Macrobenthos abundance and distribution on a spatially patterned intertidal flat

Natural ecosystems can show regular spatial vegetation patterns, which develop from small-scale ecological interactions. Some studies suggest that grazers can play a major role in controlling vegetation distribution in ecosystems with regular vegetation patterns, but the distribution of grazers and the effects of grazing on vegetation in spatially patterned ecosystems remains poorly understood. Here, we study how macrofaunal grazers are distributed on a spatially patterned intertidal flat and how they interact with benthic microalgae. The study was carried out on an intertidal flat where each year a regular spatial pattern of diatom-covered hummocks and water-filled hollows develops. In 2 consecutive years, benthic algal biomass was 5-fold lower in the hollows compared with the hummocks, whereas benthic macrofauna was equally abundant on both hummocks and hollows. In the laboratory we evaluated the interactive effect of landscape morphology (hummocks and hollows) and food abundance. Experiments with 2 abundant grazers, Corophium volutator and Hydrobia ulvae showed that food availability was the main driver for the spatial distribution of benthic herbivores rather than landscape morphology. Laboratory experiments, where we analyzed the grazing effects of H. ulvae and C. volutator on benthic algal biomass, revealed that both species increased biomass-specific primary production. This could indicate that benthic grazers can stimulate algal growth, which may explain the high grazer abundances in nearly bare hollows. Increased production of benthic algae may be a key factor uncoupling herbivore density from benthic algal biomass on a spatially patterned intertidal flat.