Herbivore-mediated increase in the photosynthetic capacity of marine biofilms: indirect effects of changing microalgal assemblage composition

The effects of grazing by the high-shore periwinkle Nodilittornia africana on the biomass, productivity, composition and diversity of microalgal assemblages were experimentally assessed on the south coast of South Africa. The results indicate that littorinids selectively removed large/filamentous and loosely attached 'overstorey' cyanobacteria, thereby reducing microalgal biomass and species diversity. As grazers also eliminated macroalgal sporelings, they restricted the upper distributional limit of Ulva spp. and Enteromorpha spp., which in the absence of grazers colonized and persisted on the high shore. The grazing-induced reduction in algal biomass resulted in a decline in overall biofilm productivity (production cm(-2)), In contrast, the photosynthetic capacity of algae (production per mug chlorophyll a) increased in grazed plots. This latter observation could be explained neither by nutrient addition from molluscan excretions in exclusion plots (mechanical disturbance induced photosynthetic capacity) nor by nutrient/light limitation in ungrazed plots (treatment effect was significant in biofilms of similar thickness). Instead, the results indicate that the physical disturbance caused by littorinid grazing changes the composition of microalgal assemblages to one dominated by more productive taxa.