Long-term development of the crustacean plankton in the Saidenbach Reservoir (Germany) – changes, causes, consequences

Abstract

The drinking water reservoir Saidenbach (Germany) has been investigated continuously from the year 1975 onwards. Since 1987, daphnids (Daphnia galeata) — an essential component in the cycle of matter and energy in the pelagic zone of reservoirs and lakes — became less and less abundant. Its total and relative biomass declined drastically and the exponential population growth began later in the year. After 1990, also the biomass of the copepods decreased significantly. Thus, the total crustacean biomass declined from 0.35 cm³ m⁻³ (mean 1975–1990) to 0.22 cm³ m⁻³ in the 1990s. Top-down effects seem to be responsible for all these changes, because stocks of both native planktivorous fishes, mainly roach (Rutilus rutilus) and perch (Perca fluviatilis), and non-native silver carps (Hypophthalmichthys molitrix) have increased over the years. A second observation confirms this conclusion: the significant temperature effects on the growth rate of Daphnia observed every year in early summer (May) in the first years under investigation were not detectable anymore after the year 1987. Since then, the exponential population development did not start until end of June/beginning of July. This delay and the substantial decrease of the mean body length of the daphnids are clear indications for the enormous predation pressure on this zooplankton group. Simultaneously, the concentration of nanoplankton increased and the Secchi depth decreased indicating the diminishing impact of crustaceans on phytoplankton in the course of the years. Thus, a significant relationship between the concentration of Daphnia and the water transparency could be obtained — a confirmation of their great importance for the water clarity. Consequently, if Daphnia shall provide a positive contribution to water quality (e.g., by effective particle elimination), a well structured fish community with regard to species composition, size structure, standing stock and functional groups will be needed as a prerequisite.