Nitrification and denitrification in a eutrophic lake sediment bioturbatedby oligochaetes

The influence on nitrification and denitrification of oligochaete Limnodril us sp. and Tubifex tubifex bioturbation was determined in eutrophic lake se diment from the Basin of Lake Ringsjon in southern Sweden. Nitrification an d denitrification activity was compared in sieved and in undisturbed sedime nt cores in laboratory mesocosms at 2 different concentrations of nitrate. Nitrification and total denitrification rates were positively correlated to oligochaete biomass between 0 and 4 g dry weight m(-2) at both 33 and 268 muM nitrate in the inflowing water. Denitrification (d(w)) was relatively m ore stimulated by the oligochaetes at high concentrations of nitrate in the overlying water than at low nitrate concentrations. Though nitrification w as enhanced by oligochaetes, this tended to reach a maximum between 3 and 5 g of oligochaete dry weight m(-2). Comparison of the enhancement of denitr ification by oligochaetes with other similar studies of denitrification in eutrophic sediment bioturbated by tube-dwelling chironomids indicates that at a similar biomass oligochaetes are less effective at mobilizing nitrate to deeper sediment layers. This is explained by the different ways oligocha etes and tube-dwelling chironomids bioturbate the sediment. Sieving and hom ogenising the sediment had no pronounced effect on rates of denitrification and nitrification compared to undisturbed sediment.