The role of physical stability on the establishment of steady states in the phytoplankton community of two Maritime Antarctic lakes

Two Antarctic lakes near Hope Bay were studied during summers 1998 and 1999. One of the lakes (Boeckella) is located near Esperanza Station and exhibits a meso-eutrophic condition due to the input of nutrients of a nearby penguin rookery. Its surface generally remains ice-free during the Antarctic summer ( December - March). The other lake (Chico) is situated on the Mount Flora shelf, is typically oligotrophic and its surface is ice-free only during brief periods in the summer season. The difference in the duration of the ice-cover insures that the wind mixes the former lake continuously throughout the summer, while the latter remains almost always stratified. X-2, X-3 and Z functional groups defined by Reynolds dominated phytoplankton in both lakes. In Lake Boeckella, Chlamydomonas spp. followed by Ochromonas sp. were the most frequently encountered taxa in the nano-phytoplankton fraction, however the latter species was dominant when the lake froze. In Lake Chico, the major contribution to this fraction was due to different genera of flagellated Chrysophyceae (Ochromonas sp., Chromulina spp., cf. Chrysidalis). In terms of density and biomass in both lakes picocyanobacteria represented a large proportion of the phytoplankton. Probably due to the typically low algal biodiversity of Antarctic lakes, both water bodies showed periods of more than 2 weeks when a maximum of only three species comprised more than 80% of the standing crop. In spite of this, Chico Lake was the only one in which no significant change was recorded in total biomass. Thus, we were able to identify equilibrium phases in the latter lake, which were confirmed by a low coefficient of variation. The presence of an almost permanent ice cover in Chico Lake generated more stable ecological conditions, allowing the development of steady state assemblages. On the contrary, the wind influence in the shallow Antarctic ice free lake (Lake Boeckella) provided continuous mixing events, disrupting the possibility of establishing a steady state.