Nutrient additions to mitigate for loss of Pacific salmon: consequences for stream biofilm and nutrient dynamics

Amy M Marcarelli,Colden V Baxter,Mark S Wipfli

doi:10.1890/es13-00366.1

Abstract

Mitigation activities designed to supplement nutrient and organic matter inputs to streams experiencing decline or loss of Pacific salmon typically presuppose that an important pathway by which salmon nutrients are moved to fish (anadromous and/or resident) is via nutrient incorporation by biofilms and subsequent bottom‐up stimulation of biofilm production, which is nutrient‐limited in many ecosystems where salmon returns have declined. Our objective was to quantify the magnitude of nutrient incorporation and biofilm dynamics that underpin this indirect pathway in response to experimental additions of salmon carcasses and pelletized fish meal (a.k.a., salmon carcass analogs) to 500‐m reaches of central Idaho streams over three years. Biofilm standing crops increased 2–8‐fold and incorporated marine‐derived nutrients (measured using 15N and 13C) in the month following treatment, but these responses did not persist year‐to‐year. Biofilms were nitrogen (N) limited before treatments, and remained N limited in analog, but not carcass‐treated reaches. Despite these biofilm responses, in the month following treatment total N load was equal to 33–47% of the N added to the treated reaches, and N spiraling measurements suggested that as much as 20%, but more likely 2–3% of added N was taken up by microbes. Design of biologically and cost‐effective strategies for nutrient addition will require understanding the rates at which stream microbes take up nutrients and the downstream distance traveled by exported nutrients.

Highlights

Transfers of organisms, energy, nutrients and organic matter that cross ecosystem boundaries, termed subsidies, can have broad consequences for recipient ecosystems and food webs (Polis et al 2004, Baxter et al 2005, Marcarelli et al 2011)
For ash-free dry mass (AFDM), there was a significant effect of period, with the highest pretreatment AFDM standing crops observed in years 1 and 3
Nutrient addition to mitigate for the loss of marine-derived nutrients delivered by salmon in the Pacific Northwest is a widely applied management strategy (Compton et al 2006) with the ultimate goal of increasing production of resident and/or migratory fish via both direct and indirect pathways

Summary

Introduction

Energy, nutrients and organic matter that cross ecosystem boundaries, termed subsidies, can have broad consequences for recipient ecosystems and food webs (Polis et al 2004, Baxter et al 2005, Marcarelli et al 2011). Fish movement and migrations are an important mechanism for subsidy delivery and disturbance in aquatic ecosystems (Flecker et al 2010), and an archetypal example is the migration of Pacific salmon (Oncorhynchus spp.). Because of their anadromous life history, Pacific salmon are vectors of nutrient and energy subsidies from marine to freshwater-riparian ecosystems (Naiman et al 2002). There is concern that reduced nutrient returns have led to decreases in primary and secondary production, in turn causing bottom-up limitation of juvenile salmon production This oligotrophication has been hypothesized to lead to a continued downward trend in salmon populations (Stockner et al 2000, Achord et al 2003). In this region and others, nutrient additions to mitigate for and restore natural salmon runs have been widely considered and implemented (Hyatt et al 2004, Compton et al 2006)

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