Using a macroalgal δ15N bioassay to detect cruise ship waste water effluent inputs

► Green macroalgae exposed to nutrient solutions exhibited changes in tissue 15N signatures. ► Macroalgae exhibited no fractionation with NO3 and slight fractionation with NH4. ► Algae exposed to cruise ship waste water had increased tissue δ15N indicating a heavy N source. ► Field bioassays exhibited decreased δ15N indicating isotopically light riverine δ15N-NO3 was likely the dominant N source. ► Algal bioassays could not detect a δ15N cruise ship waste water signal in this system.