Water masses and their seasonal variation on the Lincoln Shelf, South Australia

Abstract

Five water masses are defined for the Lincoln Shelf, South Australia using hydrographic and stable isotope data. Three water masses are present on the shelf and slope year-round. Slope Water with low temperature, salinity, and isotope values occurs perennially on the slope at depths greater than 180 m, and episodically upwells onto the shelf during summer. During strong upwelling events this water can be upwelled from 300 m to 100 m water depth on the shelf south of Kangaroo Island. This water then flows west toward Eyre Peninsula and into the mouth of Spencer Gulf. The most common shelf water is Subtropical Surface Water, a mixed water mass that is transported year-round by the eastward flowing South Australian Current. This current is heavily influenced by waters from the Great Australian Bight and local modifications, with temperature and salinity of 16–17°C and 35.8, respectively. Local summer heating and evaporation of Subtropical Surface Water on the shelf and within Spencer Gulf forms Evaporated Water, a water mass with high temperature, salinity and isotope values. The use of stable isotopes in water mass analysis permits the identification of two new water masses that form on the shelf during summer. Mixed Slope Water forms when Slope Water mixes with Subtropical Surface Water during upwelling events, and Cooled Evaporated Water is generated when surface Evaporated Water mixes vertically with cool, fresh bottom waters. Such dynamics lead to mixing of nutrient-rich mesotrophic upwelled waters with oligotrophic surface waters, which supports greater levels of primary productivity on the shelf.