Trophic structure in the pelagial of 25 shallow New Zealand lakes: changes along nutrient and fish gradients

To gain better insight into the importance of predator and resource control in New Zealand lakes we surveyed the late summer trophic structure of 25 shallow South Island lakes with contrasting nutrient levels (6–603 μg TP l–1) and fish densities. Total catch of fish per net (CPUE) in multi-mesh gillnets placed in the open water and the littoral zones was positively related with the nutrient level. Trout CPUE was negatively correlated with total phosphorus (TP) and total nitrogen (TN). Zooplankton seemed largely influenced by fish, as high fish CPUE coincided with low zooplankton and Daphnia biomass, low average weight of cladocerans, low contribution of Daphnia to total cladoceran biomass, low ratio of calanoids to total copepod biomass and low ratio of zooplankton biomass to phytoplankton biomass. However, chlorophyll a was only slightly negatively related to Daphnia biomass and not to zooplankton biomass in a multiple regression that included TN and TP. Ciliate abundance was positively related to chlorophyll a and negatively to Daphnia biomass, but not to total zooplankton biomass, while no relationships were found between heterotrophic nanoflagellates and zooplankton. The relationships between fish abundance and nutrients and fish abundance and zooplankton:phytoplankton ratio and between chlorophyll a and TP largely followed the pattern obtained for 42 north temperate Danish lakes. We conclude that fish, including trout, have a major effect on the zooplankton community structure and biomass in the pelagial of the shallow oligotrophic to slightly eutrophic New Zealand lakes, but that the cascading effects on phytoplankton and protist are apparently modest.