Biotic Control of Stream Fluxes: Spawning Salmon Drive Nutrient and Matter Export

Abstract

Organisms can control movements of nutrients and matter by physically modifying habitat. We examined how an ecosystem engineer, sockeye salmon (Oncorhynchus nerka), influences seasonal fluxes of sediments, nitrogen (N), and phosphorus (P) in streams of southwestern Alaska. The purpose of this study was to investigate whether salmon act as net importers or net exporters of matter and nutrients from streams and how these roles change as a function of salmon population density. We measured discharge and concentrations of suspended sediments and total N and P every 7-14 days for up to four summers in 10 streams spanning a gradient in salmon densities. We statistically allocated whole-season fluxes to salmon activities, such as excretion and bioturbation, and to export by hydrologic discharge. In addition, we used counts of spawning salmon to estimate nutrient and matter imports by salmon to streams. Large seasonal pulses of suspended sediments, P, and N were associated with salmon spawning activities, often increasing export an order of magnitude higher than during pre-salmon levels. Years and streams with more salmon had significantly higher levels of export of sediments and nutrients. In addition, years with higher precipitation had higher background export of P and N. Salmon exported an average of the equivalent of 189%, 60%, and 55% of total matter, P, and N that salmon imported in their bodies. The relative magnitude of export varied; salmon exported more than their bodies imported in 80%, 20%, and 16% across all streams and years for sediments, P, and N, respectively. A bioassay experiment indicated that the P exported by salmon is directly available for use by primary producers in the downstream lake. These results demonstrate that salmon not only move nutrients upstream on large spatial scales via their migration from the ocean and subsequent death, but also redistribute matter and nutrients on finer spatial scales through their spawning activities.

Linking resource availability and habitat structure to stream organisms: an experimental and observational assessment

“…Low returns of spawning anadromous fish are widespread: the flux of nutrients and energy delivered by spawning salmon to some tributaries of the North Pacific is about 95% lower than historic levels (Gresh et al 2000). These reductions alter a range of ecological processes, including nutrient dynamics, sediment flux, and trophic production, from headwater streams to near-shore habitats (Johnston et al 2004, Moore et al 2007, Wipfli and Baxter 2010. Therefore, management or restoration actions that increase the number of spawning adults or mimic their enrichment and physical effects may improve the individual and population growth of a variety of organisms, including juvenile salmon, in tributaries with low adult returns (e.g., Wipfli et al 2004).…”

“…Migrations of anadromous fish are a relatively well-studied example of a resource pulse that influences a diversity of ecological characteristics, including trophic productivity and habitat structure, through their spawning activities (Richey et al 1975, Moore et al 2007, Jones et al 2010). Though our understanding of this system is growing (Janetski et al 2009), there remain some unresolved questions including two that we address in this paper.…”

“…Specifically, while there is strong evidence supporting the direct transfer of energy (tissue and eggs) from spawning salmon to higher trophic levels (e.g., Armstrong et al 2010), the evidence supporting the indirect pathway of salmon resources fertilizing food webs via bottom-up processes is mixed (e.g., Lessard and Merritt 2006). One factor that may contribute to these inconsistent results is that most studies sample static measures, such as abundance or biomass, rather than turnover or export, which may be more responsive to salmon subsidies relative to static measures (Lessard and Merritt 2006, Moore et al 2007, Kiernan et al 2010.…”

Influence of temperature on surface sediment disturbance by freshwater fish: a microcosm experiment

“…Among this fauna, many studies have focused on fish as actors of bed surface disturbance in streams (e.g., Flecker, 1996;Flecker and Taylor, 2004;Holtgrieve and Schindler, 2010), given their key role in the flux and transfer of particles and solutes in aquatic ecosystems. The influence of spawning behaviour of salmonid species on streambed has been extensively documented (e.g., Montgomery et al, 1996;Peterson and Foote, 2000;Gottesfeld et al, 2004;Moore et al, 2007;De Vries, 2012) but knowledge of other behaviours as zoogeomorphic activity (e.g., foraging, swimming and burrowing) remains scarce, especially for non-salmonid species (but see Statzner and Sagnes, 2008;Shirakawa et al, 2013;Pledger et al, 2014).…”

From earth and ocean: investigating the importance of cross‐ecosystem resource linkages to a mobile estuarine consumer

“…A considerable body of research has investigated the effects of the salmon's unique life history, which couples offshore marine productivity to coastal ecosystems (Gende et al 2002, Naiman et al 2002. However, the net effects of spawning salmon can vary (Harding et al 2014); salmon can subsidize freshwater and riparian habitats through excretion and egg and carcass deposition (Kline et al 1990, Cederholm et al 1999, Janetski et al 2009, and can export nutrients with juvenile emigration and disturbance as adults dig and defend nests (Moore et al 2007, Kohler et al 2013. Although considerable amounts of salmon-derived nutrients imported into coastal watersheds are retained, large proportions are also exported downstream to estuaries (Gende et al 2004, Mitchell and Lamberti 2005, Cak et al 2008.…”