In the McMurdo Dry Valleys (MDV), Antarctica, benthic diatoms are common in the perennial microbial mats that are common in the glacial meltwater streams. The 45 primarily endemic diatom taxa represent the most diverse group of eukaryotes in the MDV. Furthermore, these microbial mats are hotspots of primary productivity and microbial biodiversity in this harsh polar desert. Studies by the MCM-LTER project have shown that the community composition of the benthic diatom is related community composition of both the cyanobacteria and heterotrophic bacteria that comprise the majority of mat biomass in the streams. Comparison of diatom community composition among streams has shown that diatom community composition varies with the magnitude and intermittency of the streamflow, with streams that have experienced relatively high and stable flow supporting distinct communities from those that have experienced flow for only three summers in the 22 years of record. In the majority of the 16 monitored streams the diatom community composition has been stable despite large inter-annual variation in summer flow regime which resulted in large reductions in mat biomass due to scouring. In contrast, in a few shorter streams substantial shifts occurred following summers with high flow, with smaller diatom taxa that appear to have less heavily silicified frustules becoming relatively more abundant following floods.
We hypothesized that one driver for the shift in diatom community composition may be a decrease in the availability of dissolved silica from the weathering of silicate minerals in the hyporheic zone. In the longer streams (5 km or longer) the concentration of dissolved silica increases two-fold in the downstream direction, reflecting the exchange of hyporheic water equilibrated with silicate minerals with more dilute stream water. As streamflow increases in the shorter streams, the influence of this hyporheic exchange on the chemical composition of the stream water decreases, resulting in lower silica concentrations, along with lower concentrations of major ions. Analysis of the long-term patterns in diatom community composition and the concentrations of dissolved silica and other nutrients provide support for this hypothesis. We propose that in these stream ecosystems there is a threshold concentration of dissolved silica that represents an indirect hydrologic connection to diatom community composition. These findings are relevant to understanding water quality in other stream ecosystems because benthic diatoms are used as indicators of water quality in many regions.