Recruitment dynamics of North Sea macrozoobenthos in intertidal soft bottoms: larval availability, settlement and dispersal

Abstract

This thesis investigates how larval supply, larval settlement and secondary dispersal affected recruitment and recovery of macrozoobenthos living in an intertidal sand flat located in the German Wadden Sea. The temporal availability of meroplanktonic bivalve larvae was related to the timing of sea water temperature increase and/or the timing and magnitude of phytoplankton blooms. Larval settlement of the polychaete Lanice conchilega occurred almost exclusively at exposed sites within the adult habitat and was dependend upon the density of the adults. Model simulations revealed that dense L. conchilega patches develop only in areas of high near-bottom flows. In contrast, bivalve larvae settled mainly in sheltered habitats. Both polychaetes and bivalves undergo phases of postlarval dispersal which rapidly changed the spatial patterns generated by larval settlement. In-situ staining experiments showed that secondary dispersal is strongly size/age dependent. The recovery of two macrobenthic assemblages following the severe winter was rapid (<1 year) and influenced by 1) species life histories, 2) habitat conditions, and 3) biotic interactions, namely facilitation. The temporal course of recolonisation was mainly determined by the availability of settling stages (larvae and postlarvae). This thesis elucidates that both pre- and post-settlement processes contribute to the recruitment and recolonisation of macrozoobenthic communities. In dynamic systems, flexibility of life histories, high mobility of benthic life stages and the existence of functionally distinct life stages provide multiple paths to population maintenance and a high resilience. It is highlighted that at the end tides and currents are the decisive processes because they set the boundary conditions to which organisms respond and mediate the function of many processes such as larval drift, larval settlement and secondary dispersal.