Tissue nutrient contents of the red alga Ellisolandia elongata as a tool to assess the ecological status in NW Mediterranean coastal waters

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Eutrophication is a major threat to aquatic ecosystems because excessive nutrient enrichment may result in biodiversity loss, profound functional changes and reduction of ecosystem services. Unless several water quality indexes have been developed following the implementation of the EU Water Framework Directive (WFD), there is still room for improvement. In particular, the stress-tolerant articulated red coralline alga Ellisolandia elongata (=Corallina elongata) is dominant in a wide range of environmental conditions, hampering a proper evaluation of the water quality of some of these places. Here we have checked whether E. elongata can be used as a eutrophication bioindicator by measuring the tissue phosphorus and nitrogen contents. Our results detect an increase in tissue nutrient contents in E. elongata along a gradient of nutrient enrichment and, thus, it can be used as bioindicator of water quality status. Their measurement will allow a more accurate water quality assessment in places dominated by E. elongata where WFD compliant indexes based whether on species/assemblages presence and/or abundance do not provide enough resolution.