Interaction of top down and bottom up factors in intertidal rockpools: Effects on early successional macroalgal community composition, abundance and productivity

Increasing levels of eutrophication in coastal waters and recognition of natural variation in supply of nutrients, through for example variations in upwelling intensity, have led to an increasing demand to understand the role of bottom up forcing in coastal communities. Rocky intertidal habitats are highly disturbed and are composed of a patchy mosaic of assemblages in different successional stages. The role of consumer control of algae in such habitats is well recognised but the way consumer control interacts with variation in resource supply is still debated. We examined, using a factorial manipulative experiment, the way top down control by molluscan grazers interacts with nutrient supply to determine the community composition and productivity of early successional communities in intertidal rockpools. The experiment was run over a six-month period, through the summer months, when nutrients were potentially limiting. Rockpools were cleared of biota and nutrient supply enhanced using slow release fertiliser, while the level of molluscan grazing was reduced through regular removals. Macroalgal colonisation was rapid. Positive effects of nutrient enrichment on macroalgal cover were generally lower than the negative effects of grazer control, although at the peak of algal cover, nutrient effects were apparent irrespective of grazer treatment. There were distinct differences in the identity of dominant species among nutrient treatments with green Ulva spp. dominating under enhanced nutrient conditions and red Ceramium spp. dominating under ambient conditions. These differences contributed to a mismatch between effect estimates based on cover and those based on direct measures of productivity. The use of percentage cover as a surrogate of biomass in intertidal studies is discussed. The results indicate that in a disturbed environment enhanced nutrient supply can stimulate macroalgal blooms despite strong top down control by intertidal grazers.