This paper reviews results from a monitoring programme carried out in a shallow eutrophic estuarine system. The aims of the work were to define the sources of nutrients responsible for large accumulations of plant biomass found in the system; to define and understand relationships between significant biological and physicochemical factors; and to provide data for measuring the effectiveness of proposed management strategies. The programme is of more general interest because it explores the use of long-term data bases for understanding behaviour in highly variable systems.
Estuaries show large changes from season-to-season, and large differences between years, especially in regions with highly seasonal and erratic rainfalls. The water of an estuary tends to be fresh when river discharge flushes saline water from the basin, and saline when river flow ebbs and marine water penetrates from the ocean.
Variability is also caused by stratification of the water column, a consequence of the large density difference between marine and fresh water. Even shallow estuaries may be highly stratified for days or weeks at a time, with consequent physicochemical and biological differences between surface and bottom waters.
The monitoring programme has been carried out in a coastal-plain estuary, the Peel-Harvey System in Western Australia. This consists of two large shallow basins, Harvey Estuary (into which the Harvey River discharges) and Peel Inlet (the basin into which the Murray and Serpentine Rivers discharge). Each basin has an area of approximately 66 x 106 m2, and maximum depth of 2m. The basins are linked together and have a common, narrow 'inlet channel' connecting Peel Inlet to the ocean (Figure 1).