Mollusc shell detritus affects benthic subtidal community dynamics in the Northern Wadden Sea

Abstract

A shallow subtidal area in the northern Wadden Sea was monitored over 17 years (from 2003 to 2019) for sediment parameters and macrobenthic fauna. Due to the sheltered position of the study area, sediment composition remained rather stable with only minor annual and seasonal variations in sediment granulometry. An intermittend storm (‘Kyrill’) had no significant effect on sediment composition parameters; the construction of an artificial dune along the southern border of the study area had minor transient but no lasting effects on sediment composition. Faunal species richness and total abundance showed the typical seasonality with minimum abundance in late winter and a peak after recruitment in summer. Variations between years in autumn (post-recruitment) abundances were best explained by the number of days with a seawater temperature <1°C during the preceding winter. Temperature during other seasons, salinity, chlorophyll concentration and NAO showed no significant correlations with total abundance, nor did storm (‘Kyrill’) or construction of the artificial dune. Within-site faunal variability was best explained by water depth and velocity of the tidal currents while sediment granulometry was of minor importance. However, the amount of bivalve shell detritus mixed with the sandy sediment proved to be the strongest covariant of species numbers, total abundance, and species-specific abundances. At the sediment surface, shells provide the only anchorage for epibenthic species which in turn attract associated fauna. Shells inside the sediment hamper movement of infaunal predators and epibenthic predators are handicapped in rooting about for prey. Thus, shell material is a highly significant structural factor for the macrozoobenthos in these shallow waters. In a future with increasing ocean acidification, the availability of benthos as a food source for higher trophic levels will depend on the balance between pH-driven shell destruction and the compensatory power of shell-building species.