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Carbon and nitrogen stable isotope ratios of macroinvertebrates in the littoral zone of Lake Biwa as indicators of anthropogenic activities in the watershed

Karube, Zin'ichi, Sakai, Yoichiro, Takeyama, Tomohiro, Okuda, Noboru, Kohzu, Ayato, Yoshimizu, Chikage, Nagata, Toshi, Tayasu, Ichiro
Ecological research 2010 v.25 no.4 pp. 847-855
Bivalvia, anthropogenic activities, carbon, human population, lakes, land use, littoral zone, macroinvertebrates, nitrates, nitrogen, nutrients, paddies, particulate organic matter, population density, regression analysis, rivers, snails, stable isotopes, statistical models, wastewater, watersheds
Carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) of macroinvertebrates inhabiting littoral zones of lakes can serve as useful indicators of material loading from the watershed. We collected snails (Semisulcospira spp.) and bivalves (Unio douglasiae biwae Kobelt) from 29 littoral sites in Lake Biwa near the mouths of river tributaries with various human population density (HPD) and land-use patterns. The δ¹³C and δ¹⁵N signatures were determined for three potential food sources: particulate organic matter in the pelagic zone (PPOM), riverine particulate organic matter from tributaries (RPOM) and epilithic organic matter in the littoral zone (EOM). The stable isotope mixing model revealed that snails relied mainly on EOM, and bivalves on PPOM and RPOM. Multiple regression analysis showed that intersite variation in δ¹⁵N for snails was best explained by HPD, while variation in δ¹⁵N of EOM and nitrate was explained to a lesser extent by HPD. Comparison with isotope signatures of their food sources and riverine nutrients revealed that snails assimilated anthropogenic nitrogen from wastewater in the watershed. Our results also showed that the δ¹³C value of bivalves was marginally related to the fraction of paddy fields in the watersheds. In conclusion, the isotope signatures of macroinvertebrates inhabiting the littoral zone can be useful indicators of anthropogenic impacts from the watershed.