Responses of the Mediterranean seagrass Posidonia oceanica to in situ simulated salinity increase

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Abstract
The dominant Mediterranean seagrass Posidonia oceanica inhabits sublittoral environments with very stable salinity regimes; the species is considered highly sensitive to even moderate increases in salinity caused by hypersaline effluents (brine) from desalination plants. We analysed the effect of salinity increase on seagrass vitality and survival by means of an in situ mesocosm experiment. To this end, we used the brine (70–75 psu) produced by a pilot desalination plant, which was diluted with seawater to obtain brine solutions of 1 psu (high salinity increase, HS) and 2.5 psu (very high salinity increase, VS) over the mean natural salinity (37.5±0.16 psu) and then interspersed a set of experimental units in a nearby P. oceanica meadow. At the end of the experimental period (3 months), these treatments had produced differential effects on all seagrass descriptors, i.e., intense and significant in the VS experimental units and more modest (or even negligible) in the HS ones. Seagrass meadow declined through a significant decrease in shoot density in both HS (12.4±3.4%, mean±SE) and VS (18.5±3.05%, mean±SE) in comparison to control experimental units (P. oceanica is very close to the upper limit of its natural salinity range (ca. 38 psu).