Effects of Tubifex tubifex ( Oligochaeta : Tubificidae ) on N - mineralization in freshwater sediments , measured with 1 5 ~ isotopes Silvia

Sediment cores conta~ning different dens~t ies of Tubifex tublfex ranging from 0 to 70000 ind m-' were incubated in the laboratory Rates of O2 and NO, uptake NH,' production n~tr~fication and denitnfication were determined from sediment-water fluxes Pore water NH,+was measured at the end of the expenment At natural densities -50000 ind m ', there were increased rates of O2 consumption (x2), denitnfication of water phase NO; (x3) and NH,' efflux (x26) Nitrification was stimulated at low worm densities, but inhibited at higher worm densities The transport of reduced compounds and organic matter, with the fecal pellets, to the sediment surface stimulated anoxic cond~tions in the inhabited microcosms These anoxlc cond~tions led to increased rates of denitnfication and were responsible for the decrease in nitnfication at h ~ g h e r worm densities Approximately 25% of the NO? produced by nitnfication within the sedlment was subsequently den~trlf ied Denitnfication was responslble for 25% of the NO, disappearance from the system The h ~ g h e r ates of denitrification were counterbalanced by higher rates of NH,' flux from the sediment It is likely however that the presence of T tubifex resulted in a net loss of nitrogen that could otherwise have been used by the primary producers