Modeling the influence of salmon spawning on hyporheic exchange of marine-derived nutrients in gravel stream beds

Salmon that spawn in streams deliver marine-derived nutrients (MDN) that catalyze trophic productivity and support rearing juvenile salmon. Salmon spawning also affects hyporheic exchange and movement of dissolved MDN through the stream bed by creating redd topography that induces pumping exchange and by winnowing fine sediment and loosening the bed, which alters hydraulic conductivity and bed porosity. The spatial extent of spawning within the channel likely governs the volume and rate of dissolved MDN exchanged with the stream bed through this process. To explore this issue, we used a two-dimensional groundwater model to predict changes in hyporheic volume, flux, and mean hydraulic residence time of dissolved MDN as a function of the proportion of the bed surface occupied by redds (P). Predictions indicate that hyporheic volume and flux systematically increase with P, while the mean hydraulic residence time of dissolved MDN in the hyporheic zone decreases sharply with P, from 5.79 h on an unspawned bed (P = 0) to 0.03 h for a mass-spawned bed (P = 1.0). Shorter residence time results from hyporheic flux increasing faster than hyporheic volume with higher P. Implications for uptake of dissolved MDN are explored with Damkohler numbers, defined as the ratio of the mean hydraulic residence time to a biogeochemical rate of interest. Given the considerable influence of spawning on hyporheic exchange, additional research is needed to determine conditions under which bioassimilation of dissolved MDN is limited by nutrient supply, extent of the hyporheic zone, or processing rate of MDN in stream beds.