Food niches of cyclopoid copepods in eutrophic Plußsee determined by stable isotope analysis

Feeding studies have suggested interspecific differences in the food sources and trophic position of cyclopoid copepod species. We measured δ13C and δ15N of five sympatric cyclopoid copepod species and of bulk POM from a eutrophic lake over a one-year period, to determine if stable isotope derived-data from the field supported the hypotheses from feeding studies. Cyclops abyssorum and C. vicinus differed significantly in δ15N but not in δ13C, inferring assimilation from similar carbon sources but differentiation in their trophic position. Cyclops abyssorum δ15N typically was 1– 2‰ higher than C. vicinus, suggesting a more predatory feeding mode compared to its congener. The summer species Mesocyclops leuckarti, Thermocyclops oithonoides and Diacyclops bicuspidatus exhibited considerable differences in δ13C. The lower δ13C of the hypolimnetic D. bicuspidatus compared to the two epilimnetic species indicates a food niche differentiation by vertical separation. All copepod δ13C and δ15N signatures showed seasonal fluctuations, with low values in winter and high values in summer. The δ13C of seston was generally higher than copepod δ13C. We regard selective feeding from the bulk POM, spatial separation of feeding and assimilation of 13C depleted food sources responsible for this phenomenon.