Interaction effects of flow velocity and oxygen metabolism on nitrification and denitrification in biofilms on submersed macrophytes

Effects of water flow velocity on nitrification, denitrification, and the metabolism of dissolved oxygen and inorganic carbon in macrophyte-epiphyton complexes were investigated in the present study. The metabolic rates were measured in microcosms containing shoots of Potamogeton pectinatus L. with epiphytic biofilms in the light and dark with no flow or with the flow velocities of 0.03 and 9 cm s(-1). Photosynthesis and respiration increased with increasing water flow velocity. Rates of oxygen respiration were positively correlated to the oxygen concentration of the water. Nitrification was not significantly affected by flow velocity, but nitrification was higher in light than in dark at 0.03 cm s(-1), but not at 9 cm s(-1). Denitrification was higher in stagnant water and at 9 cm s(-1) than at 0.03 cm s(-1) in the absence of oxygen, possibly due to complex effects of water flow velocity on the supply of organic matter to the denitrifying bacteria. Denitrification was always inhibited in light, and negatively correlated to the oxygen concentration in dark. Epiphytic denitrification occurred only at low oxygen concentrations in flowing water, whereas in stagnant water, denitrification was present in almost oxygen saturated water. Therefore, because there are little of water movements and high oxygen consumption in dense stands of submersed macrophytes, significant rates of epiphytic denitrification can probably be found within submersed vegetation despite high oxygen concentrations in the surrounding water. In conclusion, this study shows that the water flow and oxygen metabolism within submersed vegetation have minor effects on nitrification, but significantly affect denitrification in biofilms on submersed macrophytes.