Large variations in eutrophication among estuaries reflect massive differences in composition and biomass of macroalgal drift.

The characteristics of detached macroalgae (drift) in nearby highly eutrophic and mesotrophic estuaries in south-western Australia are compared to elucidate the magnitude and types of changes that occur in macroalgal drift when estuaries receive excessive nutrient input. Drift characteristics in the large basins of the microtidal, eutrophic Peel-Harvey and mesotrophic Swan-Canning, which is not subjected to large nutrient inputs directly from agricultural land, differed markedly. Biomass (dry weight) in mesotrophic estuary was dominated by rhodophytes (92%), particularly Laurencia and Hypnea, and in eutrophic estuary by opportunistic chlorophytes (68%), especially Chaetomorpha and Ulva. Prevalence and biomass of drift were far greater in the eutrophic estuary, particularly during summer and autumn when macroalgal growth rose sharply. Macroalgal biomass in the eutrophic estuary was positively related to salinity. These results facilitate predictions of how climatic and other anthropogenic changes influence extent of macroalgal growth and thus change the estuarine environment.