Opening regimes and salinity characteristics of intermittently opening and permanently open coastal lagoons on the south coast of New South Wales

Abstract

The physico-chemical and hydrological characteristics of two intermittently opening coastal lagoons on the mid south coast of New South Wales were studied and compared with those of a permanently open coastal lagoon in the same area. Aspects studied over a period of several years during the mid to late 1980's included seasonal patterns of catchment rainfall, fluctuations in salinity and water temperature, and the opening regimes of these lagoons to the sea. The two intermittently opening lagoons (Swan Lake and Lake Wollumboola) were found to open for durations of around one to five months at approximately two-yearly intervals over the study period. Their salinity regimes fluctuated over this period in response to a complex of factors including rainfall and catchment runoff, temperature and surface evaporation, and the morphology of their entrance channels and duration of their opening to, and thus, the amount of water exchange with, the sea. These physical, chemical and hydrological characteristics are compared and contrasted with those of the permanently open lagoon (Lake Conjola), and also those of similar intermittently opening lagoons in other areas of the Indo-Pacific region (including south-western Australia, southern Africa and eastern Mexico). The above findings, and particularly those relating to lagoon opening regimes and salinity fluctuations, will be discussed in relation to both the community structure of fish assemblages and fisheries in these two types of lagoons, and juvenile fish recruitment to them and adult fish movements into and out of them, in subsequent papers describing the ecology of fishes in these systems.