Coastal eutrophication a study of Orielton Lagoon

Orielton Lagoon, an enclosed estuary, was created in 1953 following the extension of a road causeway between Midway Point and Sorell. Over the past 30 years, decomposition of excessive macrophytic growth has been blamed, for the periodic occurrence of unpleasant, nauseating odours. The growth of macrophytes appears to be associated with rainfall patterns, temperature and nutrient levels in the lagoon. A biological survey of the lagoon indicated a highly eutrophic, hypersaline water body, rich in phytoplankton. Diatoms dominated the phytoplankton and there was a suppressed zooplankton population throughout the year. Average monthly chlorophyll levels varied between 10 and 43 J.Lg/L. The prime source of nutrients measured in 1991 was a sewage treatment plant at Midway Point which has discharged primary treated sewage into the lagoon since 1969. Nitrate levels were similar to those in the adjacent Pittwater area but phosphate levels in the lagoon were approximately double those in Pittwater. Computer modelling of water movement through the lagoon was used to estimate water exchange within the system. This highlighted the very limited water exchange with the existing restricted outlet and demonstrated the significant additional water movement that could be achieved by minor modification. The study has predicted changes in water quality that would result from both increased water flow and removal of nutrient input. To achieve a water quality similar to that in the neighbouring Pittwater area, with a chlorophyll level between 1 and 5 J.Lg/L, both the removal of the nutrient source and increased flushing of the system must be undertaken. The importance of the area as an internationally recognised wetland, important for migratory wading birds, is a prime consideration in any management plans to improve the water quality in the lagoon. The presence of a profitable aquacultural industry in the adjacent Pittwater area is also important in formulating future management practices.