Influence of Chironomus plumosus larvae on ammonium flux and denitrification (measured by the acetylene blockage- and the isotope pairing-technique) in eutrophic lake sediment

Oxygen uptake, ammonium flux and denitrification were determined in sediment from a eutrophic lake in southern Sweden. Part of the sediment was bioturbated by incubation in a laboratory mesocosm by incubation containing 2000 tube-dwelling larvae of Chironomus plumosus L. m(-2). Oxygen consumption was increased 2-fold in the bioturbated compared with the nonbioturbated sediment, some 20% of the increase could be explained by chironomid respiration. There was a net release of ammonium from the bioturbated sediment to the overlying water. Only 11-45% of this could be explained on the basis of larval excretion. With increasing nitrate concentration, denitrification of the nitrate coming from the water (d(w)) increased to a greater extent in the bioturbated than in the non-bioturbated sediment, whereas denitrification of the nitrate from the coupled nitrification-denitrification (d(n)) was unaffected. The acetylene blockage technique underestimated denitrification by 63-88% compared with the nitrogen isotope pairing technique. The results indicate that bioturbation by tube-dwelling chironomid larvae can have a major impact on the nitrogen turnover in lake sediment, mobilising the ammonium to the water and stimulating denitrification by reducing the diffusive barrier blocking nitrate from reaching anoxic zones in the sediment. Under the aerobic conditions under which the experiments were conducted, the bioturbated eutrophic sediment acted as a more pronounced sink for inorganic nitrogen compared with the non-bioturbated sediment.