Crustacean zooplankton trophic position, food item origin and relationship to fish in the temperate, subalpine Lake Savalen, central Norway, compared with similar lake ecosystems.

Lake Savalen is a subalpine, oligotrophic, dimictic and regulated lake situated in Norway, north Europe. It is located between 703-708 meters above sea level in the central part of Norway. The lake has a maximum surface area of 15.2 km2. The zooplankton was studied since it forms a crucial link in the aquatic food web. Energy and matter accumulated through primary production are incorporated into consumers and transmitted to planktivorous fish via zooplankton. The ratios of the stable isotopes of nitrogen (15N:14N) and carbon (13C:12C) were analysed. Stable carbon isotope ratio reflects the input of carbon revealing the contributions of different food sources, and nitrogen isotope indicates the trophic role of the organisms. In Lake Savalen the zooplankton species showed marked differences in their trophic levels, with the highest δ15N values in the most pronounced predator species Bythotrephes longimanus. Slightly lower δ15N values were found in the omnivorous species Arctodiaptomus laticeps and Heterocope saliens, both of which probably have a less carnivorous food choice than B. longimanus. The lowest δ15N values found in zooplankton species logically fall in with the filtrator Daphnia galeata with a significant intake of algae, bacteria and possibly a small amount of protists. A major spring flood in May/June 1995 in the closely situated Lake Atnsjø demonstrated in a most convincing way the importance of allochthonous material for the zooplankton community. The zooplankton density increased three-fold and the biomass two-fold after the flood, compared with reference years