Impact of Macrofaunal Recolonization on Benthic Metabolism and Nutrient Fluxes in a Shallow Marine Sediment Previously Overgrown with Macroalgal Mats

The impact of bioturbation by the polychaeteNereis diversicoloron 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 mats. 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 CO2and 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 introducingN. diversicolorto laboratory microcosms, followed by measurements of O2, CO2and inorganic nitrogen fluxes across the sediment–water interface regularly over a 35-day period. Presence ofN. diversicolorincreased sediment O2uptake and CO2production 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 toN. 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 CO2and NH+4to 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.