Different interrelationships among phytoplankton, bacterial and environmental variables in dumping and reference areas in the East Sea

Approximately several hundred million tons of waste, composed mostly of dredged materials and sewage sludge, are disposed annually in the world's ocean; millions of tons of waste are dumped yearly into the East Sea, Korea. To assess the impacts of such dumping on bacteria and phytoplankton in the pelagic ecosystem, and to understand the mechanisms behind any observed changes, environmental, bacterial, and phytoplankton variables in dumping and reference areas in the East Sea were measured 10 times from January 1996 to August 1998. The effects of waste on bacteria and phytoplankton were also tested in vitro by directly adding various types of waste to seawater. Physicochemical characteristics such as temperature, salinity, and concentrations of ammonium, nitrate, and phosphate were generally similar between dumping and reference areas; similar values were observed in bacterial biomass and chlorophyll a concentrations between the 2 areas. However, higher or lower bacterial production (BP) and lower primary production (PP) were frequently observed in dumping areas compared to reference sites. Microbial ecological relationships (e.g. sea surface temperature vs. BP/PP ratio, and nitrate concentration vs. PP) were significant at the reference sites but not in the dumping areas. Comparisons of values and relationships of the environmental, phytoplankton, and bacterial variables between dumping and reference sites suggested that lower BP and PP in dumping areas might be due to the disposal of inhibitory substances. This inference seemed to be supported by the waste-addition experiments, in which thymidine incorporation rates were inhibited at concentrations > 1 % of human and animal wastes and acid waste, and aminopeptidase activity was inhibited at concentrations > 1 % of human and animal wastes and > 5 % of acid waste. Thus, continual disposal of waste might impose inhibitory effects on activities of phytoplankton and bacteria in surface waters of the dumping area, but also stimulatory effects on bacteria in more diluted waters. Dumping of organic waste in the ocean has the potential to affect functions of the marine ecosystem at the bottom level of microbial food webs.