16 17In many regions, seawater desalination is a growing industry that has its impact on 18 benthic communities. This study analyses the effect on benthic communities of a 19 mitigation measure applied to a brine discharge, using polychaete assemblages as 20indicator. An eight-year study was conducted at San Pedro del Pinatar (SE Spain) 21 establishing a grid of 12 sites at a depth range of 29-38 m during autumn. Brine 22 discharge started in 2006 and produced a significant decrease in abundance, richness 23 and diversity of polychaete families at the location closest to the discharge, where 24 salinity reached 49. In 2010, a diffuser was deployed at the end of the pipeline in order 25 to increase the mixing, to reduce the impact on benthic communities. After 26 implementation of this mitigation measure, the salinity measured close to discharge was 27 less than 38.5 and a significant recovery in polychaete richness and diversity was 28 detected, to levels similar to those before the discharge. A less evident recovery in 29 abundance was also observed, probably due to different recovery rates of polychaete 30 families. Some families like Paraonidae and Magelonidae were more tolerant to this 31 impact. Others like Syllidae and Capitellidae recovered quickly, although still affected 32 by the discharge, while some families such as Sabellidae and Cirratulidae appeared to 33 recover more slowly. 34 35

Abstract. 9Desalination has become an important industry whose dense, high-salinity effluent has 10 an impact on marine communities. Without adequate dilution, brine remains on the 11 bottom increasing bottom salinity and affecting benthic communities. Amphipods 12 showed high sensitivity to increased salinity produced by desalination brine discharge. recovery, e.g. Harpinia pectinata. These differences may be dependent on the 20 organism living habits. 21

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