Reduced epibenthic predation on intertidal bivalves after a severe winter in the European Wadden Sea

Abstract

One hypothesis to explain the phenomenon of high bivalve recruitment after severe winters in coastal North Sea sediments is reduced epibenthic predation. Using predator exclusion ex- periments, I tested the hypothesis that epibenthic predation on the juvenile bivalves Cerastoderma edule, Macoma balthica and Mya arenaria was lower after a severe winter (1995 to 1996) than after a moderate (1996 to 1997) and a mild (1997 to 1998) winter. In C. edule and M. arenaria there was 2-fold evidence for reduced epibenthic predation after the severe winter: (1) significant predation ef- fects occurred only in exclusion experiments after the 2 milder winters but not after the severe winter; and (2) recruits attained larger sizes in August and October after the severe winter suggesting contin- uous growth rather than truncation of the size spectrum by predators. In M. balthica, predation ef- fects were also significant only after the milder winters but there was no effect on size. In all 3 bivalve species, recruitment at the experimental sites in the fall was higher after the severe winter than after the 2 milder ones. These results suggest that high bivalve recruitment after severe winters is primar- ily caused by the post-settlement factor of reduced epibenthic predation on the tidal flats. The strat- egy to conduct several predator exclusion experiments in both sand and mud in 3 consecutive years with differential winter conditions, and while considering migration activity and size development in juvenile bivalves, proved useful to distinguish between cage artefacts and predation effects.