Variability in predator abundance links winter temperatures and bivalve recruitment: Correlative evidence from long-term data in a tidal flat

Abstract

In the Wadden Sea, annual recruitment in the major bivalve species is mostly successful in summers following severe winters, whereas it usually fails after mild winters. On the basis of a 40 yr data series, we corroborated this relationship between winter temperatures and subsequent summer abundance of recruits in 3 bivalves: Macoma balthica, Cerastoderma edule, and Mytilus edulis. A 30 yr data series showed that annual abundance of their main epibenthic predators (0-group shrimp Crangon crangon and 0- and 1-group shore crabs Carcinus maenas) was higher in springs after mild winters compared to cold ones. Summer recruit numbers in 4 bivalve species (the above and Mya arenaria) were higher when predator biomass had been lower in the preceding spring. We argue that the latter relationship was probably of a causal nature and might explain not only the relationship between winter temperatures and bivalve recruitment success, but also the observed synchronies of annual recruitment success between bivalve species and between distant parts of the Wadden Sea. Shrimp abundance in May showed a significantly increasing long-term trend that might partly explain the declining trends in recruitment success, biomass, and annual production observed in the western Wadden Sea in M. balthica and M. edulis. We conclude that top-down effects by predation on early bottom stages of bivalves strongly affect biomass and production of some of the most important members of the benthic community of the Wadden Sea, keeping this system well below its carrying capacity for bivalves in nearly all years.