Eutrophication in the Peel-Harvey Estuarine System, Western Australia

Abstract

The most obvious symptom of eutrophication in this estuarine system is a green alga, Cladophora, which was sparse in 1966 but now accumulates and rots on the shores. The work is part of a continuing study designed to assess the relationships between nutrient input and the growth of Cladophora and phytoplankton.

The system consists of two shallow basins, interconnected and linked to the ocean by a narrow channel. It is fed by three rivers; 90 percent of river flow occurs during four winter months.

Phytoplankton and water nutrient levels are low in summer, but high during and after an input of river nutrients in winter. Nitrogen:phosphorus ratios, regression analyses, and nutrient limitation assays suggest that nitrogen is potentially limiting in summer and autumn, phosphorus in winter and spring. The Harvey typically supports higher phytoplankton levels than the Peel. Diatom populations may be replaced by blue-greens in summer in the Harvey.

Cladophora forms detached spheres of branched filaments and is only prominent in the Peel. Changes in biomass and growth of confined populations in the field, together with laboratory experiments, show that growth occurs when temperatures and light intensities are high, not in winter when water column nutrient levels are high. Water from between the algal spheres has increased levels of phosphorus compared with the water column above, emphasising the possible importance of nutrient release from decaying material below. It is suggested that phytoplankton are important in trapping water-column nutrients during and after river nutrient input, and that subsequent Cladophora and phytoplankton growth depends on nutrient recycling.