Factors that control the production rate and biomass of phytoplankton in high-mountain lakes

Summary

Clear lakes above the timber line are inhabited throughout the year by a considerable diversity of nannoplanktic algae. Only a few species are of quantitative importance, but these show a biomass (maximum in the ice-free period) greater than that of the phytoplankton of many lowland lakes. The production rate of alpine phytoplankton is low, limited by nutrients in summer and autumn, and by light in winter and spring. Throughout the winter and spring autotrophic rather than heterotrophic production dominates, the algae being adapted to those low intensities of radiant energy that enters the lake in spite of ice and snow cover.

The vertical distribution of phytoplankton is characterized by a concentration of algae in the surface layers in winter, and in deep water during summer stagnation. Light limitation and physical properties of the water cause the surface maximum in winter, whereas in the open season a better balance between availability of nutrients and energy income explains the fact that the peak of phytoplankton biomass and production rate is in the meta- and hypolimnion. Only the downward migration of Peridineae immediately before ice break is directly caused by excessive light. Ultraviolet radiation adds to the incoming energy; a specific detrimental effect of it is unlikely.