Drivers of deep water renewal in Macquarie Harbour, Tasmania

Abstract

Abstract Macquarie Harbour is a fjord-like estuary located on the west coast of Tasmania and is home to a growing aquaculture industry. The harbour is oriented with the mouth on its north west end and two main tributaries located on its northeast and southeast ends. Both of the system's major tributaries have hydroelectric power schemes controlling catchment discharge into the harbour. The harbour has a history of low oxygen concentrations and the bottom water is often oxygen poor. No study to date has described the underlying mechanisms of deep water renewal and oxygenation in the system. Approximately 5 years of data from monthly dissolved oxygen, salinity, and temperature profiles, located throughout the harbour, and 3 continuous logger stations was analysed with water level and meteorological data. These data were used to describe the drivers of water level and deep water oxygen renewal in the system. This study provides a first order estimate of the net mass of dissolved oxygen entering via these marine water intrusion events. Data analysis indicates that deep water renewal appears to occur episodically, with only 2 significant events recorded over a 5 year period, though a number of smaller events were also recorded on a monthly basis. The largest events were observed during winter and late autumn and are associated with strong north westerly winds and a sudden drop in atmospheric pressure. The drop in pressure results in an increase in water level that allows marine water to enter the harbour via the mouth. The north westerly winds act to push surface water to the southern end of the harbour allowing more water to flood over the sill into the Sophia basin. Analysis of oxygen, temperature and salinity logger data indicates that larger renewal events create water column instabilities which can provide a potential threat to the salmon aquaculture industry situated in the harbour. The frequency of renewals also has implications on the standing oxygen concentrations in the harbour, with fewer events likely resulting in greater periods of hypoxia in the bottom and mid water column.