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International audienceIn this study we develop and evaluate a macroalgal bioassay tool for monitoring the spatial extent of dissolved wastes loaded from offshore fish farms into the marine coastal ecosystem. This tool is based on the analysis of the nitrogen stable isotope ratio (δN) in tissues of several benthic primary producers (macroalgae and epiphytes) incubated, by means of incubation devices, in the water column at increasing distances (from 0 m to 1000 m) from the fish cages. The bioassays were performed in three fish farms situated in different geographical locations (the Canary Islands, Murcia and Catalonia) and we test: the suitability of the different macroalgae species used in relation with their resistance to incubation and their sensitivity to fish-farm wastes and the most appropriate incubation depth (5m or ~20m) and incubation time (2, 4 or 6 days) to detect the spatial distribution of δN around fish farms. In general terms, the results showed a significant increment of δN values towards the fish cages with respect to the reference (initial) and control values for all the species of macroalgae tested except for the red algae from Canary Islands. The magnitude and shape of the reported spatial responses varied as a function of the experimental settings analyzed as well as a function of the nutrient regime characteristics of each coastal area. The spatial gradient was more consistent in the shallow part of the water column (5m depth), than in the deeper part (~20m) and was statistically significant after an incubation period of four days. These results confirm the effectiveness and reliability of the method proposed, enabling the spatial extent of nutrients derived from fish farms to be assessed in an effective and simple manner, suitable for use in monitoring programs around offshore fish farms

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