Variation in carbon and nitrogen stable isotopes in POM and zooplankton in a deep reservoir and relationship to hydrological characteristics

Abstract

We investigated the carbon and nitrogen stable isotope composition of particulate organic matter (POM) and zooplankton to understand the effects of highly variable hydrologic inputs with the onset of the summer monsoon season in Lake Soyang, Korea. The main characteristic separating pre-monsoon and post-monsoon seasons in Lake Soyang is the dramatic difference in hydrologic inflow. Pre-monsoon δ13C-POM (range −32.4 to −21.8‰) was lower than post-monsoon δ13C-POM (range −25.5 to −19.9‰). Epilimnetic δ13C-POM was higher post-monsoon compared to pre-monsoon, except in 2007. In the metalimnion, pre-monsoon δ13C-POM varied among years, while the variation in δ13C-POM post-monsoon was small and was related to the inflow of organic matter during summer monsoon storms. δ13C-zooplankton was lower post-monsoon (range −31.9 to −24.4‰) compared to pre-monsoon (range −19.4 to −17.7 ‰), while δ15N-zooplankton showed the opposite pattern (pre- and post-monsoon average 6.6 ± 2.0‰ and 4.3 ± 1.5‰, respectively). In Lake Soyang, the seasonal variation in δ13C-zooplankton was closely related to annual succession in the phytoplankton community and δ13C-phytoplankton. Thus, the results of this study provide some insights into the use of δ13C and δ15N measurements for understanding linkages between varying hydrologic characteristics and seasonal variation in the stable isotope composition of zooplankton. The dramatic seasonal change in hydrologic inputs in the monsoonal climate and consequent changes in turbid surface water inflow is linked with changes in the phytoplankton and zooplankton communities in Lake Soyang.