In sheltered, eutrophicated estuaries, reduced nitrogen (NH x ), and pH levels in the water layer can be greatly enhanced. In laboratory experiments, we studied the interactive eVects of NH x , pH, and shoot density on the physiology and survival of eelgrass (Zostera marina). We tested long-term tolerance to NH x at pH 8 in a 5-week experiment. Short-term tolerance was tested for two shoot densities at both pH 8 and 9 in a 5-day experiment. At pH 8, eelgrass accumulated nitrogen as free amino acids when exposed to high loads of NH x , but showed no signs of necrosis. Low shoot density treatments became necrotic within days when exposed to NH x at pH 9. Increased NH 3 intrusion and carbon limitation seemed to be the cause of this, as intracellular NH x could no longer be assimilated.Remarkably, experiments with high shoot densities at pH 9 showed hardly any necrosis, as the plants seemed to be able to alleviate the toxic eVects of high NH x loads through joint NH x uptake. Our results suggest that NH x toxicity can be important in worldwide observed seagrass mass mortalities. We argue that the mitigating eVect of high seagrass biomass on NH x toxicity is a positive feedback mechanism, potentially leading to alternative stable states in Weld conditions.