Quantifying the Impacts of Instream Macrophytes on Nutrient Assimilation in a Southern California Stream: Implications for TMDLS

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 increases seasonal assimilative capacities and can reduce loadings to downstream reaches and receiving water bodies. However, plants must be harvested to achieve complete removal of nutrients from the system. Understanding and quantifying these processes can greatly aid in defining appropriate TMDL methodologies and performing supporting numerical modeling for streams with macrophytes. The study presented here aimed to quantify nitrogen and phosphorus assimilation and downstream delivery rates in a small stream dominated by large wetland emergent macrophytes. The stream is "mowed" annually for flood control purposes. Nutrient addition experiments were performed at two different stages of macophyte growth: late in the growing season, just before the reach was mechanically cleared of macrophytes (September) and then again two months later after re-growth had commenced and new shoots were established. A stream solute transport numerical model was applied that captures both transport processes and biokinetics of the system. Experimental and modeling results show that, in the short-term, mowing created a more nutrient-retentive system, compared to pre-mowing. This appears to be due to increased uptake in the re-growth (compared to old shoots) and decreased downstream solute velocities (as a result of downed vegetation). Whole-stream effective nitrogen uptake rates ranged from 12 to 16 mg-N m-2 d-1 (pre-mowing) and 40 to 57 mg-N m-2 d-1 (postmowing). Effective phosphorus uptake rates ranged from 15 to 17 mg-P m-2 d-1 (premowing) and 32 to 52 mg-P m-2 d-1 (post-mowing). These rates compare well to literature-reported rates for constructed wetlands when transport limitations are accounted for. Stream average solute velocities decreased nearly 3x after mowing (1389 vs. 482 m d-1 ), despite slightly higher flows. Both the uptake rates and solute velocities will be useful for parameterization of future watershed numerical models, including those to be used for TMDL analyses. Implications for future TMDL analyses are demonstrated.