Limited effects of grazer exclusion on the epiphytes of Posidonia sinuosa in South Australia

Abstract

The role of grazing in regulating the abundance and biomass of epiphytes of the seagrass Posidonia sinuosa Cambridge et Kuo was investigated at an oligotrophic site in lower Spencer Gulf, South Australia. Prosobranch gastropods >7 mm in size were excluded from 0.25 m 2 plots of seagrass for 3 months with four steel mesh cages, compared with four partial cages used to control for cage artefacts, and also four uncaged control plots. Abundances of epiphytic algae, invertebrates and epiphytic mass (DW, AFDW and calcium carbonate) were recorded regularly. The indirect effect of grazer exclusion on seagrass leaf mortality (necrosis) was also measured. Grazing effects on the epiphytic assemblage were detected despite some cage artefacts. Filamentous algal abundance and epiphytic biomass (AFDW) increased in response to grazer exclusion. Caging per se reduced crustose coralline algal cover and calcium carbonate during the experimental period but, after 3 months, no artefact or treatment effect was discernible. Abundances of invertebrate taxa were little affected by grazer exclusion although caging per se reduced the density of spirorbid polychaetes. Grazer exclusion did not affect seagrass necrosis. In comparison with similar studies and given the duration of exclusion, the sum of treatment effects on epiphytes and seagrass was small. Rather, the role of epiphyte grazing in this nutrient-poor area appears to be limited to the maintenance of an epiphytic assemblage that is free of filamentous algae and dominated by crustose corallines.