Diets of crustacean zooplankton, inferred from stable carbon and nitrogen isotope analyses, in lakes with varying allochthonous dissolved organic carbon content

We used stable carbon and nitrogen isotope analyses to estimate the relative proportions of three putative food sources (1) algae, (2) allochthonous organic matter (but including also heterotrophic bacteria and green-sulphur bacteria having similar isotopic values) and (3) methane-oxidizing bacteria (MOB) in the diets of crustacean zooplankton in five small boreal lakes representing a gradient of dissolved organic carbon (DOC) concentration from ca. 5 to 40 mg C l(-1). The lakes were sampled in May, after establishment of stratification, and again in October during autumnal mixing of the water column. IsoSource mixing model outputs indicated that the proportion of algae in the diets of zooplankton was generally higher in May than in October, and that bacteria contributed to the diets of both cladocerans and copepods. Our results indicate that bacteria, especially MOB, can make an appreciable contribution to zooplankton diets in these small lakes, even in those with relatively low DOC concentrations.