Sensitivity of the seagrass Cymodocea nodosa to hypersaline conditions: A microcosm approach

Marine water desalination is an emergent strategy to satisfy water demand in coastal regions. The brine originated as a by-product (60–90 psu) can have a significant impact on important marine ecosystems, yet the tolerance thresholds of most key species remain unknown. Therefore, with the aim of filling this gap and providing useful information to mitigate such impacts, we have experimentally assessed the response to hypersaline conditions of the seagrass Cymodocea nodosa. We exposed plants in microcosms to 4 treatments: control (37.2 psu), 44 psu, 54 psu and 62 psu, with three replicate aquaria per treatment for 17 days, and we measured plant response and status. We found that the seagrass shoots endured 44 psu without any apparent damage, but vitality decreased in 54 and 62 psu treatments. Plants at both 54 and 62 psu displayed reduced leaf growth and higher incidence of leaf necrosis, relative to control conditions. In addition, in plants at 62 psu, photosynthetic performances (measured as leaf quantum efficiency) dropped shortly after salt addition, and by the end of the experiment they also suffered substantial mortality. These findings should be viewed with caution, due to the relatively short time-span of the experiment, as in the long-term plants may be less tolerant than reported here. However, our results highlight the need for careful management of brine disposal to prevent deterioration of seagrass habitats; salinity should never exceed 44 psu inside the meadows, to prevent deterioration of C. nodosa habitats.