EFFECTS OF THE SEA-URCHIN ECHINOCARDIUM-CORDATUM ON OXYGEN-UPTAKE ANDSULFATE, REDUCTION IN EXPERIMENTAL BENTHIC SYSTEMS UNDER INCREASING ORGANIC LOADING

The effects of the presence of the sea urchin species Echinocardium co rdatum on benthic oxygen uptake rates, oxygen penetration depths and s ulfate reduction rates were studied in benthic boxcosms (volume ca. 25 l). Boxcosms were enriched by adding a single dose of organic materia l, collected during a natural spring bloom of Phaeocystis sp. to the s ediment surface. Three series of boxcosms without E. cordatum and thre e series with E. cordatum were enriched with respectively 20, 40 and 6 0 gC/m(2), to study the effect of increasing organic loading. Starved boxcosms (no organic matter supply) were used as a control. All experi ments showed a steep peak in oxygen uptake one or two days after organ ic matter supply. Thereafter, oxygen uptake rates remained relatively constant, at significantly higher rates in the presence off. cordatum. Increasing the organic load enhanced benthic oxygen uptake. However, under the highest loading, in the absence off. cordatum, oxygen uptake did not further increase. Oxygen penetration depths showed little var iation between different experiments. Sulfate reduction was positively correlated with organic load. Overall sulfate reduction rates were no t enhanced by E. cordatum. It was concluded that E. cordatum augments benthic oxygen uptake. Under increasing organic loading, E. cordatum p robably has a stimulating effect on both aerobic microbial respiration and reoxidation of reduced inorganic compounds.