NITROGEN CYCLING IN LAKE-SEDIMENTS BIOTURBATED BY CHIRONOMUS-PLUMOSUSLARVAE, UNDER DIFFERENT DEGREES OF OXYGENATION

Sediment cores containing different densities of Chironomus plumosus, ranging from 0 to 12 000 ind. m(-2), were incubated in the laboratory, with 100 and 39% O-2 saturation in the overlying water. Rates of O-2 uptake, and fluxes of the various inorganic N species were measured af ter addition of (NO3-)-N-15 to the overlying water. The animals enhanc ed O-2 and NO3- uptake, due to irrigation. Denitrification of NO3- com ing from the overlying water (Dw) and dissimilatory NO3- reduction to NH4+ (DNRA) represented 20-30 and 4-10% of the NO3- uptake, respective ly. Only 20-40% of the measured NH4+ effluxes corresponded to DNRA, th e rest was probably due to animal excretion. Nitrite production, mostl y from dissimilatory NO3- reduction, was detected at both 39 and 100% oxygen saturation. Higher rates of NO2- production at the lower oxygen concentrations, were probably due to a thinner oxic layer, compared t o fully oxygenated waters. The presence of Chironomus plumosus increas ed nitrification rates, relative to non-inhabited microcosms. However, nitrification rates were low compared to Dw, probably due to low numb ers of nitrifiers in the sediment. At 39% oxygen saturation, rates of nitrification and denitrification of NO3- generated within the sedimen t were not measurable.