Interacting in the dark: a study of the diel vertical migrations of pelagic plankton and fish in Lake Ladoga

The diel vertical distribution of nutrients, phytoplankton, zooplankton and fish were studied simultaneously at two pelagial stations in Lake Ladoga in August 1995. At one site (Valaam) cryptophyceans dominated in phytoplankton, while at the other (Konevits) diatoms were more common. In Valaam the algae were distributed unevenly, mainly in the surface layers, while in Konevits the algae were distributed evenly throughout the entire 0–20 m layer. In Valaam a clear diel migration pattern was observed by most of studied zooplankton taxa, while in Konevits only large Cyclops and Limnocalanus migrated. The peak total density of pelagic fish was more than twice as high in Konevits as in Valaam, while the zooplankton density was three times lower in Konevits than in Valaam. In Valaam the clear surface maximum of cryptophyceans probably affected the migration pattern of most species of herbivorous zooplankton. In Konevits the lower temperature in the metalimnion, where the daytime zooplankton maximum occurred, and evenly distributed food resources may have limited zooplankton migration, despite the existing predation pressure.