The flocculation feedback loop: Delivery of marine derived nutrients in Pacific salmon streams.

Pacific salmon contribute significant quantities of marine derived nutrients (MDN) to their natal streams. Post-spawning carcasses fertilize and stimulate stream productivity as they decay. Until now, there has been no complete description of a nutrient delivery mechanism for natal streams. The salmon-floc feedback loop proposed here is a positive feedback system that delivers salmon derived nutrients and organic matter to the streambed where they can be retained and metabolized by benthic food webs to stimulate stream productivity and provide sustenance for juvenile salmon. The three stages of the salmon-floc feedback loop were verified using the controlled environment of flumes and field verified concentrations of salmon organic matter and inorganic particulates. Stage one, floc formation was found to occur in the water column in the presence of salmon organic matter as well as salmon organic matter and clay. During the salmon organic matter treatments the particle size distribution of suspended sediment shifted toward larger particles indicating the formation of flocs. Stage two, floc sedimentation was identified by an increase in the effective particle size distribution of fine sediments in the gravel bed after the addition of salmon organic matter and clay. Water column flocs settled or were sequestered on the flume bed by advective and intergravel flow through porous, raised, gravel bed sections. Stage three, floc dissociation and nutrient release was identified by the increase in nutrient, bacterial, and biochemical oxygen demand levels of fine sediments that were captured and retained within intergravel pores. Floc formation and streambed delivery/retention required sufficient quantities of organic matter from decaying salmon, sufficient but not excessive inorganic sediment levels, bacterial populations, low-flow stream conditions, porous raised gravel bed areas, and hyporheic exchange to be an effective MDN vector. The verification of the salmon-floc feedback loop is an important step toward furt