major physical disturbances (capitals) influencing benthic community structure in the West Antarctic Peninsula and Scotia Arc region. Likely future scenarios over both short (decades) and longer (centuries) timescales are shown. The sizes of the grey arrows indicate the relative importance of each major disturbance pressure.

Download figure to PowerPoint

Changes in benthic community structure are likely to vary considerably from place to place, as they do now, but for those sites subjected to increased disturbance of any form some generalisations can be made. Perhaps crucially, from the limited number of reports available, it seems that the same suite of species (this is particularly true for polychaetes) are associated with early stages of post-disturbance recovery, regardless whether the initial disturbance was from ice scouring (Gerdes et al. 2003, Smale et al. 2007a), anchor ice (Lenihan and Oliver 1995), sedimentation (Hyland et al. 1994) or pollution (Lenihan and Oliver 1995, Conlan et al. 2004). Thus, these same circum-Antarctic pioneers are likely to become dominant at sites where disturbance pressures intensify. Frequently disturbed assemblages in Antarctica, as with elsewhere, are characterised by a low number of species, small individuals and a lack of sessile, structural taxa. We can expect these assemblages to develop at shallow sites proximal to retreating glaciers and ice fronts, of which there are many in the WAP and Scotia Arc region, in response to increased sediment deposition, ice disturbance and fresh water input. At larger scales, we can perhaps expect species distributions to alter in their bathymetry, although this will undoubtedly be highly patchy. Isolated patches of assemblages in areas of shelf protected from physical disturbance will be important sources of recruits for colonising newly available space, as it seems they have been throughout past glacial cycles (Held 2003, Allegrucci et al. 2006).

If, as is increasingly being discussed, IPCC projections of CO2, temperature and sea level rise have been very conservative and ice sheets and shelves are more dynamic than supposed, even the current generation of human observers will witness rapid, massive and catastrophic changes in ecosystems. Sometimes in the past disturbance pressures have been severe and accompanying organism responses have been termed “mass extinctions”. We suggest that the current and developing situation is analogous to these past events and will increasingly be discussed as such. The Arctic and Antarctic are our greatest repository of near-past information through ice and sediment cores, and parts of these regions are the most rapidly altering systems, with arguably the most sensitive fauna and least complex systems. We think they must represent our best prospects of understanding and projecting future disturbance and organism response through wider yet more detailed analyses and interpretations of past and current change.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Historic context of disturbance and shelf biodiversity
  4. Ice mediated disturbance
  5. Sedimentation
  6. Other disturbances
  7. Confidence of predictions in a rapidly changing world
  8. Conclusions
  9. Acknowledgements
  10. References

We thank P. Fretwell and A. Cook for assistance with mapping and K. Linse for constructive comments on an earlier draught of the paper. Thanks also to J. Gutt, R. Scrosati and one anonymous reviewer for their helpful comments on an earlier draught of the review.

References

  1. Top of page
  2. Abstract
  3. Historic context of disturbance and shelf biodiversity
  4. Ice mediated disturbance
  5. Sedimentation
  6. Other disturbances
  7. Confidence of predictions in a rapidly changing world
  8. Conclusions
  9. Acknowledgements
  10. References