Salinity fluctuation of the brine discharge affects growth and survival of the seagrass Cymodocea nodosa

•Desalination brine discharges can cause increments and fluctuations of salinity.•We detect some negative effects of a brine discharge on Cymodocea nodosa.•It was detected a leaf growth decrease and an increase in shoot mortality.•An increase in the % of horizontal respect to vertical shoots was also detected.•Not only salinity changes, but also slightly higher temperatures can cause these negative effects.

The increase of seawater desalination plants may affect seagrasses as a result of its hypersaline effluents. There are some studies on the salinity tolerance of seagrasses under controlled laboratory conditions, but few have been done in situ. To this end, Cymodocea nodosa shoots were placed during one month at four localities: two close to a brine discharge; and the other two not affected by the discharge, and this experiment was repeated four times. The results obtained showed a decrease in growth and an increased mortality at the localities affected by the brine discharge. An increase was detected in the percentage of horizontal shoots in respect to vertical shoots at the impacted localities. It is probably that not only the average salinity, but also the constant salinity fluctuations and slightly higher temperatures associated with the brine that may have caused physiological stress thus reducing C. nodosa growth and survival.