Influence of Corophium volutator and Hydrobia ulvae on intertidal benthic diatom assemblages under different nutrient and temperature regimes

Epipelic diatoms dominate the microphytobenthos of estuarine sediments, where they play important roles in ecological processes such as primary production, secondary production and sediment stability. Grazing (top-down control) and nutrients (bottom-up control) regulate the biomass and species composition of intertidal benthic diatom assemblages. However, observations of grazing/predation effects on species richness differ under contrasting nutrient conditions. We investigated the interactive effects of grazing, nutrients and temperature and compared the impacts of Corophium volutator and Hydrobia ulvae -2 species that differ in their feeding strategies and bioturbation effects. Diatom assemblages were collected from 2 estuaries (Biezelingsche Ham, Westerschelde, high nutrient, and Zandkreek, Oosterschelde, low nutrient) in The Netherlands that differ in their dominant macrofaunal grazer species. Assemblages were grown in the laboratory without (control) and with grazing activity under different nutrient and temperature regimes. C. volutator exerted a strong regulatory influence on epipelic diatoms by reducing biomass, and preferentially consuming certain dominant taxa, thereby increasing species richness, evenness and diversity. The percentage of epipsammic species increased in the presence of C. volutator, at the expense of Navicula species. Biezelingsche Ham assemblages grazed by C. volutator were not influenced by nutrient or temperature regime, while control assemblages were influenced by temperature. In contrast, differences in the structure of diatom assemblages between the treatments were far less pronounced for H. ulvaegrazed and control Zandkreek assemblages. H. ulvae appeared to be a general consumer, grazing subdominant species. Species richness was greater at low temperature, regardless of nutrient level. Macrofaunal grazing did not predictably increase or decrease species diversity, but could potentially do both, and it may mask the effects of environmental and bottom-up control.