Studies on the causes of mortality of the estuarine bivalve Macoma balthica under conditions of (near) anoxia

Survival of the bivalve Macoma balthica in (near) anoxic seawater was studied in a static system and a flow-through system and compared with emersed exposure to air and N sub(2). In the static system, a decrease in pH and exponential accumulation of sulphide in the incubation medium were observed, indicating excessive growth of (sulphate-reducing) bacteria. These changes in the chemical environment were prevented by the use of a flow-through system. However, this treatment hardly affected survival time. Median mortality times were 8.3 and 9.0 days for the static and flow-through incubation, respectively. Addition of the antibiotic chloramphenicol strongly increased survival time in both systems with corresponding values of 17.9 and 23.0 days. A similar value was obtained for survival in air (LT sub(50) = 21.7 days). In a second experiment (1 year later), we obtained much lower values for anoxic survival in a static system, although laboratory conditions, season and temperature were similar. The pH values were adjusted to 6.5, 7.2 and 8.2 by buffering the media (25 mM Tris-HCl), and the corresponding LT sub(50) values were 5.5, 5.7 and 4.7 days, respectively. In the presence of chloramphenicol the values were 10.8, 10.9 and 9.5 days, respectively. These values show that a slightly acidic medium increased survival time. Exposure to an atmosphere of N sub(2) resulted in a survival time close to that in anoxic seawater without chloramphenicol (LT sub(50) = 6.4 days). Overall the results indicate that proliferation of anaerobic bacteria associated with the bivalves was the main cause of death. Since chloramphenicol also displayed a strong positive effect in the flow-through system, which prevented the accumulation of released waste products and a decrease of pH, bacterial damage must have been by injury of the tissues of the clams and not by the release of noxious compounds to the medium. Bacterial outbreaks are a part of every anoxic event (eutrophication), and therefore, in their habitats, direct bacterial infection may also be the cause of clam mortality. It is concluded that laboratory studies on anoxic tolerance, or impact of sulphide, may produce artefacts when no precautions are taken to suppress bacterial proliferation.