Gravel Bed Delivery of Marine-Derived Nutrients During Active and Post-Spawn Periods: a Flume-Based Simulation

Pacific salmon spawning events appear to have contrasting effects on natal stream environments.  The disturbance salmon create during redd construction can reduce stream productivity while the marine derived nutrients (MDN) released during post-spawn can stimulate productivity.  To properly evaluate the ecological benefits and costs of spawning events, MDN delivery and storage processes need to be quantified.  Recent flume-based simulations identified the delivery to and storage of floc-based MDNs in gravel beds, but these studies did not quantify the magnitude of salmon organic matter delivered to the gravel bed versus that flushed downstream.  To address this gap, a mass balance study was conducted in three flow-through flumes (2m*2m*30m) over a 21-day period (consisting of four pre-treatment, four exposure, and 13 post-exposure days) to quantify the amount of salmon organic matter delivered to the gravel-bed during active and post-spawn simulations.  Each flume was exposed, via a continuous drip delivery system, to a single treatment of either, salmon only, salmon plus 0.5 mgl^-1 clay, or salmon plus 5 mgl^-1 clay.  The latter two treatments reflect post and active spawn water column conditions, respectively based upon previous field investigation.  Flume water column depth and velocity as well as gravel bed substrate size were similar to local natal salmon streams.  In each treatment, suspended particle size distributions measured in-situ showed the formation of larger flocs in the water column during the exposure period.  Post-treatment suspended sediment measurements also indicate that flocs settling during the exposure period could re-suspend and migrate further downstream.  Gravel beds within each flume were enriched with MDN following treatments but the proportion of salmon organic matter delivered to the gravel-bed was highest in the active spawn simulation.  This more effective delivery in the active spawn simulation is attributable to the higher suspended sediment concentration, which is known to enhance floc formation.  Although the active spawn simulation delivered more MDN to the gravel bed during this simulation, the post-spawn period may be equally or more important in natural streams as the post-spawn salmon decay period is longer than the active spawn and may accordingly provide more MDN to the streambed if carcasses remain in-stream.  The delivery and retention potential of MDN to spawning streambeds and the intergravel environment may be particularly important for interior streams, which experience low flow conditions during the spawning period and accordingly have the potential for hyporheic recruitment.