A RECENT report shows seagrass populations in the Oyster Harbour catchment have increased as a likely result of improved water quality after suffering severe deterioration.
A significant deterioration in the seagrass meadows was recorded in the 1980’s after the harbour had become increasingly eutrophic and the water quality deteriorated, resulting in increased algal bloom and the ecosystem significantly collapsed.
Supported by the State Government’s Royalties for Regions Program, through the Regional Estuaries Initiative, data was collected by the Department of Water and Environmental Regulation in March 2019 at 200 randomly selected sites, using underwater cameras and viewing cones to observe and assess seagrass coverage, species and height.
Areas excluded from the survey were those with aquaculture lease and the deepest parts of the estuary where seagrass unlikely grows.
Spatial software ArcGIS was then used to create maps of the seagrass extent and coverage based on field observations.
So what is so important about seagrass?
Seagrass meadows provide habitats and food for birds, fish and crustaceans while also contributing to good water and sediment quality by consuming nutrients and oxygenating the water.
Seagrass is also a key ally in the fight against climate change as it stabilises sediments, protects shorelines from erosion and store carbon whilst release oxygen.
Studies in the 1990’s and 2006 showed seagrass recovery in ‘patchy’ areas and calculated the total coverage at 540 hectares, but the most recent 2019 study found that total coverage had increased to 663 ha and the deepest observation of seagrass was at 5.5m.
Effects of human interference
Department of Water and Environmental Regulation (DoWER) responded to the Weekender stating the seagrass loss in Oyster Harbour was caused by build-up of macroalgae and epiphytes from catchment clearing and agriculture land use through the twentieth century.
This was the leading cause of poor water quality as the run-off delivered both sediment and fertiliser-derived nutrients into the harbour.
The report also stated higher rainfall have also eroded nutrient-rich, fine, silty soils from the catchment which caused flood waters to have high turbidity.
“These nutrients fuelled the prolific growth of macroalgae and epiphytes, that impacted seagrass and wider ecosystem health,” they said.
“When macroalgae and epiphytes are over abundant, they can shade and/or smother seagrass, limiting the amount of light available for the seagrass to convert into energy for growth- through photosynthesis.
“Catchment clearing and subsequent agricultural land use through the twentieth century was the leading cause of poor water quality – with run-off delivering both sediment and fertiliser-derived nutrients to the harbour.”
While the seagrass population is recovering in Oyster Harbour, nutrients from agricultural land use, urban and industrial land-uses, such as septic, garden fertiliser and stormwater outflows, continues to be the biggest threat to the health of the harbour.
Therefore management of the water quality requires managing of the multiple sources of nutrient input through the catchment.
Chang in farming practises
Thanks to early intervention in the 1990’s by Albany locals and a change in farming practises, a recovery and a restoration program led by the Oyster Harbour Catchment Group (OHCG) was put in place.
Oyster Harbour Catchment Group worked for more than 30 years implementing a catchment project and a seagrass regrowth program, collaborating alongside Great Southern farmers to reduce their fertiliser run-off into creeks and estuaries.
OHCG Communications Officer Sayah Drummond said working with farmers has been a big collaboration to clean up the waterways.
“The best way we found to naturally filter all the chemicals out of the system before it gets down to the estuary is to fence off those creek lines and rivers and re-vegetate it and that acts as a natural bio filter for flow off from paddocks,” Ms Drummond said.
“…fertiliser floods into the system and algae love it, then algal blooms block off all the sunlight getting to seagrass and then it kills seagrass and that in turns kills all the fish.”
The future of Oyster Harbour
The future of the catchment is uncertain in regard to climate change, but the Catchment Group are ready to do what they can to keep the ecosystem alive.
“…all we can do is put it back the best we can so it can do its ecosystem job and that way the whole harbour will be a bit more protected in the future, and being able to monitor it means as soon as it starts declining again we can get back onto it and figure out what’s doing it,” Ms Drummond said.
“These stories of extremely slow recovery after loss of seagrass, highlight how important it is to protect our seagrasses from becoming lost in the first place,” DoWER said.
Studies continue to be carried out in Oyster Harbour, with results of another assessment carried out in January 2021 expected to be released late 2021.
Read more about Oyster Harbour Catchment Group here https://www.ohcg.org.au/
To read the full report by the Department of Water and Environmental Regulation, go to this website https://www.wa.gov.au/government/publications/seagrasses-four-estuaries-western-australias-south-west-2017-20-report-no-86?utm_source=hootsuite&utm_medium=Twitter&utm_term=&utm_content=&utm_campaign=launch