Spring weather determines the relative importance of ciliates, rotifers and crustaceans for the initiation of the clear-water phase in a large, deep lake

Clear-water phase (CWP) is an important event in seasonal planktonsuccession. We examined the influence of all herbivorous zooplanktonon its initiation under different weather and climatic conditionsusing up to 19 years of observations from the large, deep LakeConstance (Europe) and estimates of relative clearance rates.A CWP occurred regularly, even if daphnid biomass was stillvery low. CWP was attributed to strong grazing either by a daphnid-dominatedzooplankton community or by a diverse assemblage consistingof micro- and meso-zooplankton. Both types of zooplankton communitiesoccurred with approximately the same frequency. The timing ofthe CWP was unrelated to the North Atlantic Oscillation (NAO)but correlated with the wind-dependent intensity of deep verticalmixing 3 months earlier, during early spring. Less mixing enabledearly growth of phytoplankton, ciliates and rotifers despitelow temperatures, which prevented daphnid development at thistime. This resulted in enhanced grazing of ciliates and rotifers,which increased the importance of phytoplankton less ediblefor most ciliates, rotifers and daphnids. Ciliates clearly dominatedthe grazing pressure on phytoplankton throughout spring, maintaininghigh biomasses together with the phytoplankton for up to 2 months.A CWP was observed when herbivores grazing on larger phytoplanktondeveloped in addition to ciliates.     

Predator-prey dynamics driven by feedback between functionally diverse trophic levels

Neglecting the naturally existing functional diversity of communities and the resulting potential to respond to altered conditions may strongly reduce the realism and predictive power of ecological models. We therefore propose and study a predator-prey model that describes mutual feedback via species shifts in both predator and prey, using a dynamic trait approach. Species compositions of the two trophic levels were described by mean functional traits--prey edibility and predator food-selectivity--and functional diversities by the variances. Altered edibility triggered shifts in food-selectivity so that consumers continuously respond to the present prey composition, and vice versa. This trait-mediated feedback mechanism resulted in a complex dynamic behavior with ongoing oscillations in the mean trait values, reflecting continuous reorganization of the trophic levels. The feedback was only possible if sufficient functional diversity was present in both trophic levels. Functional diversity was internally maintained on the prey level as no niche existed in our system, which was ideal under any composition of the predator level due to the trade-offs between edibility, growth and carrying capacity. The predators were only subject to one trade-off between food-selectivity and grazing ability and in the absence of immigration, one predator type became abundant, i.e., functional diversity declined to zero. In the lack of functional diversity the system showed the same dynamics as conventional models of predator-prey interactions ignoring the potential for shifts in species composition. This way, our study identified the crucial role of trade-offs and their shape in physiological and ecological traits for preserving diversity.