MEASURING THE IMPACTS OF EMERGENT MACROPHYTES ON INSTREAM NITROGEN PROCESSING

A stream's capacity for assimilating nutrients is often strongly tied to the presence of instream biota. In many small and/or ephemeral streams, emergent macrophytes can dominate nutrient processing. Nutrient uptake by these plants can reduce loadings to downstream reaches and receiving water bodies, but must be harvested to completely remove the nutrients from the system. Understanding these processes can greatly aid in defining appropriate TMDL methodologies for small streams with macrophytes.The work presented here investigates the impacts of emergent macrophytes on instream nitrogen processing using a combination of field and mesocosm experimentation. Releases of nitrate and a conservative salt to a reach dominated by macrophytes (watercress), and then to the same reach cleared of macrophytes, were performed to help quantify both the movement of solute into and around the vegetation stands and the biokinetic uptake by the plants. The field experiments are supported by mesocosm incubations that isolated the plants and allowed direct measurements of uptake rates with simultaneous measurements of water nitrate concentration, photosynthetic rates, and ambient light. Finally, hypothetical TMDL calculations were performed for the study reach to highlight the impacts of the plants on these types of analyses.Results show that macrophytes dominate instream nitrogen processing, with the reach's assimilative capacity significantly decreased following clearing of the plants. Mass-specific uptake rates are shown to be correlated with water nitrogen concentration but not significantly correlated, in the short term, with plant photosynthesis rates or ambient light levels. Uptake rates are also shown to vary greatly with time of year. Finally, results indicate that manipulating macrophyte biomass in streams, including periodic harvesting, can be an effective stream and/or watershed management practice for nutrient-impaired systems.