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 and 13C of A. pacificus ranged from 37.0 to 33.4, and 15N 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. 13C of total particulate organic matter (POM) was always higher than zooplankton with the range of 29.3 to 26.2, but the 15N of POM was between the two species, ranging from 2.8 to 1.7. 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. © 2011 The Royal Society of New Zealand.