Informed regulation of the discharge of chemical pollutants into the oceans requires knowledge of the fates and effects of such substances in the regions of concern. In order to provide a capability for realistic controlled experiments, microcosms of a plankton-based coastal marine ecosystem with a benthic component have been set up in 12 tanks outdoors, each with a 5-m deep water column, 13 tons of water and one ton of sediment. The tanks are mixed to achieve proper turbulence, and illuminated with natural sunlight. The behavior of these systems was compared to that of adjacent Narragansett Bay, the source of water and sediments. Experiments have been carried out with hydrocarbons, various tracers, and the scaling of physical parameters. Results from operations through two annual cycles demonstrate that in both chemical and biological composition and chemical and biological processes and annual cycling the systems behaved in ways remarkably similar to comparable regions in Narragansett Bay. Concentrations and fluxes of nutrients and trace metals followed closely those of adjacent Narragansett Bay. Phytoplankton, zooplankton and benthic animals were similar to the Bay in species composition. Biomass estimates were low but generally within bay ranges. The similarity in chemical cycles between microcosms and the coastal ecosystem suggests that many pollutant substances should behave in the microcosms as they do in the field. Examples of some experiments with hydrocarbons and trace metals are presented.