Microbial dynamics during the summer ice-loss phase in maritime Antarctic lakes

The dynamics of bacterioplankton and protozooplankton in two maritime Antarctic lakes (Heywood Lake and Sombre Lake, Signy Island, South Orkneys) were studied during the phase of ice break-out (December and early January 1994/95). The lakes are suffering animal-induced (fur seal) eutrophication, though Heywood Lake is most severely affected. Both lakes had morphologically diverse bacterial communities which increased during the study period, reaching maxima of 80 x 10(8) l(-1) in Heywood Lake and 31.8 x 10(8) l(-1) in Sombre Lake. Heterotrophic nanoflagellates (HNAN) reached a peak in late December with maxima of 40.6 x 10(5) l(-1) in Sombre Lake and 174 x 10(5) l(-1) in Heywood Lake. Phototrophic nanoflagellates (PNAN) peaked in late December after ice loss in Heywood Lake (63 x 10(5) l(-1)), which coincided with a peak in a bloom of Chroomonas acuta which reached abundances of 1.0 x 10(8) l(-1). In Sombre Lake, ice persisted for a longer period and here PNAN reached their highest density at the end of the study period (around 70.0 x 10(5) l(-1)). Ciliate abundance reached high levels in Heywood Lake (>6000 l(-1)), while in Sombre Lake maximum abundance was 568 l(-1). Protozooplankton diversity was greater in the less-enriched Sombre Lake. Grazing rates of ciliates averaged 70.6 bacteria indiv.(-1) h(-1) in Heywood Lake and 119.3 bacteria indiv.(-1) h(-1) in Sombre Lake. The difference was a reflection of the different taxonomic make-up of the community in the lakes. HNAN grazing rates varied between 0.51 and 0.83 bacteria indiv.(-1) h(-1) Sombre and Heywood Lakes, respectively. Specific growth rates (r) h(-1) in Sombre Lake were 0.028 for ciliates and 0.013 for HNAN, and in Heywood Lake 0.010 for ciliates and HNAN 0.012. These growth rates result in doubling times ranging between 38 and 69 h for ciliates and around 55 h for HNAN. HNAN grazing on bacteria was curtailed in Heywood Lake in early January as a result of predation by microcrustacean larvae feeding on the plankton. Thus, for a short phase top-down control was apparent in the dynamics of Heywood Lake, a feature uncommon in Antarctic lake ecosystems. The impact of natural eutrophication on these systems is discussed in relation to other unaffected Antarctic lakes.