Pelagic‐benthic coupling‐The importance of diatom interannual variability for population oscillations of Monoporeia affinis

The amphipod Monoporeia affinis (Lindström) exhibits long‐term population oscillations in oligotrophic Lake Vänern. Correlation showed profundal oxygen and temperature, and intraspecific and predatory interactions to be poor explanatory variables. Conversely, the interannual variability in food availability—diatom biovolume—was highly correlated with amphipod total density at t + 1 yr and the density of 0 + amphipods and growth (amphipod body length in August) of 0 + , 1 + , and 2 + generations. Amphipod recruitment success of 0 animals may be an important factor governing long‐term population dynamics, and strong 0 + year classes result in greater 1 + and 2 + population densities. Fish predation, although tentatively not an important population‐regulating factor, was negatively correlated with amphipod total body lengths of the 1 + and 2 age classes in May. Size‐selective fish predation pressure on juveniles and adults may facilitate coexistence of the 3‐yr age classes.