Relationships between planktivore community capacity (PCC) and cladoceran microfossils in northern Swedish lakes

Planktivorous fish are known to affect the zooplankton community both in terms of species composition and species specific morphological traits. Remains of cladocerans are deposited as microfossils in lake sediments, and the microfossil assembly hence reflects the historical impact of the planktivore community in the lake. The microfossils of cladocerans can therefore be used to reconstruct historical changes in the planktivore fish community in lakes. Catch per unit effort in numbers (CPUEn) and weight (CPUEw) have previously been used as proxies for planktivory. Although these standard measurements are useful in lakes dominated by obligate planktivorous fish, they have limits in their predictive power, particularly in lakes dominated by fish that are not planktivorous throughout their entire lifespan. In this study, we suggest a novel way of estimating the extent of planktivory in lakes, the Planktivore Community Capacity (PCC). PCC takes into account the size and species specific foraging efficiency of fish on zooplankton which gives a more accurate estimate of the intensity of planktivory, especially in lakes dominated by fish that undergo ontogenetic niche shifts from zooplankton to other prey as they grow. Such fish include, for instance, perch (Perca fluviatilis) and Arctic char (Salvelinus alpinus). To analyze the relationship between surface sediment cladoceran remains and planktivory intensity, 39 lakes were sampled regarding the fish community and microfossil assembly. We focused on Bosmina, which is a dominant zooplankton genus in the study area. Bosmina is well preserved in the sediment, and also known to exhibit changes in density and morphology depending on the planktivore predation pressure. We analyzed both the density of two Bosmina species in the sediment and morphological traits in relation to estimates of planktivory intensities. Among the variables measured, morphological traits and relationship between Bosmina longispina and Bosmina longirostris were highly correlated to PCC. PCC had a higher explanatory power than both CPUEn and CPUEw for observed patterns in microfossil characteristics in lakes. We suggest that estimates of Planktivore Community Capacity (PCC) is a powerful approach for reconstructing historical changes in planktivory in lakes.