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

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 δ13C and δ15N values (–18.5‰ and 13.9‰, respectively) than resident freshwater omnivorous fishes (–25.7‰ and 11.8‰, respectively). In a tidal stream supporting abundant spawning Alosa spp., 13C and 15N enrichment was observed in stream amphipods (Gammarus fasciatus Say, 1818) coincident with the spawning migration of Alosa spp. The δ13C value for G. fasciatus increased from –28.5‰ to –26.0‰ from early to late April then fell to –28.1‰ in early June. A similar trend was observed in mayflies (Heptageniidae). Particulate organic matter and sediments from both streams remained depleted in 13C (between –29‰ and –28‰) and 15N (between 0.5‰ and 3.0‰) 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.

Physical factors affecting the relative abundance of native and invasive amphipods in the St. Lawrence River

The Ponto-Caspian amphipod Echinogammarus ischnus (Stebbing, 1899) is reportedly replacing the North American amphipod Gammarus fasciatus Say, 1818, in the lower Great Lakes, but the two species appear to coexist in the upper St. Lawrence River several years after invasion by E. ischnus. A multi-site survey in the river between Lake Ontario and Montreal (Quebec) found that E. ischnus and G. fasciatus respond differently to substrate characteristics, water chemistry variables, and current velocity. Both species increase in abundance in the presence of dreissenid mussels. However, E. ischnus density is positively correlated with current velocity and an increasing proportion of gravel-sized sediment, while G. fasciatus density is positively correlated with benthic filamentous algal (Cladophora spp.) biomass, macrophyte biomass, and pH. Habitat heterogeneity within the river may be promoting the coexistence of native and exotic amphipods by allowing them to segregate along physicochemical gradients.

Genotypic variation among Gammarus fasciatus (Crustacea: Amphipoda) from the Great Lakes - St. Lawrence River: implications for the conservation of widespread freshwater invertebrates

We examined the population genetic structure of the amphipod Gammarus fasciatus (Crustacea: Amphipoda) over two years from a total of 11 sites within the Great Lakes - St. Lawrence River system using allozyme electrophoresis. In 1995, we examined differences within the St. Lawrence River (Cornwall to Quebec City), and in 1996, we focused on larger scale, whole-system differences from Lake Superior to Lake Champlain. We found very low levels of genetic differentiation among sites (Wright's FST < 0.03). This was not unexpected for an active disperser in a relatively contiguous system. However, we also found low levels of genetic variability within sites (Hobs = 0.07- 0.15), a result that was not anticipated. We surveyed the available literature on gammarid and other amphipods and found that limited genetic variability is characteristic of continuously distributed taxa, contrary to the expectations of current conservation theory. This is potentially of concern, especially in light of recently reported declines of G. fasciatus at several locations within the St. Lawrence River and Great Lakes. Given the importance of species such as G. fasciatus in aquatic food webs, further work is clearly needed on the consequences of population genetic structure to species' responses to novel environmental changes (e.g., exotic species introductions).