In the McMurdo Dry Valleys of Antarctica, perennial microbial mats are common in glacial meltwater streams that flow during the austral summer. Studies by the McMurdo Dry Valleys Long Term Ecological Research project have shown that interannual persistence of mats is central to stream ecosystem function and that community compositions of cyanobacteria, heterotrophic bacteria, and diatoms that comprise the mats are interrelated. Shorter streams flow every summer, have lower solute concentrations, and support communities dominated by lightly silicified diatoms, while longer streams have lower and less regular flow, higher solute concentrations, and are dominated by more silicified diatoms. To evaluate how mat communities respond to changing flow and water quality, we conducted a multi-year transplant experiment in which we moved intact mat patches from one stream to another, with mats moved within the same stream acting as controls. These mats grow slowly, with regrowth of biomass from scraped rocks taking about five summers. This experiment involved two short streams (Green and Bowles Creeks) and two long streams (Delta and Von Guerard Streams). Our hypothesis was that competitive interactions within microbial mats for nutrients, especially dissolved silica, would result in eventual turnover in the mat diatom and bacterial communities.
Results/Conclusions
Chlorophyll a, nitrogen and carbon contents, and C:N ratios in mat material were similar across the initial transplanted mat patches, with C:N matching the Redfield ratio. One year after establishing the experiment, we observed significant differences in the persistence of the transplanted mat patches among streams at the beginning of summer. Mats from the two long streams, Delta and Von Guerard Streams, retained high coverage, averaging about 60 to 70% of the initial coverage of the transplanted rock or sediment, regardless of whether they were moved within the same stream or to another long or short stream. In contrast, there was low persistence (only about 5% or less) of mats from Green and Bowles Creeks moved within the same short stream. By the end of summer, mat patches transplanted to the two long streams retained their coverage, and mats transplanted from Green Creek to a short stream exhibited regrowth. Chlorophyll a content was generally stable in mat patches that retained coverage and lower in regrown patches. These findings suggest that the differences in microbial communities in the mats influences the adhesion of the mats to the substrate, and thus their interannual persistence under varying flow regime and water quality .