Condition of Oyster Harbour

Download 2016/17 technical report
 

The Regional Estuaries Initiative supported the ongoing water quality, seagrass and macroalgae monitoring of Oyster Harbour since 2016 with monitoring continuing through Healthy Estuaries WA.

Below is the latest data about the condition of Oyster Harbour.

Macroalgae in Oyster Harbour

Macroalgae (or seaweeds) are a vital and natural part of many estuarine and marine ecosystems. They provide important habitat and food for many creatures, and perform essential ecological functions like oxygenating water and absorbing nutrients. However, an overabundance of macroalgae can become a problem, and often indicates too many nutrients in the water. Macroalgae can therefore be indicators of water quality. Assessing macroalgae in estuaries forms part of our integrated understanding of ecosystem health.

There are many different macroalgae species (which should not be confused with seagrasses). These are generally grouped based on their appearance or pigments in their tissue. The groups are red algae (Rhodophyta), brown algae (Ochrophyta) and green algae (Chlorophyta).

Excessive macroalgae growth was previously an issue in Oyster Harbour. A historical survey from the late 1980s showed excessive growth of Cladophora species – a green macroalgae. This species dominated the estuary, making up more than 90% of the algae biomass. Other algae observed during this period included green algae such as Chaetomorpha and Ulva species, as well as comparatively small amounts of unidentified red and brown algae. This overabundance resulted in macroalgae harvesting in the 90s, which saw more than 30,000 tonnes of algae removed over a period of seven years.

The macroalgae was harvested to reduce some of its negative effects, such as oxygen depletion and foul odours, as well as increase the likelihood of seagrass recovery. Another option was to manage the source of the problem, rather than dealing with the consequences, by reducing nutrient inputs to the harbour. This became the preferred plan, and eventually replaced harvesting. In the decades since, there has been a remarkable recovery of seagrasses in the harbour and a reduction in nuisance algae growth.

As part of the Department of Water and Environmental Regulation’s seagrass monitoring, a snapshot study occurs during the summer months in the Oyster Harbour in survey years. In the summer of 2019, two types of brown algae were recorded: Neptune’s necklace (Hormosira banksii) and Sirophysalis species. At the time of the survey, macroalgae were observed on 5% of occasions with very low cover (approximately 5%). Macroalgae were confined mostly to the eastern parts of the basin, as well as near areas used for aquaculture and close to the river mouths. Aquaculture leases were not included in the survey.

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Water quality data 2016/17

Since October 2016, the Department of Water and Environmental Regulation 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. Healthy Estuaries WA continues to support and focus on efforts that address nutrient inputs from agricultural and urban sources.

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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