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A healthy, biodiverse estuary depends on low nutrient inputs flowing in from the catchment.
Vegetation near streams, drains and rivers acts as a natural filter removing nitrogen and phosphorous from the water. Low nutrient concentrations entering the estuary limits the amount of algae growth that can occur.
With low algae growth and low amounts of organic matter in the waterway, water is clear, allowing seagrasses to adsorb the sunlight, providing oxygen and stabilising sediments. Seagrasses also provide important habitats to aquatic life and provide food for waterbirds.
A eutrophic system has large amounts of nutrients entering the waterway through fertiliser run-off, animal effluent run-off and the release of nutrients from sediment under certain conditions.
The large amounts of nutrients cause excessive growth of algae, which can be microscopic (phytoplankton) or up to several meters long and visible with the naked eye (macroalgae).
Eutrophic conditions become a threat to the health of the estuary and aquatic life when they persist for long periods of time.
Bacteria that breaks down algae consumes dissolved oxygen out of the water. The lack of dissolved oxygen can leave aquatic life with insufficient oxygen to survive and can cause chemical changes resulting in the release of more nutrients from the sediment.
This can cause a cycle which is difficult to rectify; excess nutrients cause an algal bloom, algae breaks down consuming dissolved oxygen and lack of oxygen causes the release of excess nutrients.
Algae and organic matter also block sunlight, stopping it from reaching seagrass. Seagrass helps stabilise sediment and oxygenate the water, and the loss of seagrass can contribute the cycle of eutrophication.