Limnology of Pyramid Trough
Based on their water chemistry, the ponds in the Pyramid Trough region can be divided into two main groups, and within each group, a gradient is evident. The first group is characterized on a more recent melt history, with low NO3–N concentrations, a high proportion of HCO3 and lighter water isotopic signatures. Within this group, a gradient of increasing conductivity corresponds to increasing Cl/HCO3, consistent with the removal of carbonate salts. Calcite (CaCO3) was observed at the margin of Tut pond; a salt known to precipitate early in the evaporation sequence (Healy et al., 2006). Group 2 appears to contain ‘older’ water, characterized by high NO3 and low proportion of HCO3 and a heavier isotopic signature, all indicating a greater degree of evaporation in these ponds. The increasing conductivity gradient in group 2 was attended by increasing Cl/SO4 and Na(+K)/Ca, consistent with the precipitation of Ca sulphate salts. Salts precipitating around the margin of group 2 ponds were observed to be primarily gypsum (CaSO4) with minor mirabilite (Na2SO4); sulphate salts typically precipitated in the later stages of pond evaporation (e.g. Healy et al., 2006).
Chemical divergence of the two groups of ponds appears to follow classical wetland paradigms in that it is owing to shifts in the balance of evaporation and inundation. Group 1 ponds are likely to be receiving the highest loading of recent meltwater. During our sampling, Cleopatra Pond was receiving dilute ice-melt flowing in from the Upper Alph River, and whilst Egypt Pond was not connected, it was sufficiently close to the river to suggest that at times of exceptionally high discharge, it may receive such flow. Tut Pond, the third member of group 1 was, however, remote from direct inflow, residing in a closed basin but was stratified, so only part of its bulk chemistry is reflected in the surface layer plotted in Fig. 2. If mixed, it seems likely (from the isotopic signature for example) that Tut would have plotted as a group 2 pond on the ternary diagrams. All group 2 ponds have a closed basin catchment and evaporative evolution of the water in them, coupled perhaps with periodic redissolution of peripheral salts explains their higher conductivity and Cl–SO4 anion dominance. Colin Pond provides an instructive outlier, which contains high NO3, consistent with its upland location, but was also relatively dilute in other ions with HCO3 a significant anion and an isotopic signature that suggested a significant meltwater loading. High nitrate concentrations have been noted previously in inland ponds in this part of Antarctica (Vincent & Howard-Williams, 1994; Webster-Brown et al., 2010) and have been ascribed to accumulation of nitrates produced by upper atmospheric processes.
Environment–biota relationships in Pyramid Trough
As in many other Antarctic freshwater ecosystems (e.g. Sabbe et al., 2004), mat-forming Cyanobacteria were the most conspicuous member of the benthic biota of Pyramid Trough wetland. The cyanobacterial taxonomic diversity identified by microscopy and 16S rRNA gene sequence clone libraries were similar (17 vs. 21 ‘taxa’), and there was a general agreement in the types of organism the two methods returned. As is frequently the case in Antarctica (Quesada et al., 2008), filamentous members of the Oscillatoriales were abundant in the mat communities, although the abundance and diversity of Chroococcales and Nostocales featured here are striking. There were several morphotypes that were not recovered in the 16S rRNA gene surveys, particularly within the Chroococcales. These differences could be due to differential PCR amplification, over-representation of multi-cellular filamentous morphotypes and incomplete screening of the cyanobacterial 16S rRNA gene diversity. This highlights the need for a polyphasic approach when analysing cyanobacterial diversity (Taton et al., 2003). Deep coverage sampling using high-throughput sequencing of cyanobacterial diversity at many sites, (Sogin et al., 2006), would have improved resolution of the diverse assemblages and provided a better quantitative representation of community composition.
Differences in community composition between the two groups of ponds are evident, with some taxa, Schizothrix, Calothrix/Dichothrix occurring in all group 1 ponds but not in group 2. The presence of organisms from the Dichothrix-Calothrix complex in group 1 ponds also explains the high concentrations of scytomenin, as these taxa have a prevalence of producing the dark sheath pigment used to screening excess irradiance including UV (Quesada et al., 1999). Similarly, high abundance of Calothrix has also been reported from James Ross Island in maritime Antarctica (Komárek et al., ) and some streams in the Dry Valleys, including areas close to Pyramid Trough (Howard-Williams et al., 1986, 1989). A relationship between overall salt content and cyanobacterial richness and community composition was observed to different degrees with ARISA, light-microscopy analysis and 16S rRNA gene sequence clone libraries. Conductivity has been previously reported to be of importance in shaping cyanobacterial assemblages in Antarctic terrestrial aquatic ecosystems including Laresman Hills, Prydz Bay and Ross Ice Shelf regions (Broady & Kibblewhite, 1991; Sabbe et al., 2004; Jungblut et al., 2005; Taton et al., 2006; Verleyen et al., 2010). Several taxa occur in only one or a few ponds such as Chroococcales ribotype Ptcy20 in Pettway, which interestingly clustered with environmental sequences from a hypersaline ecosystems. However, while this extreme habitat was distinctive, there was no clear evidence of taxa separating out systematically along the environmental gradients that we have identified. Sabbe et al. (2004) and Sutherland (2009) reported a similar lack of clear separation of cyanobacteria along conductance gradients in Antarctic lake systems.
With a few exceptions, the cyanobacterial community did not separate along environmental gradients, as might be expected if community structure was linked with environmental preference. The notable exceptions were the nitrogen fixers that were most abundant in the low-nitrate ponds. The more chaotic assembly seen in both morphotypes and ribotypes is more consistent with that expected from neutral rather than niche-driven dynamics. Neutral processes (Hubbell, 2008) tend to dominate community assembly when sympatric species show a degree of ecological convergence that minimizes the effects of niche-driven processes, and dispersal, chance and juvenile survival play significant roles in determining composition. Reasons why neutral processes may be important in Antarctic microbial communities, mostly relate to their common need for a high degree of tolerance of multiple stressors.
Our sampling was restricted to a single time-point at the peak of the summer growth season. Such a sampling strategy is not new to comparative limnology in Antarctica (e.g. Sabbe et al., 2004) and poses a number of limitations on interpretation. These include the possibility of species succession, although unlikely in the short summer growth periods and high biomass communities, and the failure to fully document the dynamics of the environment. We suggest that the latter is more serious, and information has recently emerged on the conditions in Antarctic ponds outside the relatively benign summer period, which confirms that organisms must tolerate a cascade of stressors during the summer–winter transitions that are never experienced in summer (Hawes et al., 2011a, b). These include exposure to high conductivities even in relatively dilute ponds, and it seems likely that a broad environmental tolerance may be a more important attribute for polar cyanobacteria than adaptation to one point on an environmental gradient. Vincent (2000) suggested that selection pressures would favour these multi-stress-tolerant taxa in the Antarctic cyanobacteria, and we suggest that selection for stress tolerance results in a degree of ecological convergence amongst Antarctic cyanobacteria.
Viewed within another limnological continuum, polar ponds are an unusual subset of temporary waters, one in which water is withdrawn by freezing rather than drying. Within temporary waters, increasingly stressful habitats (infrequent, short, unpredictable inundations) tend to result in dominance by increasingly stress-tolerant taxa, forming simple ecosystems with few trophic linkages (de Meester et al., 2005; Vanschoenwinkel et al., 2009). The microbial mats meet this description, and again, selection for stress tolerance may have led to a degree of ecological convergence that makes sorting along environmental gradients unlikely to dominate local biogeography.
In summary, our data show that the ponds of the Pyramid Trough capture a range of habitat diversity, with two distinct groups of ponds present and distinct gradients within each group. These different groups appear to relate to different predominant sources of water and salts. However, biological variables did not fall out clearly along the same lines as the habitats. Differences between communities in the ponds could not be related to habitat differences or the gradients in variables between ponds and groups of ponds. Taxa present appear to be a mix of those that can tolerate a broad, overlapping range of habitats, suggesting a prevalence of neutral assembly rules in this high-stress environment. These taxa present include both those representative of the wider Antarctic region area and others that are rare. The inclusion of this area in that part of the McMurdo Dry Valley Antarctic Specially Managed Area that receives an enhanced degree of protection from human activities, and the apparent importance of numerous varied habitats for optimizing appears fully justified for the purposes of protecting regional diversity.