Seasonal changes in vertical distribution, biomass and faecal production of tubificids in the profundal region of a shallow Japanese lake

Two tubificid species Limnodrilus hoffmeisteri and L. claparedeianus formed more than 93% of the total number of oligochaetes in the profundal. Limnodrilus spp. worms were found down to 33 cm in the sediment but in great numbers in the upper zone in June and October. Worms confined to the top 15 cm of sediment accounted for 53-92% of the total number. There were two annual maxima in population density and biomass, one in late spring (66000 inds m^?2, 17 g wet wt m^?2) and the other in mid autumn (97000 inds m^?2, 176 g wet wt m^?2). Two regression lines describing the effect of temperature on faecal production rate were obtained; Log F = 0.0604 T (°C) ?0.7660 (below 15°C), Log F = 0.0266 T – 0.2170 (above 15°C). In total 26.8 kg dry wt m^?2 of sediment was defecated annually by Limnodrilus spp. The sediment in the 0–10 cm stratum may pass through the guts of the worms 2.3 times a year. Sedimentation rates in profundal region were very low with respect to the faecal production rates of the tubificids.