Water residence time as a driving force of zooplankton structure and succession

We studied zooplankton dynamics in a groundwater-fed, montane lake during four consecutive years and assessed the importance of water residence time for zooplankton dynamics. Crustacean abundance and biomass were significantly correlated with water residence time and temperature, but showed no significant correlation with phytoplankton biovolume. We hypothesised that temperature depended on water residence time (τ), and therefore we further investigated the functional relationship of crustacean dominance with the latter by logistic regression analysis. Water residence time values above a threshold value (τ = 193 days) determined crustacean biomass dominance while values below determined rotiferan dominance. Our results indicated that water residence time was an important factor structuring zooplankton succession in this lake that showed large fluctuations of τ values (median 263 days; range 23 – 786 days for the four year period) compared to other lakes. We suggest that crustacean biomass was directly controlled through water residence time as found for riverine systems, whereas rotifer biomass was controlled through exploitative competition with crustaceans for phytoplankton. The importance of water residence time may have been underestimated in lakes when explaining zooplankton community structure and succession, because studies usually focus on other factors such as temperature, predation, or food limitation.