A novel interaction between nutrients and grazers alters relative dominance of marine habitats

Strong top-down control by grazers is considered a moderating influence on the negative effects of elevated nutrients on marine algae. Little experimental work has been done in a system that has weak grazing pressure (weak top-down control), which is also subjected to elevated nutrients. We experimentally elevated nutrient concentration to test (1) the effects of nutrient enrichment on algal assemblages in the presence and absence of canopies (Ecklonia radiata) and (2) the interactive effects of nutrients and molluscan grazers (meso-grazers) on algal assemblages in the absence of canopies. We established that the loss of canopy-forming algae is likely to be a key precursor to nutrient-driven changes of assemblages of benthic algae, because nutrients had no effects on algal assemblages in the presence of canopy-forming algae. In the absence of canopy-forming algae, space was monopolised by filamentous, turf-forming algae, and it was only in the presence of grazers that nutrients caused a change to the relative covers of algal habitat that monopolise canopy-free space. When grazers were present at natural densities, elevated nutrients reduced the monopoly of turf-forming algae in favour of foliose algae. These results demonstrate a novel interaction between nutrients (bottom-up control) and grazing pressure (top-down control), which are fundamental to predictions about management of human activities that continue to reduce densities of herbivores and increase nutrient availability on temperate coasts.