Marine nutrient transport: anadromous fish migration linked to the freshwater amphipod Gammarus fasciatus

摘要

Migrating anadromous fish may transfer marine-derived nutrients to oligotrophic tidal fresh water. River herrings (genus Alosa Linck, 1790) are the dominant anadromous genus in Virginia, USA. This study investigates whether marine nutrients derived from spawning Alosa spp. were incorporated into benthic invertebrates by using the stable isotopes of carbon and nitrogen. Spawning Alosa spp. had higher delta C-13 and delta N-15 values (-18.5 parts per thousand and 13.9 parts per thousand, respectively) than resident freshwater omnivorous fishes (-25.7 parts per thousand and 11.8 parts per thousand, respectively). In a tidal stream supporting abundant spawning Alosa spp., C-13 and N-15 enrichment was observed in stream amphipods (Gammarus fasciatus Say, 1818) coincident with the spawning migration of Alosa spp. The delta C-13 value for G. fasciatus increased from -28.5 parts per thousand to -26.0 parts per thousand from early to late April then fell to -28.1 parts per thousand in early June. A similar trend was observed in mayflies (Heptageniidae). Particulate organic matter and sediments from both streams remained depleted in C-13 (between -29 parts per thousand and -28 parts per thousand) and N-15 (between 0.5 parts per thousand and 3.0 parts per thousand) during the spawning run of Alosa spp. It is estimated that between 5% and 35% of amphipod biomass may be derived from marine carbon brought to tidal fresh water by spawning Alosa spp.