Seasonal changes in hydrology and the distribution of plankton in the Bathurst Harbour estuary, southwestern Tasmania, 1988-89

The hydrology and abundance of planktonic organisms (>20 m size) were surveyed at various sites in the Bathurst Harbour/Port Davey estuary on five occasions between October 1988 and July 1989, and the hydrology again surveyed in February 1991. In winter, the period of peak freshwater outflow, a brackish surface layer(= 15%o) was present to a depth of ~4 m throughout the estuary. This surface layer was present to a reduced depth in other seasons, except during summer, when the halocline broke down and surface salinities of=29%o in 1989 and =25%o in 1991 prevailed. Bottom waters were close to fully marine (~33%0) throughout the year. Surface waters in Bathurst Harbour were found to be extremely depleted in nitrates «0.1 11M). Oxygen levels were also very low (~35% saturation) during the 1989 summer, which was unusually dry, but were near full saturation during the other sampling periods. The plankton community in Bathurst Harbour contained very few taxa, particularly during the winter months. The dinoflagellate Dinophysis acuminata comprised >99% of total phytoplankton numbers in October 1988 and July 1989, while the copepods Gladioferens inermis and Oithona australis and the appendicularian Oikopleura sp. numerically dominated the woplankton community. Dinoflagellates and copepods both occurred in high densities. The distribution of diatoms within the estuary was inversely related to the distribution of dinoflagellates; diatoms occurred in low densities relative to dinoflagellates in Bathurst Harbour outside the summer months, but occurred in much higher densities than dinoflagellates in the waters of Port Davey throughout the year