EFFECTS OF TUBIFEX-TUBIFEX (OLIGOCHAETA, TUBIFICIDAE) ON N-MINERALIZATION IN FRESH-WATER SEDIMENTS, MEASURED WITH N-15 ISOTOPES

Sediment cores containing different densities of Tubifex tubifex, rang ing from 0 to 70 000 ind, m(-2), were incubated in the laboratory. Rat es of O-2 and NO3- uptake, NH4+ production, nitrification and denitrif ication were determined from sediment-water fluxes. Pore water NH4+ wa s measured at the end of the experiment. At natural densities, similar to 50 000 ind. m(-2), there were increased rates of O-2 consumption ( x2), denitrification of water phase NO3- (x3) and NH4+ efflux (x26). N itrification was stimulated at low worm densities, but inhibited at hi gher worm densities. The transport of reduced compounds and organic ma tter, with the fecal pellets, to the sediment surface stimulated anoxi c conditions in the inhabited microcosms. These anoxic conditions led to increased rates of denitrification and were responsible for the dec rease in nitrification at higher worm densities. Approximately 25% of the NO3- produced by nitrification within the sediment was subsequentl y denitrified. Denitrification was responsible for 25% of the NO3- dis appearance from the system. The higher rates of denitrification were c ounterbalanced by higher rates of NH4+ flux from the sediment. It is l ikely, however, that the presence of T. tubifex resulted in a net loss of nitrogen that could otherwise have been used by the primary produc ers.