The impact of bioturbation by the polychaete Nereis diversicolor on sediment metabolism and exchange of dissolved inorganic nitrogen across the sediment-water interface was studied in different sediment types from a shallow estuary, Kertinge Nor, Denmark. In addition, potential effects of a possible recolonization of benthic infauna to previously defaunated organic-rich sediment was evaluated. Three study sites were situated along a transect, representing a distinct gradient in sedimentary organic content and faunal composition within a relatively short distance. The inner location was rich in benthic infauna, and composed of sandy sediment of relatively low organic content (<1.4% LOI) originating mainly from benthic microalgae. The outer location was muddy and organic rich (5-14% LOI) due to frequent coverage and deposition of filamentous macroalgae. The benthic infauna was sparse here as a result of poor oxygen conditions below dense algal mars. Another intermediate location was situated in the transition zone between the others. Variations in sediment C:N ratios of the bulk organic pool and in C:N stoichiometry of organic matter mineralization, estimated from porewater profiles of CO2 and NH4+, indicated distinctly different sources of organic matter input to the sediments, and differences in the nutritional quality of detritus available for decomposers. A recolonization event was simulated by introducing N. diversicolor to laboratory microcosms, followed by measurements of O-2, CO2 and inorganic nitrogen fluxes across the sediment-water interface regularly over a 35-day period. presence of N. diversicolor increased sediment O-2 uptake and CO2 production by 2.1-2.6 and 1.5-1.9 times, respectively. Release of total inorganic nitrogen was stimulated 1.5-2.4 times due to N. diversicolor. Worm respiration accounted for only 32-46% of the increase, indicating enhanced microbial activity in the sediment; the stimulation being highest in the organic-rich sediment. Burrow construction and ventilatory activities following macrofaunal recolonization in the long-term defaunated organic-rich sediment resulted in a massive pulse release of accumulated porewater CO2 and NH4+, to the overlying water. The results indicate that macrofaunal recolonization will have a pronounced long-term influence on benthic metabolism and nutrient exchange, leading to a reduction of the large internal pool of dissolved and particulate nutrients in the sediment. (C) 1997 Academic Press Limited.
