PS 3-28
Benthic microbial mat expansion and nutrient uptake during lake level rise in ice-covered Lake Vanda, McMurdo Dry Valleys, Antarctica
Lake Vanda is a perennially ice-covered lake in the McMurdo Dry Valleys, Antarctica. The lake lacks macroscopic metazoans, but hosts benthic photosynthetic microbial mats with dm-scale pinnacles formed by microbial growth. Lake Vanda had a stratified water column with two convecting layers separated by a pycnocline from 24 to 27m in 2013. The lake has no outflow, and its water budget consists of inflow from the Onyx River and ice ablation from the lake surface. The water balance responds to regional climate change, and recent lake level rise has submerged new areas, which are colonized by microbial mats. The microbial mats are P limited, and P annually sequestered in mat growth shallower than 19m was comparable to influx from the Onyx River at ~1.4 µg P m-2 yr-1 from 1998-2010 (Hawes et al., 2013). To constrain microbial influences on P distribution, we document drop camera surveys of benthic microbial mats. In addition, depth soundings, historical aerial photographs, and GPS shoreline tracks were georeferenced to a digital elevation model. Measurements were referenced to records of lake level to refine bathymetry and calculate change in benthic area through time.
Results/Conclusions
Thirty four drop camera sites across the lake demonstrate that pinnacles in microbial mats are present at depths of up to ~50m, but are absent to poorly developed below this depth. The bathymetric reconstruction combines 2013 GPS shoreline tracks, new and preexisting depth soundings, and aerial photographs from 1964, 1972, 1983, and 1994. The resulting bathymetric model indicates that lake level rise has increased the wetted area of the lake at an average rate of ~6x104 m2 yr-1 since 1964, while lake volume has increased by 2x106 m3 yr-1 on average. For microbial mat growth rates to have remained constant with increased surface area, P flux must have increased by 8x104 µg yr-1. This calculated increasing P flux indicates that either 1) growth rates per m2 surface area decreased during the observed period of growth or 2) there was an increase in P flux. An increased P flux could come from either the Onyx River or diffusion across the density gradient from deeper parts of the lake where increasing lake level may have shifted the rate of decay relative to primary productivity.