Stable carbon and nitrogen isotope composition of co-existing herbivorous zooplankton species in an oligo-dystrophic lake (Shirakoma-ike, Japan)

Abstract

We studied carbon and nitrogen stable isotopes in particulate organic material and zooplankton in an oligotrophic humic lake, Lake Shirakoma-ike (Japan). The phytoplankton density was low and the zooplankton community was dominated by two species: a cladoceran (Daphnia longispina) and a copepod (Acanthodiaptomus pacificus). Though the two species have similar body size and shared same phytoplankton food sources of low diversity, they showed significant differences in isotope compositions; δ13C of D. longispina ranged from −30.9 to −28.2‰ and δ13C of A. pacificus ranged from −37.0 to −33.4‰, and δ15N of D. longispina and A. pacificus varied from 0.9 to 1.5‰ and −1.6 to −0.2‰, respectively. This implies that they rely on different food sources and food quality. δ13C of total particulate organic matter (POM) was always higher than zooplankton with the range of −29.3‰ to −26.2‰, but the δ15N of POM was between the two species, ranging from −2.8 to 1.7‰. The patterns of δ13C and δ15N suggest that A. pacificus might feed more on autochthonous organic matter compared with D. longispina. The differences in isotopic composition among co-existing herbivorous zooplankton species may be useful tools in investigating distinct energy pathways in unproductive lakes.