Atlantic anadromous fishes are important vectors of pulsed marine-nutrient subsidies to freshwater ecosystems
Anadromous fishes deliver marine-derived nutrients (MDNs) as they migrate into freshwater, stimulating the production and growth of algae, aquatic invertebrates and fish, and shifting reliance to marine-derived resources. Marine-derived nutrients can also increase the quality of food resources by making available essential fatty acids that are required for optimal physiological performance, but cannot be synthesized by freshwater organisms. The focus of our research was to evaluate the role of different timing and spawning strategies on supplementary resource use in freshwater food webs in Atlantic rivers. The objectives were to (i) determine the degree to which MDNs transported to rivers by rainbow smelt, alewife, sea lamprey, or Atlantic salmon are incorporated into freshwater organisms, (ii) determine the relative contribution and temporal shifts of MDNs in the diet of resident salmonids, and (iii) determine whether fatty acids delivered by Atlantic salmon are transferred to consumers. We predicted that MDNs would be incorporated differentially into the tissues of freshwater organisms depending on the anadromous species spawning; primarily through direct consumption of eggs and carcasses and indirect assimilation when excretory products are the primary nutrient source.
Results show that MDN subsidies are incorporated into all trophic levels across streams receiving spawning rainbow smelt, alewife, sea lamprey, or Atlantic salmon. Peak δ15N values in the biofilm (1.2-5.4‰), macroinvertebrates (0.0-6.8‰), and resident salmonids (1.2-2.6‰) were synchronous with spawning, however the magnitude and temporal patterns of enrichment differed depending on the timing and spawning strategy of the anadromous fish present. The SIAR mixing model also displayed a strong pattern of reliance on MDN resources (33.1-47.0%) in the diet of resident salmonids. Increases in lipid content among biofilm (4.3%), marcoinvertebrates (6.8%), and parr (0.0-15.5%) coincided with exposure to spawning Atlantic salmon. Direct consumption of marine subsidies was indicated by the presence of marine fatty acid biomarkers in consumers. Omega-3 and omega-6 polyunsaturated fatty acids comprised of 30, 45, and 50% of the total fatty acids in parr brain, gonad, and muscle tissues respectively; compared to 45% in salmon eggs. Fatty acids of this series were primarily represented by 20:5n-3, 22:6n-3, 20:4n-6, and 18:2n-6. The high degree of spatio-temporal heterogeneity in marine-nutrient subsidies lead to both direct and indirect pathways of incorporation into stream food webs, suggesting organisms at multiple trophic levels derive substantial proportions of their energy from marine resources.