ALBERT

Description: The exchange of inorganic nutrients; ammonium, nitrate and reactive phosphate between burrows of the infaunal polychaete Nereis virens Sars and the overlying water was assessed using V-shaped sediment cores. Exchange was determined by monitoring ventilation current and nutrient concentration of in- and excurrent water. Ammonium supply appeared independent of overlying water concentrations, showing a constant release of 0.5 μmol·h−1 (for a 2-g individual + burrow system) at concentrations from 2 to 87 μM. Of this release ≈40% originated from worm excretion, and the rest from microbial mineralization. Nitrate and phosphate exchange appeared very sensitive to overlying water concentrations, having equilibrium (zero flux) at 10–15 and 3 μM, respectively. Below these concentrations nitrate showed a slight release (due to nitrification), whereas phosphate was released at a rate of 3.2 × 10−2 μmol·h−1 at 1 μM (mineralization and desorption). Above equilibrium they both were removed during water passage through worm burrows, reaching 0.4 μmol·h−1 for nitrate at 107 μM (nitrate reduction) and 3.7 × 10−2 μmol·h−1 for phosphate at 5.6 μM (adsorption processes). The burrow system apparently acted as a buffer for phosphate and, to some degree, nitrate in the overlying water. At the study site (Norsminde Fjord estuary) nereid burrows were estimated to increase the sediment-water interface 150%. About 17% of the water column was cycled through the sediment by Nereis each day. The worm + burrow system was estimated to release 95 μmol· m−2·h−1 ammonium to the overlying water, which was ≈76–90% of the total release of ammonium from the sediment (30–36% was worm excretion).