Two large, shallow (2m) coastal lagoons of south western Australia have become eutrophic during the last 20 years. Abundant benthic green algae (principally Cladophora aff. albida and Chaetomorpha spp.) accumulate on the shores of one and there are dense phytoplankton blooms of the blue-green Nodularia spumigena in the other. The eutrophy is caused by a great increase in the amount of nutrients, especially phosphorus (phosphatic fertilizers). Tidal range is small and exchage between estuary and ocean is restricted; evaporation is high and salinity varies seasonally from 5 to 50%; significant river flow is confined to about two winter months (July-August). At this time inorganic nutrient concentrations are high in lagoon water, but low temperature and low light conditions limit growth of benthic algae. However, diatom blooms retain nutrients and they are recycled to detrital sediment to become available to benthic algae and Nodularia when temperatue and light are favorable for their growth in summer.