How tides significantly alter the flushing of two large neighboring inverse estuaries

A three-dimensional hydrodynamic model is applied to study the flushing of two large neighboring inverse estuaries, Spencer Gulf and Gulf St. Vincent, South Australia, with and without tides. A density-driven overturning circulation, peaking in austral late winter and spring, controls the flushing of both gulfs. Tidal effects significantly affect the flushing of both estuaries with seawater from the ambient continental shelf. Tidally enhanced friction slows the mean flow and significantly reduces the flushing of the Upper Spencer Gulf by ∼9 months. Here, the tidal effect indirectly creates a more hypersaline, dynamically sheltered environment that ecologically supports the world's only mass aggregation of the Giant Australian Cuttlefish ( Sepia apama ). On the other hand, tidal mixing prevents the accumulation of dense water in a seafloor depression in the lower Gulf St. Vincent which increases the flushing of this region by ∼6 months. Findings of this study reveal the significance and opposite roles that tides play in the unique marine environment of South Australian gulfs.