The effect of migratory fish on freshwater ecosystem nutrient dynamics

Anadromous fish are often important vectors for nutrient fluxes between marine and freshwater ecosystems. These nutrients, termed as marine-derived nutrients (MDN), enter the stream in the form of eggs, sperm, waste and adult carcasses. Small freshwater streams often have a low primary production due to heavy shading, so these marine derived nutrients play an important role in increasing productivity of the streams food web. Pacific salmon (Oncorhynchus spp.) are known to contribute with MDN in the freshwaters of North America where they migrate to spawn and later die. Atlantic salmon (Salmo salar) and sea trout (S. trutta) are iteroparous, meaning that adults may return to the ocean after spawning. Therefore it is less clear if these anadromous fish can significantly contribute to spatial translocation of nutrients from the Baltic Sea to the freshwater rivers. The aim of this study was to investigate if transported nutrients incorporate into the tissues of aquatic communities and riparian plant communities. Stable isotopes were used to track marine-derived nutrients into the biota of freshwater. Field work along with laboratory analyses was done to measure the ratio of 13C/12C (δ13C) and 15N/14N (δ15N) in benthic invertebrates and primary producers (algae and riparian vegetation) collected from five streams. Two streams supported Atlantic salmon and sea trout, two streams could potentially receive other anadromous fish and one stream had a migratory obstacle. Moreover, δ13C and δ15N values were estimated in tissues and eggs of reproductive individuals of Atlantic salmon and sea trout. The results from the study revealed that organisms from nearly all trophic levels are enriched with 15N and 13C, in the streams where Atlantic salmon spawn and the overwinter mortality occurs, indicating a significant contribution of Atlantic salmon and sea trout to nutrient dynamics of boreal stream