Abstract
Estuaries are amongst the world’s most productive ecosystems, lying at the intersection between terrestrial and marine environments. They receive substantial inputs from adjacent landscapes but the importance of resource subsidies is not well understood. Here, we test hypotheses for the effects of both terrestrial- and salmon-derived resource subsidies on the diet (inferred from stable isotopes of muscle tissue), size and percent nitrogen of the soft-shell clam (Mya arenaria), a sedentary estuarine consumer. We examine how these relationships shift across natural gradients among 14 estuaries that vary in upstream watershed size and salmon density on the central coast of British Columbia, Canada. We also test how assimilation and response to subsidies vary at smaller spatial scales within estuaries. The depletion and enrichment of stable isotope ratios in soft-shell clam muscle tissue correlated with increasing upstream watershed size and salmon density, respectively. The effects of terrestrial- and salmon-derived subsidies were also strongest at locations near stream outlets. When we controlled for age of individual clams, there were larger individuals with higher percent nitrogen content in estuaries below larger watersheds, though this effect was limited to the depositional zones below river mouths. Pink salmon exhibited a stronger effect on isotope ratios of clams than chum salmon, which could reflect increased habitat overlap as spawning pink salmon concentrate in lower stream reaches, closer to intertidal clam beds. However, there were smaller clams in estuaries that had higher upstream pink salmon densities, possibly due to differences in habitat requirements. Our study highlights the importance of upstream resource subsidies to this bivalve species, but that individual responses to subsidies can vary at smaller scales within estuaries.
Highlights
Cross-ecosystem resource linkages can structure and stabilize recipient communities [1,2]
Zone and location did not have any interaction effects with salmon density or watershed size, clams below streams had higher %N than those in control locations (Fig 8C) and this disparity was most apparent in lower clam beds (Fig 8D)
This study demonstrates the complex relationships between terrestrial and marine resource subsidies and traits of sedentary estuarine consumers
Summary
Cross-ecosystem resource linkages can structure and stabilize recipient communities [1,2]. Terrestrial-derived resources are often thought to be of lower quality than estuarine or marine sources [21] They can elevate organic content in estuaries [19,29,30], few studies have attempted to detect productivity responses from them [31,32,33] and even fewer have tested the degree to which responses can vary across landscapes [26]. We test hypotheses on how terrestrial and salmon resource subsidies, in addition to individual traits, explain the diet (inferred from stable isotopes of nitrogen and carbon), size, and percent nitrogen of soft-shell clams (Tables 1 and 2). 1) Moving outward from upper to lower zones (increasing distance from stream outlet) will reduce the effect of both salmon density and watershed size on mass and % N. Salmon spawn in streams throughout BC’s central and north coasts, which can produce over half of the wild salmon stocks in this region, and account for over 30% of total populations within BC and the Yukon [54]
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