Past and present grazing boosts the photo-autotrophic biomass of biofilms

Little is known about the long-term consequences of grazing effects on microphytes. This study tested density-dependent responses to grazer removal on the biomass (chlorophylla, chla) and composition of natural high rocky-shore biofilms over a 7 mo period. Gastropod snailsMelarhaphe neritoidesgraze entirely within circular halos generated in biofilms surrounding their refuges. The experiment crossed 3 levels of original snail density per halo with 3 levels of grazing intensity (generated by 100, 50 and 0% snail removal). Areas inside halos from which all snails had been removed sustained significantly higher chlathan never-grazed control areas outside the halos. This effect of grazing history was still present after 7 mo, suggesting that past grazing had an enduring positive influence on biofilm biomass. Against expectation, chla-biomass was not increased by removing snails, regardless of original grazer density. Half- and fully-grazed halos peaked to a higher chlathan ungrazed halos in spring. Grazing did not affect the presence of major biofilm taxonomic groups, although it did alter their relative contributions. Never-grazed areas were covered by a thick biofilm-detritus complex and had proportionally more filamentous cyanobacteria than grazed areas, which sustained abundant clusters of coccoid cyanobacteria and lichen within micro-pits inaccessible to snail radulae. The present study shows that effects of grazing history are not exclusive to macrophytic systems. Grazers boosted the concentration of micro-autotrophs relative to non-chlabiofilm constituents, probably by removing an unproductive biofilm canopy and facilitating light and nutrient penetration for new growth.