Responses of microbial phototrophs to late-Holocene environmental forcing of lakes in south-west Greenland

1.The biological structure of arctic lakes is changing rapidly, apparently in response to global change processes such as increasing air temperatures, although altered nutrient stoichiometry may also be an important driver. Equally important, however, are local factors (e.g. landscape setting, hydrological linkages and trophic interactions) that may mediate responses of individual lakes at the regional scale. Despite general acknowledgement of the importance of local factors, there has been little focus on among-lake variability in the response to environmental change. 2.Sedimentary pigments, organic carbon and nitrogen, and biogenic silica (BSi) in 210Pb and 14C-dated sediment cores from three contrasting lakes in the Kangerlussuaq area (c.67 degrees N, 51 degrees W) of south-west Greenland were used to reconstruct algal and phototrophic bacterial ecological change during the late-Holocene. Water chemistry for the individual lakes varies in terms of conductivity (range: 303000Scm1) and stratification regimes (cold monomictic, dimictic and meromictic), linked with their position along the regional climate gradient from the coast and to the present ice sheet margin. 3.Despite essentially similar regional climate forcing over the last c.1000years, marked differences among lake types were observed in the phototrophic communities and their temporal variability. Considerable short-term variability occurred in an oligosaline, meromictic lake (SS1371), dominated by purple sulphur bacterial pigments, most likely due to a tight coupling between the position of the chemocline and the phototrophic community. Communities in a lake (SS86) located on a nunatak, just beyond the edge of the present ice sheet shifted in a nonlinear pattern, approximately 1000cal.years BP, possibly due to lake-level lowering and loss of outflow during the Medieval Climate Anomaly. This regime shift was marked by a substantial expansion of green sulphur bacteria. 4.A dilute, freshwater coastal lake (SS49) dominated by benthic algae was relatively stable until ca.1900 AD when rates of community change began to increase. These changes in benthic algal pigments are correlated with substantial declines (1.30.44 parts per thousand) in 15N that are indicative of increased deposition of atmospheric inputs of industrially derived NOx into the atmosphere. 5.Climate control on lake ecosystem functioning has been assumed to be particularly important in the Arctic. This study, however, illustrates a complex spatial response to climate forcing at the regional scale and emphasises differences in the relative importance of changes in the mass (m, both precipitation and nutrients) and energy flux (E) to lakes for the phototrophic community structure of low-arctic Greenland lakes.