Original paper

Seasonal vertical distribution of phytoplankton and copepod species in a high-mountain lake

Winder, Monika; Buergi, Hans; Rudolf Spaak, Piet


Many zooplankton species migrate to deeper water layers during the day to reduce light-dependent mortality risk at the surface layers. Theories explaining diel vertical migration (DVM) of zooplankters share the assumption that the surface waters are more food-rich compared to the deep-water layers and that zooplankters return at night to the surface to profit from the warmer, food-rich environment. However, evidence suggests that the resources in deep-water layers can be as profitable as in the upper water layers for zooplankton. Those deep-water food maxima have often been overlooked in freshwater. In this study we investigated the vertical distribution of phytoplankton together with the vertical day and night distribution of copepod species in a high-mountain lake with a deep-water phytoplankton maximum dominated by cryptophytes and blue-greens. All copepod species, including the small-sized individuals avoided the upper strata during daytime. Adult Cyclops abyssorum migrated almost throughout the entire year. In contrast, the Eudiaptomus gracilis and copepodites of C. abyssorum and nauplia of both species did not migrate but remained in the same water layers day and night. Secchi depth transparency significantly correlated with daytime depth of adult copepods. Daytime depths are usually associated with high concentration of algal biovolume and moderate temperature. Although food conditions are better in deeper strata, for some species the warmer temperatures in the surface waters may be the reason why they migrate to shallower depths during the night.