Salinity tolerance of the Mediterranean seagrass Posidonia oceanica: recommendations to minimize the impact of brine discharges from desalination plants

Desalination of seawater has become an important and growing industry due to the present water shortage in some Mediterranean countries. This activity may result in some environmental impacts, mainly generated by the discharge into the sea of the brine produced, which can affect marine benthic communities. Posidonia oceanica (L.) Delile is an endemic seagrass of the Mediterranean Sea, where its meadows represent a relevant ecosystem susceptible to being affected by these desalination effluents. The present study was planned in order to investigate the effects of hypersaline discharges on P. oceanica and its main associated organisms, and to provide adequate knowledge to help managers in the taking of decisions related to hypersaline discharges potentially affecting Posidonia meadows, particularly in the Spanish Mediterranean.The research included three different approaches conducted by distinct research teams whose results indicate that P. oceanica is very sensitive to salinity increases. Laboratory and field experiments showed significant effects on seagrass structure and vitality at salinities of 39.1 and 38.4, respectively. Due to this high sensitivity of P. oceanica to salinity increases, we recommend to avoid design and construction of brine discharges in areas where these ecosystems or others that are potentially sensitive occur. However, when this option is not possible, we suggest not to exceed neither 38.5 psu of salinity in any point of the meadow for more than 25% of the observations (on an annual basis) nor 40 psu of salinity in any point of the meadow for more than 5% of those observations.