Role of acute metal toxicity in structuring seagrass fauna near a lead smelter

Field acute toxicity tests over 21 d periods demonstrate that 2 species of vagile epibenthic seagrass fauna (the fish Neoodax sp. and the crustacean Cymodoce longicaudata) are acutely affected by Cd, Cu, Pb or Zn in the effluent from a lead smelter. By comparison with the laboratory LC₅₀ data in the literature for other marine fishes and crustaceans it appears that Zn is the metal mainly responsible for these effects. Although C. longicaudata is acutely affected by the metals, its abundance is normal in some parts of the highly contaminated area. A third species (the fish Helotes sexlineatus), whilst not acutely affected by the metals in these experiments, was shown by a previous survey to be reduced in abundance in the highly contaminated area. Since the distribution of these latter 2 common species cannot be explained by the results of acute toxicity tests it is unlikely that acute toxicity of these metals is the major determinant of community structure in the highly contaminated area even though the concentrations of Zn in the smelter effluent appear to be near laboratory-derived LC₅₀ concentrations for other marine species. Outside the highly contaminated area acute toxicity of these metals is unlikely to have any effect on community structure. It is concluded that acute toxicity plays only a minor role in structuring the seagrass faunal communities of the contaminated area and that acute toxicity tests using dissolved metals are unable to predict ecological effects. Therefore, such tests are probably unsuitable as bases for determining water quality criteria for protection of seagrass fauna.