A portable in situ continuous-flow apparatus for measuring metabolic activities of littoral ecosystems is described. Oxygen, temperature, light, and flow rate are continuously monitored by a microcomputer that redirects the water flow through the measurement chamber from acrylic jars containing the communities. The apparatus is especially designed for experimental purposes, since it is possible to alter the incoming waters by adding various toxicants. The application of the method is presented by an example showing the response of a filter-feeding community to cadmium.

The community showed statistically significant changes in respiration (oxygen) and excretion (nitrogen or phosphorus) as a result of treatment. Calculated ratios—such as the oxygen/nitrogen ratio, oxygen/phosphorus ratio, and perturbation index (PI), which is the relative change due to treatment, taking into account changes in the controls—were more sensitive measurements of sublethal stress than specific values of oxygen, nitrogen, or phosphorus. These indexes are independent of biomass, and PI normalizes synchronic changes in the environment due to light, temperature, and salinity variations, which affect both treated and control systems equally. The design of the system allows the communities to be placed back in the environment after being tested and also allows repeated measurements to be made of possible delayed effects or recovery from perturbations. In this paper, the authors introduce a new measurement of disturbance, the absolute disturbance index (ADI). ADI is the absolute distance between the metabolic activity of an undisturbed system and a disturbed point in a multidimensional space where each dimension represents the PI of a measured parameter.