A long-term study of the recovery of the macrozoobenthos on large defaunated plots on a tidal flat in the Wadden Sea

The process of recovery of the macrozoobenthos community on tidal Rats afte r disturbance by anoxia was studied by monitoring changes in the macrozoobe nthos over periods of up to 4.5 year within and around eleven defaunated sq uares of about 120 m(2) each. Regardless of the season, nearly all species that were present in the surrounding area had settled again in the central parts of the plots within 6 months, though their numbers were generally low if only a winter half-year had elapsed. The total numbers of macrobenthic animals reached values similar to those of the surrounding areas as soon as the recovery period had included at least one summer. Numerical recovery t hus proceeded much faster if it started in spring than in autumn. As oppose d to species richness and total number of animals, biomass recovery needed several years, as did the recovery of the size and age structure of the lon g-lived species. Most species settled primarily as early postlarvae in summ er, a few species (Macoma balthica, Arenicola marina) also in high numbers as juveniles in winter. Moreover, several species showed slow colonisation in all seasons by adults transported by tidal currents or moving across the sediment. Seasonal migration abilities of the constituent species and thei r life stages appear to be the most important mechanism explaining the cour se of the recovery of the infaunal community on tidal Bats. The extraordina rily successful settlement of larvae of several species within still sparse ly populated plots led to (sometimes substantially) higher densities of the se species within than outside the plots. Such abundance overshoots were no t only frequently observed in short-lived opportunistic species (such as Py gospio elegans and Capitella capitata) but also in the juveniles of several long-lived species (such as Macoma balthica and Mva arenaria). It is concl uded that inhibition outside rather than facilitation within the plots was an important cause of these abundance overshoots. (C) 1999 Elsevier Science B.V. All rights reserved.