With the dramatic declines in Atlantic anadromous fishes over the past century it is important to identify the relative roles marine-derived nutrients (MDNs) delivered by these fishes play in influencing freshwater food web dynamics. Rivers in New Brunswick and Nova Scotia, Canada containing Atlantic salmon (Salmo salar), alewife (Alosa pseudoharengus), rainbow smelt (Osmerus mordax), or sea lamprey (Petromyzon marinus) as the primary anadromous species, were chosen to study their effects on i) primary production/productivity; ii) trophic interactions; and iii) resource quality. To understand the linkages between freshwater and marine ecosystems, observational studies, experimental frameworks, and analytical techniques (including stable isotope and fatty acid analysis) were employed. Biofilm communities followed a predictable response pattern to MDN inputs, regardless of the fishes spawning strategy, timing, or MDN load being delivered. Biofilm community standing crop and gross primary productivity were greater in sites receiving MDN subsidies than reference sites. The 13C and 15N data showed that MDNs were incorporated into all trophic levels (biofilm, invertebrates, and salmon parr) across streams with anadromous fish spawning. Community-wide niche space (i.e. the trophic diversity among food webs) shifted toward the marine-nutrient source, however the total ecological niche space (i.e. magnitude of trophic diversity) did not always increase with MDN inputs. Exposure to MDN resources from spawning Atlantic salmon led to improved nutritional quality for all biota, as indicated by increased lipid stores in all trophic levels and incorporation of fatty acids. The variability in fatty acid profiles was accredited to inherent differences between trophic groups combined with assimilation of marine-derived fatty acids in the MDN treatments. Precipitous declines in fish populations have resulted in a net loss in MDN loading to a point that may no longer sustain elevated levels of productivity needed for sustaining large fish populations. The current trend of declining anadromous fish populations in Atlantic Canada means fewer nutrient-rich marine subsidies for stimulating trophic production in these river systems. Marine-derived subsidies (nutrients and lipids) benefit multiple trophic levels of freshwater organisms as well as provide a cross-ecosystem spatial subsidy. In order to maintain ecosystem function and productivity, it is critical to include MDNs for effective ecosystem management and river restoration strategies.
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