The effect of summer monsoon on pelagic and littoral food webs in a large regulated reservoir (Lake Paldang, Korea): A stable isotope approach

Abstract

To investigate the effect of summer monsoon-derived inputs of particulate organic matter (POM) on patterns of stable isotopes in the food web in a reservoir, we traced dietary sources of carbon and nitrogen in pelagic zooplankton and herbivorous littoral zoobenthos following a monsoonal disturbance in Lake Paldang, a large regulated reservoir in Korea. Carbon and nitrogen stable isotopes were analyzed in POM, zooplankton, sedimentary organic matter, zoobenthos, and piscivorous fishes in pre- and post-summer monsoon periods. Sharp depletion of C-13 and enrichment of N-15 of POM along with elevated POC/Chl a ratios occurred following the summer monsoon period, reflecting the rapid reduction of phytoplankton biomass and a pulsed supply of terrestrial organic detritus. Herbivorous zooplankton C-13 values, which were initially similar to POM C-13 values in the pre-summer monsoon period, decreased to nearly that of terrestrial organic matter, providing isotopic evidence of allochthonous contribution to zooplankton in the post-summer monsoon period. In contrast, herbivorous zoobenthos in the littoral area and higher fishes appeared to have a similar stable isotope composition between the pre- and post-summer monsoon period. Our findings indicate that although monsoon storms dramatically decrease phytoplankton biomass, the augmented allochthonous material after the summer monsoon offsets this effect temporarily by subsidizing pelagic zooplankton production. Allochthonous subsidies to herbivorous littoral zoobenthos and piscivorous fishes are relatively limited.