A search for eutrophication in the seagrass meadows of Rottnest Island and a base line study for further monitoring

Abstract

During a 17 day study conducted at Rottnest Island, Western Australia the leaf biomass and productivity of Posidonia sinuosa and the biomass of its epiphytes were sampled in three bays to test for indications of eutrophication. In one of these bays, the aboveground biomass and stem epiphytes were sampled from Amphibolis antarctica and A. griffithii to provide a baseline for future studies. Leaf biomass in Longreach Bay (380 g m⁻²) was significantly less than in the other two bays (550-580 g m⁻²), whereas the shoot density (390 shoots m⁻²) in Nancy Cove was less than in the other two bays (about 530-600 shoots m⁻²). Areal productivity (2.1 - 2.3 g m⁻² d⁻¹ ) of Posidonia sinuosa was not different in each bay. In the only bay sampled for Amphibolis, the leaf biomass of A. antarctica (265 g m⁻²) was lower than that of A. griffithii (390 g m⁻²) but the stem biomass was similar (260 g m⁻²). The biomass of epiphytes from the stems was also similar for each species (about 200 g m⁻²). Two of the sampled bays were used by moored boats and had shore development, yet there was no sign of higher than ambient concentrations of nutrients in any of the bays. It is concluded that, at the time of this study, there was no discernible effect of anthropogenic inputs on seagrasses in the three bays at Rottnest Island.