Brine discharge that is produced by seawater reverse osmosis desalination into seagrass meadows, elicited public concern in Western Australia (WA) when the second large desalination plant was constructed at Binningup in 2011. This was due to the presence of a seagrass meadow located near the area where the outfall was to be situated. Seagrass meadows are highly productive communities, providing valuable ecological and socio-economic functions and services to estuarine and marine coastal ecosystems. These meadows support a diverse community of invertebrates, including bivalves and finfish. Notably, almost one third of the world’s seagrass species are found in southwestern Australia.
There is evidence to suggest that seagrass meadows and the communities they support may be vulnerable to changes in salinity regimes induced by the hypersaline brine discharge from desalination plants. Specific knowledge, however, of the tolerance of Australian marine species to rapid alterations in salinity and temperature is limited and any potential synergistic stress response that may occur due to hypersalinity is also unknown. Currently there are no environmental guidelines based on species tolerances to brine discharge for nearshore coastal habitats.
Conduct a survey of the benthic communities located in Geographe Bay, near the site of the Binningup Southern Seawater Desalination Plant outfall. Investigate the tolerances of key marine species to brine discharge, identify biomarkers and develop risk assessment tools.
Two surveys were conducted by divers between March and April in 2012 and 2013. Seagrass meadows, invertebrate fauna and reef pavement habitats were characterised at nine sites along a 70km stretch of coastline, including three sites at the outfall area of the desalination plant. There were some marked differences between the surveys, such as higher faunal diversity and an abundance of pavement relative to sand in 2012, indicating a potential loss of stability and shelter in 2013. Seagrass meadows identified near the Binningup outfall, while present but not extensive in 2012, were no longer present in 2013. Additionally, there was a notable absentee from the sampling site closest to the outfall in 2013, the purple sea urchin Heliocidaris erythrogramma, which is known to be highly sensitive to salinity changes.
Laboratory studies of the response of the native seagrass Posidonia australis and the bivalve Mytilus edulis to salinity changes were conducted. The seagrass demonstrated a very high tolerance to salinity, even when the concentration was increased rapidly and without long periods of acclimation. Leaf growth was not significantly influenced by elevated salinity, however a number of other metabolic stress responses were observed; root growth was reduced after six weeks at 46 psu and 54 psu and photosynthesis was significantly reduced over a two to four week interval after exposure to 54 psu. Seedlings exhibited high sensitivity to brine exposure in concentrations above 42 psu but recovery occurred seven days after exposure ceased.
Similarly, M. edulis, displayed reduced metabolic activity at concentrations of 45 psu and above and food uptake was reduced when exposed to concentrations greater than 50 psu. A reduction in gaping activity was observed at and over 50 psu, indicating the mussels were closed for extended periods risking elevated carbon dioxide build up in the tissues. Mortality of the mussels occurred at higher concentrations; 33% mortality at 45 psu, and 58% mortality occurring at 54 psu.
Biomarkers that are recommended for examining environmental stress in seagrass at the organism and cellular levels include:
- blackened patches on the leaves or ‘salt scorch necrosis’,
- reduced leaf growth,
- lower shoot densities,
- accumulation of salts,
- elevated sugar content, and
- elevated amino acids, particularly proline and potentially tyrosine.
Biomarkers that are recommended for examining environmental stress in bivalves include:
- thinning of the digestive tubule wall, and
- abnormal to oxygen consumption.
Subsequent annual surveys could be achieved using video footage of the growth on the diffusers and immediate area.
Total Value: $1,774,352 (cash and in-kind contributions)
Principal Investigator: Dr Julie Mondon
Title: Assessing and mitigating environmental impacts of seawater reverse osmosis outfalls on key benthic marine organisms
Length: 46 months
Personnel: 5 collaborators contributing 0.5 FTE
- 2016. Assessing and mitigating environmental impacts of SWRO outfalls on key benthic marine organisms. Technical Report.