Ecotoxocicological assessment of antifouling biocides and non-biocidal antifouling paints

6. Conclusion

The following conclusions may be drawn on the basis of the present study:

Bioavailable copper is very toxic to aquatic organisms. The potential toxic effect of copper on the aquatic environment is reduced by sorption to organic matter and sediments which causes the actual bioavailability of copper to be low. Disturbances of the sediment and the consequent change in oxygen conditions may, however, remobilize sequestrated copper and such changes may probably cause effects on sensitive organisms in the vicinity of harbours and dumping sites.

DCOI is rapidly transformed into metabolites in seawater in which half-lives of between 11 and 14 hours have been found. The transformation of DCOI is considerably quicker in aquatic sediment as half-lives of less than 1 hour have been demonstrated. DCOI is very toxic to aquatic organisms as the lowest effect concentrations (EC/LC50) are lower than 10 µg/L. The aquatic toxicity of the stable metabolite, N-(n-octyl) malomanic acid, is several orders of magnitude lower as the lowest effect concentrations (LC50) are estimated to be between 90 and 160 mg/L.

On the basis of realistic worst-case scenarios, risk quotients (PEC/PNEC) for DCOI have been calculated at 8.7 for the pleasure craft harbour and 0.1 for the navigation route. Based on the calculation prerequisites, it is estimated that, within the pleasure craft harbour, there is a risk of chronic ecotoxic effects as DCOI is assumed to be applied constantly by the leaching from bottom paints. The risk quotient for DCOI out of the pleasure craft harbour is less than 1, and here the risk of ecotoxic effects is considered to be low. The calculated exposure concentrations (PEC) are based on realistically conservative assumptions, which means that, in practice, the calculated PEC values are seldom exceeded. When the pleasure craft are taken out of the water at the end of the sailing season, DCOI will probably be rapidly eliminated as DCOI is either transformed in the water phase or sorbs to the sediment, in which it is transformed with a very short half-life.

Zinc pyrithione is very rapidly transformed by photolysis and biodegradation. Zinc pyrithione is very toxic to aquatic organisms as the lowest effect concentrations (EC/LC50) are lower than 10 µg/L. The toxicity of the stable metabolites, omadine sulfonic acid and pyridine sulfonic acid, is several orders of magnitude lower as the lowest effect concentrations (LC50) for these compounds are 36 and 29 mg/L, respectively.

By using the same realistic worst-case scenarios as for DCOI, the risk quotients (PEC/PNEC) for zinc pyrithione have been calculated to be 5.6-17 for the pleasure craft harbour and 0.05-0.22 for the navigation route. The lowest risk quotients are based on PEC values in which transformation of zinc pyrithione by photolysis is included in the calculations while the highest risk quotients are based on calculations in which photolysis is totally ignored. Based on the calculation prerequisites, it is estimated that, within the pleasure craft harbour, there is a risk of chronic ecotoxic effects as zinc pyrithione is assumed to be applied constantly by the leaching from bottom paints. The risk quotient for zinc pyrithione out of the pleasure craft harbour is less than 1, and here the risk of ecotoxic effects is considered to be low. The risk quotient out of the pleasure craft harbour is probably closest to 0.05, in which photolysis has been included in the calculation of PEC as permanent shadow effects are not expected on a normal navigation route. As described for DCOI, the realistically conservative assumptions mean that, in practice, the calculated PEC values are seldom exceeded. When the pleasure craft are taken out of the water at the end of the sailing season, zinc pyrithione will probably be rapidly eliminated as a result of its short half-life in water and sediment.

Water samples from the leaching tests with High Protect 35651 caused no inhibition of the growth of S. costatum and chronic effects on A. tonsa were observed only in undiluted leachates (No Observed Effect Concentration, NOEC = 100 mL/L). Water samples from the leaching test with the experimental 86330 paint showed toxicity towards S. costatum and in acute and chronic tests with A. tonsa (NOEC, acute <100 mL/L; NOEC, chronic <10 mL/L). However, some factors seem to indicate that variations in the production or painting process may influence the leaching of substances from this type of paint. These problems should be further examined before a final assessment is made of the environmental properties of this paint. For both non-biocidal paints, water samples from leaching tests have significantly less effect than water samples from similar tests with the commercial paint, Hempel's Antifouling Nautic 76800. Leachates from High Protect 35651 and the experimental 86330 paint caused NOEC values for A. tonsa that were at least 1,000 and 100 times, respectively, higher than the corresponding NOEC values for leachate from the organotin-based paint.