Macroalgae (often referred to as seaweed) is a group of aquatic plants that occur in brown, red and green forms. The different colours of macroalgae are due to the different pigments they use to convert sunlight into energy through photosynthesis. Macroalgae grows fully submerged in most estuaries in New South Wales and is typically attached to hard substrates like rocks and firm sediment.
Macroalgae comes and goes from estuaries. It responds to nutrient availability, water clarity, water temperature, salinity and water level. Sometimes macroalgae completely take over shallow areas of estuaries for weeks or months at a time, and then disappear following a change like the estuary entrance opening to the ocean or a big rainfall event delivering lots of freshwater or sediment to an estuary.
These plants have basic structures and derive nutrients from surrounding water. They thrive in estuaries that receive abundant nutrients from the surrounding catchment area; therefore, macroalgae can be excellent indicators of an estuary that is nutrient enriched.
Macroalgae is important in estuarine ecology because it can provide habitat and food for juvenile fish and other species, especially when there is no other complex habitat available in that estuary. They can also have direct and indirect impacts on estuaries. For example:
- they can out-compete seagrass for available light which can lead to the loss and decline of seagrass
- they can remain in estuaries once they die, sinking to the bottom and decomposing which releases stored nutrients and depletes the oxygen in the water column, potentially leading to fish kills
- decomposing macroalgae washes up on shorelines and can cause nuisance odours and sights for local residents.
Macroalgae habitats are easily damaged. Loss of macroalgal habitat in estuaries can impact fish diversity and abundance. Physical damage that can directly remove or destroy macroalgae includes:
- boat moorings
- propeller scars
- some fishing activities.
Macroalgae can also be indirectly impacted by sedimentation, coastal development and aquaculture. Sea level rise, the intensity and frequency of large storm events, and changes in water quality in response to climate change have the potential to impact macroalgae.
For more information on seaweed and macroalgae in our estuaries, see the Macroalgae fact sheet on the NSW Department of Primary Industries website.