WINTER PHYTOPLANKTON DYNAMICS IN A SUB-ALPINE LAKE, COLORADO, USA

The temporal dynamics of phytoplankton were examined in The Loch, a su balpine lake in Rocky Mountain National Park, over the winter seasons of 1987-88 and 1988 - 89. The Loch was ice-covered from early November until early to mid May. The pattern of phytoplankton biovolume during ice-cover was consistent between the two years with maxima occurring in November/December and February/March. This pattern resulted princip ally from the contribution of Asterionella formosa HASS. Other dominan t phytoplankton species in terms of biomass (Dinobryon sertularia EHRE NB., Cryptomonas ovata EHRENB., and Peridinium cinctum (MULL.) EHRENB. ) collectively contributed from 10 to 90% of the total cells. Algal co mposition changed throughout the winter and individual species varied in abundance with depth. The same dominant (and most of the rare) taxa were present both years. They varied in time of occurrence and abunda nce, but did not occur at the same time in both years. Phytoplankton s pecies composition continually fluctuated throughout the winter. Becau se of the stability afforded by ice-cover, algal species succession wa s not driven by thermal regime or by wind induced changes in the mixed depth. Nor did grazing by the winter zooplankton assemblage, composed nearly exclusively of cyclopoid copepods and rotifers, adequately exp lain the phytoplankton dynamics. Freeze concentration of water (concen tration as ions are excluded in the formation of ice) in early winter may be responsible for the early phytoplankton bloom.