Abstract
Nutrient transport across ecosystem boundaries by migratory animals can regulate trophic and biogeochemical dynamics of recipient ecosystems. The magnitude and direction of net nutrient flow between ecosystems is modulated by life history, abundance and biomass, individual behavior, and body element composition of migrating individuals. We tested common assumptions applied to nutrient transport models regarding homeostasis of species' body element composition across space and ontogenetic stage. We quantified whole body phosphorus (P) concentration of three life stages of wild Atlantic salmon (Salmo salar L.) from three distinct populations in Newfoundland, Canada, to evaluate the importance of river of origin and life stage as predictors of salmon %P. We found that life stage was a more important predictor of salmon %P than river of origin, and that %P of post‐spawn adults migrating downstream to the ocean (i.e., kelts) was more similar to %P of juveniles migrating downstream to the ocean (i.e., smolts) than it was to %P of adults migrating upstream to spawn. We then compared nutrient flux for the three rivers over a 20‐year period calculated with body composition values extracted from existing literature and our direct measurements to evaluate how assumptions regarding spatial and ontogenetic homogeneity in salmon %P influenced the observed P fluxes. We demonstrate that assuming equality of kelt %P and adult %P results in an overestimate of net nutrient flux to rivers by Atlantic salmon and the erroneous conclusion that Atlantic salmon populations are unconditional sources of nutrients to their natal watersheds. Instead, Newfoundland's salmon populations are conditional sinks of freshwater P, which is the opposite functional role of Pacific salmon. Our results highlight that a better understanding of intraspecific variation in body element composition of fishes is a prerequisite to determining their role in global biogeochemical cycling.
Highlights
Nutrient transport by organisms can be an important ecosystem process (Vanni 2002, Bauer and Hoye 2014), as flows of nutrients influence trophic dynamics and biogeochemical processes in recipient ecosystems (Seale 1980, Leroux and Loreau 2008, Childress et al 2014)
To determine the sensitivity of P flux estimates to variation in whole body %P that may occur when applying nutrient content values obtained from distant systems or assuming kelt and adults %P are equivalent, we calculated P flux using three different sets of salmon %P values: (1) means of population specific %P as a percentage of wet mass, (2) a regional value calculated as the mean of %P of all individuals sampled from the three study rivers, and (3) %P values published in Lyle and Elliott (1998), which assumed that adult and kelt %P is equal
A model including both life stage and river provided a slightly better fit to the data than did the model including life stage as the sole predictor, it came at the cost of an additional parameter (Table 2)
Summary
Nutrient transport by organisms can be an important ecosystem process (Vanni 2002, Bauer and Hoye 2014), as flows of nutrients influence trophic dynamics and biogeochemical processes in recipient ecosystems (Seale 1980, Leroux and Loreau 2008, Childress et al 2014). A basic model calculates nutrient flux as the difference between nutrients imported by spawning adults and nutrients exported by smolts and kelts migrating to the ocean (Moore et al 2011). Subscripts t, a, s, and k refer to year, adult, smolt, and kelt, respectively This nutrient flux model is data intensive and long term datasets that include all parameters are rare, especially for iteroparous fishes, which require additional information about the kelt export pathway. Studies focused on iteroparous salmonids explicitly assume that kelts exiting the river have a wet weight nutrient content equal to that of incoming spawning adults (Lyle and Elliott 1998, Moore et al 2011), but measurements of artificially reared Atlantic salmon show clear changes in %P during this portion of their life cycle (Shearer et al 1994). We examined the sensitivity of flux estimates to (1) variation in %P among populations and (2) the assumption that %P of kelt and spawning adults are equal (see Lyle and Elliott 1998, Jonsson and Jonsson 2003a, Moore et al 2011) to test the hypothesis that small differences between assumed and measured %P values compound to influence ecosystem flux estimates
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