Assimilation of marine-derived nutrients from anadromous Rainbow Smelt in an eastern North American riverine food web: evidence from stable-isotope and fatty acid analysis

Abstract

Movement of anadromous fishes from marine to freshwater habitats provides a seasonal pulse of marine-derived nutrients (MDNs) to freshwater biota. In northeastern North America, iteroparous, anadromous Rainbow Smelt (Osmerus mordax Mitchell, 1814) migrate in high densities to freshwater spawning grounds in early spring. To assess whether 1) MDNs from migratory Rainbow Smelt were incorporated into multiple trophic levels and 2) total fatty acid (FA) compositions of Brook Trout changed during the Rainbow Smelt migration, we compared stable 13C and 15N isotope values and FA profiles of biota in the Pisquid River, Prince Edward Island, Canada. We sampled periphyton, benthic macroinvertebrates, and stream-resident Brook Trout (Salvelinus fontinalis Mitchell, 1814) at a downstream site encompassing Rainbow Smelt spawning grounds and an upstream site inaccessible to Rainbow Smelt. During the Rainbow Smelt migration at the downstream site, predatory caddisflies (Rhyacophilidae) and Brook Trout displayed a 3.1‰ and 1.3‰ increase in δ13C and δ15N, respectively, relative to isotope levels before the migration. Less consistent evidence of MDN assimilation was found in periphyton and nonpredatory macroinvertebrates (Heptageniidae, Elmidae). Increases in δ13C values for Rhyacophilidae and Brook Trout indicated direct consumption of tissue, most probably eggs based on limited stomach content analysis in Brook Trout. FA composition differed between Rainbow Smelt and Brook Trout sampled down- and upstream. Transfer of marine ω-3 FAs was evidenced by statistically similar amounts of eicosapentaenoic and docosahexaenoic acid between Rainbow Smelt and Brook Trout sampled on spawning grounds. We found connections between Rainbow Smelt-based MDNs and stream food webs and a nutritional benefit to Brook Trout.