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 C-13-POM (range -32.4 to -21.8 parts per thousand) was lower than post-monsoon C-13-POM (range -25.5 to -19.9 parts per thousand). Epilimnetic C-13-POM was higher post-monsoon compared to pre-monsoon, except in 2007. In the metalimnion, pre-monsoon C-13-POM varied among years, while the variation in C-13-POM post-monsoon was small and was related to the inflow of organic matter during summer monsoon storms. C-13-zooplankton was lower post-monsoon (range -31.9 to -24.4 parts per thousand) compared to pre-monsoon (range -19.4 to -17.7 parts per thousand), while N-15-zooplankton showed the opposite pattern (pre- and post-monsoon average 6.6 +/- 2.0 parts per thousand and 4.3 +/- 1.5 parts per thousand, respectively). In Lake Soyang, the seasonal variation in C-13-zooplankton was closely related to annual succession in the phytoplankton community and C-13-phytoplankton. Thus, the results of this study provide some insights into the use of C-13 and N-15 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.