The spatial and temporal dynamics of deep chlorophyll layers in high-mountain lakes: effects of nutrients, grazing and herbivore nutrient recycling as growth determinants

Abstract

Deep chlorophyll layers (DCL) are a common feature of oligotrophic lakes, yet the mechanisms that form and maintain them are not understood fully. These phytoplankton populations occur in the metalimnia of lakes where light levels are moderate to low, and where nutrient levels and zooplankton grazing pressure are different than in the epilimnion. To test the importance of nutrients and grazing pressure for algal growth in different lake strata, microcosm experiments and monitoring were conducted in two oligotrophic lakes in the Rocky Mountains of North America that contain DCL. In situ microcosm experiments with natural phytoplankton communities from three depth strata were conducted with macronutrient additions and with and without the natural zooplankton grazing communities. Alkaline phosphatase assays and the in situ microcosm experiments indicated less nutrient limitation in the metalimnia than in the epilimnia of both lakes. Zooplankton grazing in the experiments decreased algal population growth rates by as much as 6% day –1 , with impacts shifting to progressively deeper strata over the summer. Zooplankton grazing losses, however, were partially offset by nutrient recycling that increased algal growth rates. Depthdifferential nutrient deficiency and zooplankton grazing and recycling interacted to maintain the DCL in these lakes.