Peak flows in the wet season, and their variability from year to year, determine the structure of channels and floodplains, regulate primary production on floodplains and in riparian zones, and provide hydrological connectivity for transporting nutrients, sediments and organic matter. They also provide opportunities for the movement and recruitment of biota between reaches that are isolated during the dry season. The wet season is the time in the annual hydrograph when floodplains are reconnected with their rivers, an important phase given the relatively large proportion of catchment area occupied by floodplains across the region. There is a continuum in the inundation period, or flood residence time, of floodplains across the region: some floodplains, such as those in Kakadu National Park and the Daly River (Fig. 1), generally flood each year and some areas can be inundated for up to 6 months at a time (Pettit et al., 2011). Others, such as the Fitzroy River (Western Australia) and the Mitchell River (Queensland; Fig. 1), have floodplains that may only be inundated for days to weeks, even in years of high rainfall (Fig. 5). On floodplains that experience short periods of inundation lasting from days to weeks, there has been little development of aquatic plant communities adapted to long periods of inundation, and aquatic primary production is largely limited to permanent waterholes. Primary production of terrestrially-adapted plant communities can be initially suppressed and then increase following the recession of floodwaters when soils are recharged with nutrients and moisture. In these latter systems, there is only a relatively short period available for nutrient transfer and use, aquatic–terrestrial fluxes in food resources and the active movement of organisms across the floodplain (Douglas et al., 2005). These floodplains clearly have a different ecology to those inundated for long periods, but we are unable to find any literature on equivalent systems elsewhere that bears on the observations we report here. The ecology of these systems, particularly the ability of biota to take advantage of short inundation periods, represents a significant knowledge gap for northern Australia. Consequently, much of the ensuing discussion refers to the more studied systems in the Northern Territory that are characterised by relatively long inundation periods.
Figure 5. Percentage of floodplain area on the Daly (black) and Mitchell (grey) River floodplains, inundated by floodwaters during 2008 (D. P. Ward, unpubl. data).
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Rapid pulses of wet season discharge can have high erosional power through the bedrock-constrained valleys, headwaters and steep gorges that are features of Australia’s northern escarpment country, resulting in little material entrapment in these reaches (Brodie & Mitchell, 2005). A large proportion of the sediment in Australian rivers, including those in the northern wet–dry tropics, is derived from channel and gully erosion (Prosser et al., 2001; Wasson et al., 2010). Although the escarpments are generally restricted to Arnhem Land in the Northern Territory and the Kimberley Plateau in Western Australia (Fig. 1), and most of the region lies below 400 m altitude, erosional forces can still be high in low-gradient, sand-bed rivers (Brooks et al., 2009). For example, since 1990, more frequent overbank flows have contributed to increased riverbank erosion and slumping, channel-widening and sedimentation in the Daly River, Northern Territory (Wasson et al., 2010). On floodplains that are inundated for long periods, rainfall and overbank flows redistribute and deposit sediments according to floodplain topography, and as water velocity dissipates over the extent of the floodplain, the inundation extent being related to flow magnitude (Steiger & Gurnell, 2002; Naiman, Décamps & McClain, 2005; Wasson et al., 2010). On floodplains with short inundation periods, floods are rapid and episodic, occurring as ‘sheet flow’ over an essentially terrestrial system, but can still exert significant erosional force and move a considerable amount of sediment (D. P. Ward, unpubl. data). Variability in annual rainfall and flood peaks, combined with landscape topography, can thus dictate the structural heterogeneity of both channels and floodplains, and consequently the presence and persistence of aquatic refugia in channels and floodplain waterholes through the dry season (Richards, Brasington & Hughes, 2002; Bunn et al., 2006).
Wet season peak flows can also structure riparian vegetation communities and produce distinct lateral zonation largely controlled by bank height, sediment deposition and fluvial disturbance (Pettit, Froend & Davies, 2001; Lamontagne et al., 2005; Petty & Douglas, 2010). Attenuation of floods and flood variability, as has occurred in the lower Ord River (Western Australia; Fig. 1) because of the dual impoundments of Lake Kununurra and Lake Argyle, can lead to reduced structural heterogeneity in the riparian vegetation community and narrowing of the riparian zone (Pettit et al., 2001). Wet season floods also restructure channels through the provision and transport of wood and other organic matter that contribute to substratum heterogeneity as well as habitat and food resources for instream biota (Pusey & Arthington, 2003). Recent research in the Daly River indicates that the annual fluvial disturbance of riparian zones can lead to high turnover of wood deposited in channels; up to 50% of instream wood can be translocated over a single wet season, suggesting that the instream habitat for biota can be highly dynamic from year to year with interannual flood variability (N.P. Pettit & D.M. Warfe, unpubl. data). Fish communities associated with wood patches in the Cinaruco River (Venezuela) reassemble after wet season floods in a non-random manner, according to specific habitat patches regardless of the variability of those patches (Arrington & Winemiller, 2006). Such fidelity to habitat patches remains to be investigated in northern Australian rivers, but early research suggests that instream wood plays a significant role in structuring fish assemblages (N.E. Pettit, D.M. Warfe, M.J. Kennard & B.J. Pusey, unpubl. data).
Wet season flows are characterised by low sediment loads and nutrient concentrations because of the highly weathered, ancient geological nature of Australia’s tropical soils (Moliere et al., 2004; Brodie & Mitchell, 2005), and rivers across Australia’s wet–dry tropics are predominantly heterotrophic and nutrient-limited (Webster et al., 2005; Ganf & Rea, 2007). In contrast, while typically still nutrient-limited (Burford et al., 2011), estuaries appear to be more autotrophic owing to the higher aquatic productivity of mangrove forests (Alongi, Clough & Robertson, 2005; Burford et al., 2008a). In Darwin Harbour, nutrients are predominantly provided by tidal rather than tributary inputs and primary production is dominated not only by mangroves but also by benthic algae and phytoplankton, albeit to a lesser degree (Burford et al., 2008a). In the central Gulf of Carpentaria, wet season nitrogen inputs via river flows do not appear to contribute much to primary productivity, which seems instead to be supported by cyanobacterial nitrogen fixation (Burford, Rothlisberg & Revill, 2009). Nevertheless, riverine nutrient inputs during the wet season, while low, could still potentially contribute to primary production closer to the coast (Burford et al., 2011).
Wet season hydrology appears to be a main driver of productivity on floodplains, and, consequently, rates of primary production are variable across the riverine landscape (Davies, Bunn & Hamilton, 2008). Floodplains with extended periods of inundation can support substantial aquatic primary production during the wet season (Pettit et al., 2011). These floodplain dynamics are consistent with the Flood Pulse Concept, which proposes that seasonal inundation and subsequent drainage are the primary drivers of ecological processes in large floodplain rivers (Junk et al., 1989; Winemiller, 1996; Tockner et al., 2000). Estimates of carbon production on the Magela Creek floodplain in Kakadu National Park (east of Darwin, Northern Territory) show that primary production shifts from being predominantly algal-based and restricted to refugial waterholes during the dry season to extensive macrophyte production during the wet season (Pettit et al., 2011). However, despite dominating wet season primary production and biomass on the floodplain, macrophytes do not appear to contribute directly to the aquatic food web. Rather, they provide a large surface area for the attachment of epiphytic algae, which support most of the secondary production on the floodplain (Davies et al., 2008; Pettit et al., 2011). Similar observations have been made on the Orinoco floodplain in South America (Hamilton, Lewis & Sippel, 1992; Lewis et al., 2001). There is also evidence that algae produced on the floodplain during the wet season, even on floodplains with short inundation periods, may subsidise food webs in upstream reaches via the movement of fish consumers (T.D. Jardine, B.J. Pusey, S.K. Hamilton, N.E. Pettit & S.E. Bunn, unpubl. data). Such subsidies may be important for upstream food webs, as primary production in rivers during the wet season is generally low because of the scouring effect of high flows (Townsend & Padovan, 2005).
Extensive macrophyte growth can also provide important habitat for floodplain fauna. Magpie geese (Anseranas semipalmata Latham) are widespread and abundant across tropical Australian floodplains, particularly in Kakadu National Park (Bayliss & Yeomans, 1990; Morton, Brennan & Armstrong, 1990). In years of early monsoonal rainfall, there is subsequent high growth of wild rice (Oryza spp.) and water chestnut [Eleocharis dulcis (Burm.f.) Trin. ex Hensch], the latter providing nesting sites during the wet season and abundant food for adults and fledglings during the drawdown phase, and the former providing food for newly hatched goslings (Bayliss & Yeomans, 1990; P. Bayliss, unpubl. data). Hence, there is a general positive relationship between wet season rainfall, peak floods and both nesting success and dry season survival of magpie geese, particularly in years of early rainfall (Whitehead & Saalfeld, 2000). Magpie goose populations also appear to exhibit decadal trends that are tightly coupled with decadal trends in rainfall and river flows. On average, wetter years are followed by drier years over an approximate 20-year period that is mirrored in magpie goose numbers across the ‘Top End’ of the Northern Territory (Bayliss, Bartolo & van Dam, 2008).
Peak flows towards the end of the wet season often extend the persistence of aquatic habitats and are positively correlated with the abundance of plotosid and ariid catfish (Madsen & Shine, 2000). The abundance of catfish is in turn positively correlated with the body condition and yearling abundance of their main predator, the aquatic filesnake (Acrochordus arafurae McDowell) (Madsen & Shine, 2000). Conversely, the extended inundation period afforded by late season flooding reduces the available habitat, and thus the abundance, of the dusky rat (Rattus colletti Thomas), which results in poorer body condition and reduced reproduction in its main predator, the water python (Liasis fuscus Peters) (Madsen et al., 2006). These unusually strong relationships illustrate that variability in both the magnitude and timing of annual peak flows can have far-reaching effects on the composition of tropical floodplain communities, well into the following dry season. These relationships are unlikely to be as strong or prevalent on floodplains of short inundation periods.
An important consequence of wet season flows is that they provide both lateral and longitudinal connectivity throughout the entire drainage system and the opportunity for aquatic invertebrates, fish and reptiles to move between reaches to spawn (Douglas et al., 2005). Preliminary evidence from the Daly River catchment suggests that the emergence of aquatic insects peaks during the wet season, as does the input of terrestrial arthropods into streams (E. A. Garcia & M. M. Douglas, Charles Darwin University, unpubl. data). The same pattern has been observed in tropical rivers in Hong Kong, where the lateral flux of aquatic and terrestrial insects across the riparian zone also peaks during the wet season and provides an important link between aquatic and terrestrial food webs (Chan, Zhang & Dudgeon, 2007, 2008). The emergence of mature macroinvertebrates during the wet season has been suggested to be an evolutionary response to flood-induced mortality of large larvae, as well as providing available habitat for new recruits (Dudgeon, 2000; Jacobsen et al., 2008).
Of the 90 fish species recorded from freshwaters of the Daly River, one-third moves between freshwater and estuarine reaches to spawn, and one-third area vagrant estuarine species, such as bull sharks (Carcharhinus leucas Müller & Henle), and can be found hundreds of kilometres upstream (B.J. Pusey & M.J. Kennard, unpubl. data). Many of the remaining species move between different freshwater reaches during the wet season for spawning. Many of these fish species are widespread across northern Australia but, because most rivers are intermittent, their movements generally only occur during wet season flows when river, floodplain and estuarine reaches are connected. In the Magela Creek system (northeast of the Daly River in Kakadu National Park), sooty grunter (Hephaestus fuliginosus Macleay) move downstream from escarpment refugia during the wet season to spawn (Bishop, Pidgeon & Walden, 1995). Plotosid catfish move upstream into tributaries (Pusey, Kennard & Arthington, 2004), and juveniles of a number of species such as freshwater sole (Leptachirus triramis Randall) and barramundi (Lates calcarifer Bloch) also use wet season flows to move upstream, potentially escaping predation pressure in more open downstream reaches (Pusey et al., 2004; Staunton-Smith et al., 2004). Like barramundi, the giant freshwater prawn, Macrobrachium rosenbergii (De Man), is catadromous (Short, 2004) and local anecdotal evidence and observations from the Daly River catchment indicate that the juveniles move upstream en masse during the late wet season, constituting a ‘sushi train’ along the littoral margins of the main channel (B. J. Pusey, N. E. Pettit & D. M. Warfe, pers. obs.).
Peak flows also have effects beyond the freshwater reaches of tropical Australian rivers and floodplains, clearly illustrating the importance of longitudinal connectivity through these systems. Analysis of commercial penaeid prawn and finfish fisheries in northern coastal waters suggests that fisheries catches are good in years of high wet season inflows (Loneragan & Bunn, 1999; Robins et al., 2005). The commercial catch of both banana prawns (Penaeus merguiensis Fabricius) and barramundi (Lates calcarifer) has been shown to be positively related to years of high freshwater inflows (Vance, Staples & Kerr, 1985; Bayliss et al., 2008; Balston, 2009). Furthermore, the recruitment of king threadfin salmon (Polydactylus macrochir Günther) and the recruitment and growth of barramundi are also positively related to wet season peak flows, which likely increase hydrological connectivity to estuarine habitats and therefore provide greater access to estuarine nursery areas (Staunton-Smith et al., 2004; Robins et al., 2006; Halliday et al., 2008).
Wet to dry season transition
The transition from the wet to the dry season is the period when rainfall ceases and flows steadily decrease, where floodwaters recede on long-inundation floodplains, and intermittent rivers start to become hydrologically disconnected. This is also a key time ecologically: aquatic plant biomass is at its peak on river floodplains that have been inundated for some time, aquatic biota respond to receding waters by moving into refugial reaches to wait out the coming dry season, and waterbirds congregate in large numbers as aquatic resources become more concentrated in diminishing aquatic habitats.
Floodplains with long inundation periods are characterised by extensive macrophyte growth and diversity (Davies et al., 2008; Pettit et al., 2011); biomass generally peaks in the late wet season when floodwaters begin to recede (Finlayson, 1991). A similar pattern has been observed on floodplains with short inundation periods, but macrophyte growth in these systems is largely restricted to floodplain waterholes and terrestrial grasses, such as Dicanthium spp., dominate the floodplain instead (N. E. Pettit, pers. obs.). The biomass of attached algae, namely epiphytic diatoms, is also greatest in the late wet season, before declining as the water recedes and causes the macrophytes on which they grow to senesce and the available aquatic habitat to contract (Pettit et al., 2011). Aquatic macroinvertebrates associated with macrophytes on the floodplain reflect this pattern of plant production, peaking in abundance during the late wet season and transition into the dry season (Marchant, 1982; Outridge, 1988; Douglas & O’Connor, 2003). A conceptual model, developed from data from South American and African floodplains, suggests there is a peak in microbial activity on the floodplain during the late wet season as senescent macrophyte material is consumed (Winemiller, 1996). Microbial dynamics on northern Australian floodplains represent a major knowledge gap for northern Australia, but it appears that fire, rather than microbial activity, is a major consumer of plant material during the wet to dry season transition (Pettit et al., 2011).
The transition between the wet and the dry season appears to be a key time for the large-scale movements of organisms. Unlike spawning movements during the wet season, movements during this transition period are more likely to be associated with finding refuge as aquatic habitats begin to disconnect and contract. For example, saltwater crocodiles (Crocodylus porosus Schneider) that have moved onto the floodplains during the wet season return to channel reaches as floodwaters recede (Jenkins & Forbes, 1985). Melanotaeniid rainbowfish and ambassids, among other species, moved from floodplain waterholes to upstream refugial areas towards the end of the wet season in the Magela Creek system in Kakadu National Park (Bishop et al., 1995). Recent research on fish movement in tributaries of the Daly River shows large numbers of melanotaeniid rainbowfish and plotosid catfish moving downstream during the transition from the wet to the dry season, particularly in intermittent rivers that were in the process of becoming disconnected (D.M. Warfe & N.E. Pettit, unpubl. data). Fish assemblage structure in the remaining waterholes of rainforest streams in Queensland has been shown to be influenced by the magnitude of the preceding wet season, which in effect ‘sets up’ the assemblage that will persist through the dry season (Perna & Pearson, 2008). Evidence from streams in Australia’s wet tropics indicates that, while macroinvertebrate structure is not affected by season (Cheshire, Boyero & Pearson, 2005), the rate of macroinvertebrate colonisation peaks during the late wet season flows as early instars disperse to suitable habitats (Benson & Pearson, 1987). We are analysing data on benthic macroinvertebrate assemblages to determine whether these patterns hold in Australia’s wet–dry tropical rivers.
There are also increased aggregations of waterbirds on the floodplain during the transition from the wet to the dry season, as floodwaters contract to waterholes and aquatic resources become more concentrated. Darters, cormorants, pelicans and grebes that feed on aquatic invertebrates and fish become more abundant around waterholes (Franklin, 2008), and assemblages can shift as water depths decrease (Chatto, 2000). Magpie geese move to areas of high resource availability during this period (Traill, Bradshaw & Brook, 2010), occurring in their highest densities where a range of macrophyte species required for nesting and feeding occur (Bayliss & Yeomans, 1990), and can represent the largest proportion of waterbird biomass on the floodplain (Pettit et al., 2011).
The dry season is a period of limited resources as aquatic habitats become disconnected and contract across most of the region. Isolated waterholes on floodplains and in intermittent rivers become critical for sustaining aquatic biota and play an important refugial role during the dry season (Bunn et al., 2006).
There appears to be considerable variation in water quality and primary production between isolated waterholes, both on the floodplain and along river channels (Butler, 2008). Some waterholes are naturally turbid, and the dominant primary production supporting their aquatic food webs is the narrow band of benthic algae in the littoral zone (Bunn, Davies & Winning, 2003) or potentially the phytoplankton in the water column (Robertson et al., 1999). Other waterholes can support a very high biomass of macrophytes and benthic algae and have clear, deep water (Finlayson, 1991; Butler, 2008; Davies et al., 2008). In the latter case, it appears that the epiphytic algae attached to these macrophytes support the aquatic food web (Hamilton et al., 1992; Douglas et al., 2005; Pettit et al., 2011). However, on floodplains with only short inundation periods, terrestrial animals such as wallabies, horses and cattle have been observed consuming macrophytes when terrestrial vegetation becomes scarce at the end of the dry season (S. K. Hamilton, pers. obs.).
Reflecting the isolated nature of refugial waterholes, the species richness of dry season fish assemblages in intermittent rivers and reaches in the Daly catchment tends to be lower than in perennial reaches (B.J. Pusey & M.J. Kennard, unpubl. data). Macroinvertebrate communities differ between intermittent and perennial reaches, with assemblages comprising more lentic and lotic taxa, respectively (Humphrey, Hanley & Camilleri, 2008; Leigh & Sheldon, 2009). Trophic diversity within aquatic food webs can narrow (D.M. Warfe, N.E. Pettit, E.A. Garcia & M.M. Douglas, unpubl. data), and the diets of resident fish can also narrow and become poorer in quality (Balcombe et al., 2005) as consumers are forced to become more dependent on local food resources (T.D. Jardine, D.M. Warfe & N.E. Pettit, unpubl. data). Narrowing of fish diets owing to limited resource availability during the dry season also appears to be a common feature in Central and South American rivers (Winemiller, Agostinho & Caramaschi, 2008). In perennial rainforest rivers in Queensland, feeding links between fish and their food sources do not vary greatly between the wet and the dry season, supporting the hypothesis that intermittency can lead to more limited resources and more striking seasonal changes in food webs (Rayner et al., 2010).
As the dry season progresses, the available habitat contracts, increasing the potential for predation and competition, as has been shown in both Neotropical floodplain rivers (Winemiller, 1996; Rodriguez & Lewis, 1997) and Australian arid-zone rivers (Arthington et al., 2005). In shallow waterholes (<3 m), habitat reduction can be accompanied by a deterioration in water quality that can contribute to fish kills when flow resumes (Townsend, Boland & Wrigley, 1992). It is possible that fish mortality may fuel algal and bacterial growth towards the end of the dry season, as has been shown in Australian arid-zone rivers (Burford et al., 2008b). Furthermore, late dry season flowering of riparian species such as Melaleuca leucadendra L. (Pettit, 2000) can attract insects and flying foxes (Pteropus alecto Temminck) (Vardon et al., 2001), potentially contributing riparian subsidies to depleted waterholes and supporting juveniles of species that breed at the end of the dry season, for example barred grunter (Amniataba percoides Günther) (Pusey et al., 2004; N. E. Pettit & D. M. Warfe, pers. obs.).
In the few perennial rivers within northern Australia, dry season baseflows are maintained by groundwater discharge and their typically oligotrophic nature (e.g. Townsend & Padovan, 2005) reflects the low nutrient concentrations in the supporting aquifers. For example, in the Daly River, as groundwater contributes a larger proportion of flow over the course of the dry season, nitrate concentration and turbidity decrease, and the potential euphotic zone exceeds the river depth (Townsend & Padovan, 2005; Townsend et al., 2011). The clear water and low nutrient concentrations common to these perennial rivers can make them vulnerable to algal blooms as a result of nutrient enrichment (Ganf & Rea, 2007). In the Daly River and its major tributaries, photosynthesis increases over the dry season because of the accumulation of primary producer biomass rather than higher incident radiation (Webster et al., 2005; Townsend et al., 2011). While photosynthesis has been shown to be light-limited in these generally open-canopy rivers, the accumulation of primary producer biomass is probably nutrient-limited within the hydraulic and other physical constraints of the river (Webster et al., 2005; Townsend et al., 2008). Phytoplankton is likely to briefly contribute a significant proportion to primary production in the early dry season, before the subsequent growth and accumulation of benthic algae and submerged macrophytes dominate primary production (Townsend & Padovan, 2005; Townsend et al., 2011). As the dry season progresses, producer biomass generally increases before being scoured out during the first flows of the early wet season (Townsend & Padovan, 2005), a pattern also apparent in the low-nutrient wet–dry rivers of the Neotropics (Cotner et al., 2006).
While bottom-up controls probably limit primary producer biomass, in common with the Neotropics (Cotner et al., 2006; Roelke et al., 2006), there is evidence that benthic algae in Australia’s wet–dry tropics are also partly regulated by grazing pressure from macroconsumers such as freshwater prawns and catfish, particularly during the dry season when there is no disturbance from floods (Douglas et al., 2005; M.M. Douglas, unpubl. data). Such top-down control of algal resources has been observed in other tropical regions, e.g. Costa Rican streams (Pringle & Hamazaki, 1997), Andean streams (Flecker & Taylor, 2004) and the Cinaruco River in Venezuela (Winemiller et al., 2006), suggesting that it is a characteristic pattern of tropical streams and rivers (Douglas et al., 2005). There are suggestions that the meiofauna may also regulate benthic algae in tropical sand-bed rivers (e.g. Winemiller et al., 2006), but their importance in northern Australian rivers remains to be investigated (Humphreys, 2008).
Perennial rivers also provide important habitats within the dry season landscape for flow-dependent aquatic species. Juveniles of numerous fish species, such as sooty grunter (Hephaestus fuliginosus) and Butler’s grunter (Syncomistes butleri Vari), are predominantly restricted to riffle areas where they can escape predation pressure from larger fish, but also where there is more food (Pusey et al., 2004). Perennial reaches support distinctive macroinvertebrate communities consisting of groups such as baetid mayflies, hydropsychid caddisflies and hydrophilid beetles that prefer flowing water (Humphrey et al., 2008; D.M. Warfe & N.E. Pettit, unpubl. data). Together with microalgae, these macroinvertebrates are a source of benthic food that supports higher trophic levels (Douglas et al., 2005; Townsend & Padovan, 2009).
Other stream channel features can also become important as they become exposed or accessible during the dry season. Periods of low flow expose channel sediments that can become important sites for the germination and establishment of riparian seedlings (Pettit et al., 2001). The pig-nosed turtle (Carettochelys insculpta Ramsay) is restricted to perennial rivers, such as the Daly River, which are deep enough to allow extensive movement (up to 14 km) by females as they search for suitable egg-laying sites (Georges et al., 2003). The females target fluvially reworked sand banks in the inside of bends, at tributary mouths or behind large boulders and wood aggregations, and lay their eggs at the end of the dry season after water temperatures have begun to rise (Georges et al., 2003).
Dry to wet season transition
The major feature of the transition from the dry to the wet season is the occurrence of storm run-off events and the hydrological reconnection of isolated stream reaches. The first flow events of the wet season are often of poor water quality as they flush organic material and suspended particulates through the system and into the estuary, sometimes creating hypoxic conditions and causing fish kills in rivers and floodplain waterholes (Townsend et al., 1992; Townsend & Edwards, 2003; Butler, 2008). Tropical waters generally have a higher oxygen demand, and much lower oxygen saturation concentrations, than temperate waters for a given organic loading, and thus they are potentially vulnerable to anthropogenic organic loading (Lewis, 2008). Surface run-off events during the transition from the dry to the wet season can also bring a pulse of organic material, nutrients and sediments from the surrounding catchment (Schult et al., 2007); consequently, the impacts of fire and cattle grazing on aquatic systems are likely to be most concentrated at this time (Townsend & Douglas, 2000). Similarly, mobilisation of trace metals and reduced substances from floodplain soils may cause fish kills (Hart & McKelvie, 1986). This is especially likely in areas where acid sulphate soils occur on former marine sediments or as a result of past mining activities, e.g. in the Finniss River, south of Darwin (Taylor, 2007).
Run-off events during the early wet season were an important factor regulating mechanical breakdown, and consequent biotic breakdown, of leaf litter in an Amazonian floodplain stream (Rueda-Delgado, Wantzen & Tolosa, 2006). In general, decomposition rates of litter in tropical streams are strongly influenced by water temperature, turbidity, pH, salinity, dissolved organic carbon, nutrients and oxygen (Wantzen et al., 2008). Early evidence from streams in the Daly River catchment also suggests that flow regime, leaf species and the physical character of streams play a stronger role in leaf litter breakdown than microbial activity (T. Davies, N.E. Pettit & P.F. Grierson, unpubl. data). However, knowledge of microbial processes in Australian wet–dry tropical rivers is scant.
The reconnection of isolated refugia within rivers is an important time in the annual hydrograph for the movement of biota, as flows commencing after the initial poor-quality flush provide an opportunity for fauna to move into more favourable reaches and throughout the river system. Reaches in rivers and on floodplains that act as refugia during the dry season represent important sources of recolonising biota (Outridge, 1988; Perna & Pearson, 2008). Macroinvertebrates can rapidly colonise re-wetted reaches from upstream perennial reaches and floodplain waterholes via the dispersal of aerial adult stages, as well as from the hyporheic zone where microcrustaceans, oligochaetes and dipterans appear to aestivate over the dry season (Paltridge et al., 1997). The early wet season is a key time for saltwater crocodiles, Crocodylus porosus, to disperse throughout the river system and into floodplain waterholes (Jenkins & Forbes, 1985); their nesting effort is positively correlated with high rainfall and relatively cool weather over this period (Webb, 1991). Juvenile barramundi move from estuarine habitats to upstream floodplains and tributaries (Pusey et al., 2004). Males can spend between 3 and 5 years in freshwater riverine habitats before returning to estuaries for spawning, and there is also evidence for considerable movement throughout riverine habitats during this time (Griffin, 1987; Pusey et al., 2004). Recent research on fish movement in tributaries of the Daly River showed that rainbowfish (Melanotaenia australis Castelnau) and hardyheads (Craterocephalus stercusmuscarum Günther) moved upstream after flushing flows and were markedly more abundant where flow had ceased during the dry season rather than at perennially flowing sites (D.M. Warfe & N.E. Pettit, unpubl. data).