Testing the effect of Limnodrilus so. (Oligochaeta, Tubificidae) on organic matter and nutrient processing in the hyporheic zone: a microcosm method

A set of slow filtration columns was developed to test the effect of macrof auna on hyporheic zone functioning. In a first step, the suitability of the experimental system was tested by measuring the variability in water flux, sediment redistribution, dissolved oxygen, number of ETS-active bacteria, and number of bacteria among columns. The results indicated a low variabili ty between columns filled with heterogeneous sand-gravel sediments and with out macrofauna. In a second step, the effect of the tubificid worm Limnodri lus sp. on organic matter and nutrient processing was investigated in exper imental conditions. Physicochemical parameters (pH, specific conductance, d issolved oxygen, dissolved organic carbon, ammonium, nitrite, and nitrate c ontents) and microbial parameters (number of ETS-active bacteria, number of bacteria, and hydrolytic activity) were measured at different depths and d ates in four columns. A number of 100 individuals of Limnodrilus sp. was in troduced into three of the four columns whereas the last column without mac rofauna served as a control. The test with oligochaetes demonstrated a stim ulation of microbial activities at all depths. Tubificid worms tended to en hance nutrient and organic matter processing in the experimental system. Th ese results suggest that tubificid worms play a specific role at the hyporh eic interface due to their particular mode of feeding.