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

摘要

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; ~(13)C of D. longispina ranged from 30.9 to 28.2 and ~(13)C of A. pacificus ranged from 37.0 to 33.4, and ~(15)N 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. ~(13)C of total particulate organic matter (POM) was always higher than zooplankton with the range of 29.3 to 26.2, but the ~(15)N of POM was between the two species, ranging from 2.8 to 1.7. The patterns of ~(13)C and ~(15)N 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.