EFFECTS OF THE SEA-URCHIN ECHINOCARDIUM-CORDATUM ON BACTERIAL PRODUCTION AND CARBON FLOW IN EXPERIMENTAL BENTHIC SYSTEMS UNDER INCREASING ORGANIC LOADING

Effects of the sea urchin Echinocardium cordatum on benthic bacterial production were studied in marine sediment boxcosms, to which differen t amounts of organic matter (30, 60 and 90 g C m(-2)) were added. Bact erial production was estimated from incorporation rates of H-3-labelle d leucine. The distribution of organic carbon in the boxcosm sediments was monitored. Measurements were done over a period of 27 days after the introduction of organic matter. Non-enriched boxcosms were used as controls. In macrobenthos-free boxcosms, the bacterial response to or ganic-matter additions mainly took place in the organic-matter layer o n the sediment surface and in the upper 3 mm of the sediment. in the p resence of E. cordatum, more organic matter was transported into the s ediment, and the bacterial production rates in deeper sediment layers were higher. Increasing the organic loading did not further enhance th ese rates. It is concluded that E. cordatum stimulates bacterial produ ction in deeper sediment layers by increasing the transport of organic matter into the sediment and perhaps also by promoting its degradatio n. At an increasing load with Organic matter, the relative importance of the effects of E. cordatum decreases.