Water mass connectivity and mixing along the southern margin of Australia : hydrographic and stable isotope analyses

Abstract

This study is the first to characterise the hydrographic properties and depth range of the Flinders Current and confirm its influence on shelf ecosystems of the Kangaroo Island upwelling region. Four water masses are identified in the top 1000 meters water depth (mwd) from Cape Leeuwin to Tasmania, using hydrochemistry and stable isotopes of seawater. Three water masses are identified from previous literature on the southeast Indian Ocean: Subtropical Surface Water (STSW), Tasmanian Subantarctic Mode Water (TSAMW) and Tasmanian Intermediate Water (TIW), and one is newly identified and named: South Australian Basin Central Water (SABCW). STSW is transported east by the Leeuwin Current system and is modified by heating and evaporation along the subtropical continental shelf. SABCW is formed at the subtropical front within the South Australian Basin at 40degS, TSAMW is formed within the Subantarctic Zone southwest of Tasmania, and TIW is formed from mixing of two different types of Antarctic Intermediate Water west of Tasmania. The Flinders Current transports SABCW, TSAMW and TIW west along the Australian continental slope. The top surface of SABCW delineates the interface between subantarctic water transported by the Flinders Current and subtropical water transported by the Leeuwin Current system. This interface is typically 300 mwd during winter and 250 mwd during summer, but can be as shallow as 150 mwd during summer in the Kangaroo Island upwelling region and off western Tasmania. Stable isotope values show these water masses continue north along the Western Australian slope, identifying connectivity between the Flinders Current and Leeuwin Undercurrent. Deep upwelling events in the Kangaroo Island upwelling region source SABCW from depths of 300 m or more, which is the first evidence that upwelling supplies Flinders Current water to shelf ecosystems. Stable isotopes of seawater identify the formation of a mixed water mass as SABCW mixes with STSW on the shelf. Spatial distribution of this water mass suggests that upwelled water is transported west towards Eyre Peninsula and north into Spencer Gulf, and vertical mixing allows upwelled nutrients to be brought into the photic zone to be utilised by primary producers. Strong upwelling events during February and March 2008 and February and March 2010 recorded temperatures/salinities as low as 10.4degC/34.85, and NOx/phosphate concentrations as high as 13.35/0.94 umol/L, on the shelf. New results for nutrients show average values of NOx and phosphate during months of strong upwelling to be 6.1 times and 4.6 times greater, respectively, than during winter months, and that upwelled water can have nutrient concentrations up to 90 times higher than those in summer surface waters, which is higher than values recorded previously for the Bonney Coast. Upwelled water was also low in silicate, a signature of Southern Ocean water masses, which has implications for phytoplankton community structure and diatom abundance on the shelf. Identifying nutrient signatures of upwelled water, as well as water mass interactions during upwelling events, has implications for mixing of nutrient-rich upwelled waters with oligotrophic surface waters, a situation that supports greater levels of primary productivity on the shelf. Show more