Condition of Oyster Harbour
2016/17

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Summary data
 

The Regional Estuaries Initiative is dedicated to protecting and improving the health of six estuaries in the state’s south west, including Oyster Harbour. Scientific understanding is key to effective management.

Since October 2016, the Regional Estuaries Initiative has carried out monthly monitoring of estuary water quality at seven sites and algal activity at three sites. This data has been compiled to give a comprehensive overview of the annual condition of the estuary.

In 2016/17, Oyster Harbour was a relatively healthy system that is still sensitive to nutrient inputs.

Nutrient concentrations were within healthy guidelines for the majority of the year with an occasional increase after winter river flow washed more nutrients in from the catchment.

The relatively low nutrient levels coincided with low algae activity for the majority of the year with an increase in algae activity in spring and summer caused by hot temperatures and the increase of nutrients from the catchment. This increase in activity in spring and summer is a typical pattern seen in healthy ecosystems.

Algal blooms can be a sign of an unhealthy, unbalanced ecosystem and are characterised as either nuisance blooms or harmful blooms. Nuisance blooms discolour the water and may leave residue on the skin of swimmers. Harmful algae can produce toxins and when present are a threat to aquatic and human health.

For the current monitoring period there were no nuisance algal blooms in Oyster Harbour. There were four occasions throughout the monitoring period when levels of potentially harmful algae was found in quantities that exceed safe levels for shellfish consumption.

In September 2017 there was a 150 kilolitre sewerage spill from the Albany wastewater treatment facility into Yakamia Creek. An elevation of nutrients and algae concentration was not picked up by routine monthly monitoring following the spill. Bacteriological sampling at estuary sites has been included in in the next period of monitoring to provide baseline information on bacteria from stock and wastewater.

Approximately 35 per cent of Oyster Harbour has a healthy recovering seagrass habitat due to improvements in water quality and direct transplantation efforts since substantial seagrass loss in the 1990s.

Nutrients entering the Harbour from the catchment remain the highest risk to the social and environmental values of Oyster Harbour. REI continues to support and focus on efforts that address nutrient inputs from agricultural and urban sources.

Summary data

North = Northern sites (AOH-4, AOH-5, AOH-6 and KRLB)
South = Southern sites (AOH-1, AOH-2 and AOH-12)

Salinity

The salinity of Oyster Harbour is close to marine due to the permanent opening to King George Sound.

There is a small difference in salinity particularly at surface water in the north. This is caused by the freshwater inflow after rain from the Kalgan and King rivers.

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Dissolved oxygen

Dissolved oxygen is the amount of oxygen dissolved in the water through absorption from the atmosphere and/or as a result of photosynthesis by seagrass or algae.

Dissolved oxygen concentrations range from 7-8 mgL-1 which indicates excellent water quality throughout the harbour.

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Water clarity

Water clarity is a measure of how far light penetrates into the water relative to the depth of the bottom.

The water clarity of the north sites at Oyster Habour is good for shallow water, which naturally has more suspended organic matter.

At the southern sites light penetrates approximately 4.5 meters which is excellent for estuaries.

*Water clarity is measured as Secchi depth

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Nitrogen

Nitrogen levels in Oyster Harbour were well below guideline values. The guideline values are an indication of the approximate nitrogen levels that may impact the health of an estuary.

There was a slight elevation in nitrogen for the northern sites at the surface. This is from nutrients being washed in from catchment land uses through the King and Kalgan rivers.

*Guideline is the ANZECC (2000) guideline value for south-west estuaries

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Phosphorus

Phosphorous levels in Oyster Harbour were below the guideline values. The guideline values are an indication of the approximate phosphorous levels that may impact the health of an estuary.

There was a slight elevation in phosphorous for the northern sites at the surface. This is from nutrients being washed in from catchment land uses through the King and Kalgan rivers.

*Guideline is the ANZECC (2000) guideline value for south-west estuaries

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Algae

Algae levels in Oyster Harbour were below the guideline values. The guideline values are an indication of the approximate algae levels that may impact the health of an estuary.

There was more algae at the northern site, likely caused by the increase in nutrients at the northern sites. The prevailing winds in the harbour may also push organic matter north concentrating algae.

*Guideline is the ANZECC (2000) guideline value for south-west estuaries

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