Nutrients in Salmonid Ecosystems: Sustaining Production and Biodiversity

<em>Abstract</em>. The depressed status of many Pacific Northwest (PNW) salmonid stocks has focused considerable attention on the role of marine-derived nutrients (MDN) in maintaining the productivity of salmonid ecosystems and created a strong interest in stream and lake enrichment as an important salmon restoration technique. This paper reviews some of the technical and more applied aspects of stream, river, and lake enrichment as currently practiced in British Columbia and elsewhere in the world. The first step when considering potential stream and lake enrichment is to determine the ambient nutrient concentrations, stock of biogenic biomass, and trophic status of the candidate ecosystem by conducting nutrient bioassays, synoptic surveys, low-level nutrient analyses, and qualitative assessments of the ecosystems. Phosphorus is considered limiting when concentrations in composite stream or epilimnetic lake samples during the growing season are less than 1µg/L SRP and less than 2–3 µg/L TDP. Nitrogen is considered limiting in streams when dissolved inorganic nitrogen (DIN) concentrations during the growing season are less than 20 µg/L and in lakes when spring epilimnetic concentrations are less than 30 µg/L. At minimum, a 1–2 year pre-treatment study is required to determine the nutrient status and requirements of the ecosystem. Water users and regulatory agencies must be notified in advance of plans to add nutrients to rivers, lakes, or streams, and provincial, state, and/or federal permitting processes and guidelines should be followed. All nutrient enrichment programs must consider seven key variables: (1) desired concentration of nutrients, (2) type of nutrients, (3) seasonal timing of application, (4) frequency of nutrient addition, (5) location of application sites, (6) DIN:TDP (total dissolved phosphorus) ratio of nutrients to be added, and (7) application technique. Enrichment programs should attempt to mimic the anadromous salmon ‘nutrient pump’ where applicable and implement a nutrient prescription that assures the production of edible phytoplankton/periphyton and avoids the occurrence of nuisance algae. In addition, sufficient funds should be secured to monitor the ecosystem responses.