The Regional Estuaries Initiative (REI) is dedicated to protecting and improving the health of six estuaries in the state’s south west, including Wilson Inlet. Scientific understanding is key to effective management.
Since October 2016, REI has monitored Wilson Inlet’s water quality at seven sites every two weeks. The first 12 months of data (October 2016 to September 2017) has been compiled to give a comprehensive overview of the annual condition of the estuary.
In 2016/17 Wilson Inlet monitoring data show a healthy and productive estuary. However, it does show impacts of excess nutrients flowing in from the catchment at particular times of year.
Wilson Inlet’s water quality is strongly influenced by the freshwater inflows from rivers and by the sandbar separating the inlet from the ocean. The bar is usually dredged in winter to stop low-lying lands flooding after winter rainfall. It naturally closes due to waves depositing sand to reform the bar in late summer or early autumn each year. In 2017 the bar was closed for 6 months, from 23 February to 24 August.
2017 was an average rainfall year, however most of the rain fell in late winter and early spring and therefore the sandbar was opened in late winter.
When the bar was closed the water was brackish, well-mixed, oxygenated from the surface to the bottom with low concentrations of nutrients and algae – all signs of healthy conditions. In contrast, when the bar was open, salinities ranged from fresh to marine, nutrient concentrations were higher due to the nutrient-rich inflows from the catchment and consequently there was more algal growth.
When the bar was open, marine water entered the estuary and formed a heavier, salty layer underneath the brackish estuary water (stratification). When stratified, nutrients were released from sediments due to chemical changes caused by low oxygen conditions (hypoxia) in the bottom layer.
Overall microalgae densities were low, however, three periods of higher algal densities were observed; two of them during the bar-open period in spring 2016 and 2017 and one in autumn 2017. There were no reported fish kills or harmful algal blooms.
For the current monitoring period there were no nuisance algal blooms in Wilson Inlet. There were four occasions throughout the monitoring period when levels of potentially harmful algae exceeded safe levels for shellfish consumption.
Approximately 35 per cent of Wilson Inlet has a healthy seagrass habitat mainly consisting of the species Ruppia megacarpa. Ruppia plays an important role in absorbing nutrients from the water column and the sediment. With the current nutrient run-off into the estuary, if Ruppia was absent it would likely be replaced by less-desirable macroalgae or microalgal blooms.
Despite some high nutrient inputs, the estuary is resilient for several reasons – the sediments have good phosphorus-binding capacity, the abundant seagrass (Ruppia) absorbs nutrients, and strong prevailing wind regularly mixes the water, breaking down the stratification.
Nutrients entering the Inlet from the catchment and reduced winter flows due to climate change remain the highest risks to Wilson Inlet. REI continues to support and focus on efforts that address nutrient inputs from agricultural and urban sources.