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; 13C of D. longispina ranged from -30.9 to -28.2 parts per thousand and 13C of A. pacificus ranged from -37.0 to -33.4 parts per thousand, and 15N of D. longispina and A. pacificus varied from 0.9 to 1.5 parts per thousand and -1.6 to -0.2 parts per thousand, respectively. This implies that they rely on different food sources and food quality. 13C of total particulate organic matter (POM) was always higher than zooplankton with the range of -29.3 parts per thousand to -26.2 parts per thousand, but the 15N of POM was between the two species, ranging from -2.8 to 1.7 parts per thousand. The patterns of 13C and 15N 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.