Bidirectional delivery of organic matter between freshwater and marine systems: the role of flocculation in Pacific salmon streams

The literature regarding freshwater and marine exchanges of organic matter (OM) focuses predominantly on the unidirectional delivery of allochthonous and autochthonous material from freshwater to the marine environment. Another ecologically significant exchange occurs when anadromous organisms move into coastal and interior watersheds and bring marine-accrued OM and its incorporated marine-derived nutrients (MDN). We use the example of Pacific salmon streams to discuss bidirectional transfers of OM in the upstream and downstream directions with specific reference to the role of flocculation. A fish-floc feedback loop has recently been presented as a mechanism that allows transfer of salmon OM to the gravel-bed matrix in the form of flocs. In the proposed fish-floc feedback loop, the OM source is salmon, but the OM source will change with fish species. Once fish-based flocs enter the gravel bed they can be stored or used by benthic organisms. A salmon disturbance regime that includes redistribution of gravel, fine sediment, and biofilm during redd construction and release of salmon OM to the stream is integral to the fish-floc feedback loop because it provides the inorganic fine sediment and OM required for floc formation. The MDN subsidy provided by returning salmon is important for natal watershed functioning as illustrated by a conceptual OM feedback loop that links these freshwater and marine exchanges. Retention of floc-bound OM in the gravel bed afforded by settling flocs allows slower downstream transfer of nutrients en route to the ocean and a correspondingly increased period for uptake in food webs downstream.