Mixing, hypersalinity and gradients in Hervey Bay, Australia

Abstract

Hervey Bay, a large coastal embayment situated off the central eastern coast of Australia, is a shallow tidal area (average depth = 15 m), close to the continental shelf. It shows features of an inverse estuary, due to the high evaporation rate (approx. 2 m/year), low precipitation (less than 1 m/year) and on average almost no freshwater input from rivers that drain into the bay. The hydro- and thermodynamical structure of Hervey Bay and their variability are presented here for the first time, using a combination of four-dimensional modelling and observations from field studies. The numerical studies are performed with the Coupled Hydrodynamical Ecological model for RegioNal Shelf seas (COHERENS). Due to the high tidal range (> 3.5 m) the bay is considered as a vertically well-mixed system and therefore only horizontal fronts a likely. Recent field measurements, but also the numerical simulations indicate characteristic features of an inverse/hypersaline estuary with low salinities (35.5 psu) in the open ocean and peak values (> 39.0 psu) in the head water of the bay. The model further predicts a nearly persistent mean salinity gradient of 0.5 psu across the bay (with higher salinities close to the shore).