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. Intensive tidal stirring significantly slows the flushing by ∼9 months in the gulfs’ upper reaches. This tidal stirring weakens lateral density gradients that otherwise would support an exchange circulation enhancing the flushing of the region. To this end, the tidal effect indirectly creates a more hypersaline, dynamically sheltered environment in the Upper Spencer Gulf that ecologically supports the world’s only mass aggregation of the Giant Australian Cuttlefish ( Sepia apama ). On the other hand, strong tidal stirring across a narrow passage, Backstairs Passage, increases the flushing of the southern portion of Gulf St. Vincent 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. • Applies a hydrodynamic model to study the flushing of two large neighboring inverse estuaries with and without tides • Finds that tidal effects significantly alter the flushing of both estuaries • Tidal effects increase the flushing in one estuary while slowing it in the other • Changes in the flushing times are of the order of 6-9 months. This study explores the role of tides in the flushing of two large inverse estuaries, Spencer Gulf and Gulf St. Vincent, South Australia, with ambient seawater. This study reveals that tidal effects significantly affect the overturning circulation and flushing in both estuaries. Intense tidal stirring significantly slows the flushing of the upper reach of Spencer Gulf by 9 months which ecologically supports the world’s only mass aggregation of the Giant Australian Cuttlefish. Conversely, intense tidal stirring in a narrow passage, Backstairs Passage, increases the flushing of Gulf St. Vincent by 6 months.