Marine-Derived Nutrients and Nutrient Loss Mitigation: Where We've Been and Where We Might be Headed

Investigations over the past three decades have produced a plethora of information on the extent anadromous fishes affect biota and food webs in recipient watersheds.  Much of this work is centered on marine-derived nutrients (MDN) associated with Pacific salmon in western North America.  MDN provided by spawning salmon have been found in a multitude of aquatic and terrestrial plants and animals, and are documented to increase food web productivity in these typically oligotrophic freshwater systems.  Their influence on productivity comes in many forms and includes a direct food resource for consumers, incorporation of nutrients by biota following adult excretion and carcass decomposition, and through habitat modification.  Spawning salmon support animal species that often times congregate in very high numbers to feed upon this annual resource subsidy.  They can also act as ecosystem engineers, with disturbance from redd building obliterating aquatic productivity responses under certain circumstances.  Food webs and biota respond variably to salmon returns, depending upon geographic region, magnitude and timing of returns, and site conditions.  Based on the assertion that MDN increases productivity, nutrient supplementation products, ranging from liquid fertilizer to carcass analogs, are being developed and tested as mitigation for the loss of MDN in watersheds with reduced salmon runs.  Past studies and current mitigation projects suggest these nutrient supplements are effective in increasing aquatic productivity in some settings.  Evidence also suggests that high-quality forms of carbon-based compounds in carcasses and some marketed nutrient-augmentation products (e.g., pasteurized salmon carcass analogs) also drive productivity responses.  Regardless, most of the work has been short-term or case studies, and aquatic-focused.  We caution that nutrient supplementation should be viewed as a short-term tool for temporarily boosting basal food resources that support upper trophic levels, and is most practical in aquatic systems known to be nutrient-limited (N, P).  We recommend that managers use an adaptive approach and proceed with caution when considering stream fertilization.  Achieving the maximum benefit and minimizing any negative effects is dependent on a good understanding of the role MDN play in a given watershed and on implementing a well-informed and monitored adaptive management approach that includes adequate pre and post treatment monitoring of both aquatic and terrestrial responses. As our understanding of MDN continues to evolve so will our ability to potentially mitigate anthropogenic disruption of this natural phenomenon.