INFLUENCE OF CHIRONOMUS-PLUMOSUS LARVAE ON AMMONIUM FLUX AND DENITRIFICATION (MEASURED BY THE ACETYLENE BLOCKAGE-TECHNIQUE AND THE ISOTOPE PAIRING-TECHNIQUE) IN EUTROPHIC LAKE SEDIMENT

Oxygen uptake, ammonium flux and denitrification were determined in se diment from a eutrophic lake in southern Sweden. Part of the sediment was bioturbated by incubation in a laboratory mesocosm by incubation c ontaining 2000 tube-dwelling larvae of Chironomus plumosus L. m(-2). O xygen consumption was increased 2-fold in the bioturbated compared wit h the nonbioturbated sediment, some 20% of the increase could be expla ined by chironomid respiration. There was a net release of ammonium fr om the bioturbated sediment to the overlying water. Only 11-45% of thi s 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 t he sediment. Under the aerobic conditions under which the experiments were conducted, the bioturbated eutrophic sediment acted as a more pro nounced sink for inorganic nitrogen compared with the non-bioturbated sediment.