Zooplankton successions in neighboring lakes with contrasting impacts of amphibian and fish predators

Two pairs of neighboring subalpine lakes located in the Northern Calcareous Alps of Austria were investigated. Each pair comprised a deeper lake containing European minnows (Phoxinus phoxinus), and a corresponding shallower lake harboring Alpine newts (Triturus alpestris) as top predators. Plankton successions within fish and amphibian lakes differed markedly from each other. Throughout the year rotifers numerically dominated within the minnow lakes, while pigmented copepods (Genera Heterocope, Acanthodiaptomus, Arctodiaptomus, Mixodiaptomus) and Daphnia were prominent in the amphibian lakes, at least early during the ice-free period. We argue that size-selective predation by minnows was the ultimate reason for this predominance of smaller zooplankton. While one of the minnow lakes was characterized by a succession of spatially and temporally segregated rotifer species, the other minnow lake permitted the development of populations of small-sized Bosmina and Ceriodaphnia during summer, probably due to the existence of a strong oxycline allowing zooplankton crustaceans to avoid predation from shore-based shoals of minnows. Once trout were introduced into this lake, minnows were visibly reduced in abundance. Bosmina and Ceriodaphnia disappeared and Daphnia together with a predacious copepod (Heterocope) emerged either from egg banks or arrived from nearby source populations. We argue that the crustacean communities within the fishless lakes were adapted to the comparatively weak predation rates of Alpine newts.