Seasonal and Inter-annual variability in Chemical and Biological parameters in Storm Bay, Tasmania. I. Physics, Chemistry and the Biomass of components, of the food chain

Abstract

As south-east Tasmania lies close to the subtropical convergence, the waters of the shelf regions are a complex mixture of subtropical and subantarctic water masses. Temperature, salinity and nutrient data from the waters off Maria Island have been recorded for 40 years and show considerable inter-annual variability. In a complementary study, physical, chemical and biological parameters were measured in Storm Bay over 3 years. The enlarged data base gives a detailed picture of the seasonality and inter- annual variability in south-east Tasmanian waters and the effects of such variability on the food chain. The temperature and salinity measurements show that the water in Storm Bay is strongly influenced at different times of the year by water of subtropical origin from the east and by water of subantarctic origin from the west. Satellite images and drift-card data confirm these findings. Chemical parameters (dissolved inorganic and organic nutrients) showed the expected temperate seasonal trends, but some features of the seasonal cycle differed greatly in magnitude and duration between years. Differences between the years are shown to be due to inter-annual differences in the westerly wind stress, which affects nitrate concentration. In years of strong westerlies, the phytoplankton biomass and productivity increases and the spring bloom lasts longer. In such years, zooplankton biomass increases by a factor of ten in late spring, and salps replace euphausiids as the dominant organisms.