Developing biomarkers to assess desalination brine effects on seagrass

A large desalination plant using reverse osmosis of seawater has been built on the Western Australian coast 200 km south of Perth, following predictions of a drying climate with a 30% reduction in rainfall. Hypersaline... [ view full abstract ]

A large desalination plant using reverse osmosis of seawater has been built on the Western Australian coast 200 km south of Perth, following predictions of a drying climate with a 30% reduction in rainfall. Hypersaline wastewater is discharged into the wave-exposed nearshore environment approx. 10 m deep where seagrasses, rock pavement and sand form a shifting mosaic. Impacts are difficult to define in situ in such environments. We therefore carried out a series of mesocosm experiments on seagrass with the aim of developing biomarkers appropriate to testing the effects of brine discharge on temperate seagrass occurring widely on the southern half of Australia. We examined the effect of raised salinity (46 and 54 psu) compared to seawater controls (37 psu) over 4 weeks on the seagrass species, Posidonia australis. Salinity had a significant inhibitory effect on maximum quantum yield of PSII (chlorophyll a fluorescence emissions on dark adapted leaves). Plants avoided dehydration by decreasing their leaf water potential (Ψw) and osmotic potential (Ψπ) while maintaining turgor pressure (Ψp) as salinity increased. Cellular ion concentrations changed with increasing salinity (increase in Na, Cl, decrease in K, Ca) whereas concentrations of some sugars (mainly sucrose) and some amino acids indicated the formation of compatible solutes in the osmoregulatory process. These experiments indicated that the seagrass tolerated brief exposures to very high salinity, corresponding to the highest salinity of brine released from the desalination plant outfall into the ocean. Biomarkers showing responses to raised salinity include photosynthesis, water relations, ion concentrations, sugar and amino acid concentrations.