Nitrogen Removal in a Wastewater Reservoir: The Importance of Denitrification by Epiphytic Biofilms on Submersed Vegetation

The aim of this study was to examine the importance of epiphytic denitrifying bacteria on submersed vegetation in removing N from a shallow nutrient‐enriched freshwater ecosystem. The investigation was conducted during the summer of 1994 in a surface flow reservoir receiving municipal tertiary‐treated wastewater. The submersed vegetation in the reservoir was dominated by Potamogeton pectinatus L. and filamentous green algae (FGA). The N loading was 2300 mg N h−1 m−2 and the N removal, calculated as the mean difference between influent and effluent N, was 190 mg N h−1 m−2 (8%). The majority of influent N consisted of NH+4, but the main part of the N removal was due to the removal of NO−3 whereas no net retention of NH+4 was found. Mean total soluble solids and BOD7 retention was 69 and 38%, respectively. Denitrification measurements were conducted in darkness at in situ temperature in microcosms with P. pectinatus, FGA, or intact sediment cores. Epiphytic denitrification ranged between 0.21 to 7.0 mg N h−1 m−2 reservoir surface area depending on the abundance of the submersed vegetation (5–140 g DW m−2). Sediment denitrification was 4.7 mg N h−1 m−2 reservoir surface area. The mean assimilative N uptake of the submersed vegetation and epiphyton was 3.4 and 1.6 mg N h−1 m−2 reservoir surface area, respectively. Measured N removal rates through plant uptake and denitrification could only account for a minor part of the N removal observed by mass balance. However, microcosm denitrification measurements underestimate actual denitrification. Thus, the major part of the N removal was most likely due to denitrification. In conclusion, this study indicates that denitrification in epiphytic microbial communities on submersed vegetation can be of significant importance for the N removal in nutrient‐enriched freshwater ecosystems.