Physical, chemical and microbial community characteristics of lakes of the Larsemann Hills, Continental Antarctica

Abstract

Numerous freshwater lakes have developed on the three small ice-free peninsulas that constitute the Larsemann Hills, Princess Elizabeth Land, Antarctica. All the lakes are ultra-oligotrophic and ice covered for 9-11 months each year, resulting in a photosynthetically distinctive seasonal environment with uniformly low water temperatures. The benthic communities are dominated by thick cyanobacterial mats in the deeper parts of all but the few shallow brackish lakes. In these brackish lakes, the photosynthetically active mats are located in the lake margins as the deeper parts of these lakes are strongly anoxic under winter ice cover. In the more freshwater lakes, a depth-related zonation of mat type was observed, which showed pigment modifications to a substantial depth consistent with measured penetration of PAR and UV radiation, even under the extensive ice cover. The thick organic deposits and intact mat structure at depth suggest slow decomposition rates within the mats. The plankton are almost entirely microbial, with two forms of planktonic rotifer and the cladoceran, Daphniopsis studeri present in very low numbers. A desmid, Cosmarium, occurred in relatively large numbers in Heart Lake, a coastal system. The desmid group is very rare in continental Antarctica. Abundances of bacteria, heterotrophic and phototrophic nanoflagellates, and ciliates were consistently low. Microbial community diversity is low when compared to other sites in continental Antarctica (e.g. Dry Valleys) but broadly comparable to that of freshwater lakes in the nearby Vestfold Hills. The invertebrate grazer is present in such low numbers that food web structure can be considered to have a single trophic level based on the microbial loop. In view of the sparse autotrophic plankton and barren catchments, DOC supply to the microbial heterotrophs was probably derived in large part from the benthic mats rather than from planktonic photosynthetic exudates.