INTERNAL SEICHES IN LAKE CONSTANCE - INFLUENCE ON PLANKTON ABUNDANCE AT A FIXED SAMPLING SITE

During thermal stratification, pronounced internal seiches occur in Lake Constance with periods of 4-6 days. The amplitudes reach 12 m at the sampling site in Uberlinger See (northwestern part of Lake Constance). Seiches change the thickness of the warm and generally biologically rich epilimnion and, thus, also the areal abundance of planktonic organisms at a fixed sampling site. The potential impact of seiche-driven changes of epilimnetic thickness on observed plankton abundance was calculated using continuously recorded temperature profiles and weekly vertical profiles of plankton biomass. Observed plankton biomass was recalculated by taking into account the effect of changing layer thickness in order to give a more realistic picture of biomass changes caused by waxing and waning of populations. Our calculations imply that changes of strata thickness do not mask the general seasonal trend in population dynamics (e.g. spring bloom and clear water phase); estimates of average yearly standing stock are not significantly affected if sampling is done at least weekly. However, changes of strata thickness can alter areal plankton abundance at the sampling site by a factor of 2 within 2 days. During pronounced seiches, areal abundance of phytoplankton, which generally exhibit a stronger vertical gradient than most other plankton, may be changed by a factor of 3-4 within hours. Hence, at our sampling site, the impact of changes in strata thickness cannot be ignored for detailed analysis of population dynamics of autotrophs involving comparisons between single observations.