Whole-lake dissolved inorganic 13C additions reveal seasonal shifts in zooplankton diet

Sustained whole-lake additions of C-13-enriched dissolved inorganic carbon (DIC), intended to increase experimentally the delta C-13 of DIC in the epilimnion of a small lake with high dissolved organic carbon (DOC), were made during three seasonal periods (spring, summer, and autumn). Coupled with carbon and nitrogen stable isotope analysis of zooplankton and several of their putative food sources, these additions were used to investigate seasonal changes in the relative contributions of different food sources to zooplankton diet in the lake. Four main potential food sources were considered: phytoplankton, heterotrophic bacteria (HB), methanotrophic bacteria (MOB), and green sulfur bacteria (GSB). Because the number of potential food sources exceeded the number of isotopes analyzed, a computer program (IsoSource) was used to estimate the range of possible contributions of the various food sources. During all three periods the added inorganic C-13 quickly increased the epilimnetic DIC delta C-13 by between 18 parts per thousand and 21 parts per thousand above the initial value of approximately -21 parts per thousand. This C-13 enrichment of DIC was rapidly transmitted to the particulate organic matter (POM), which included photosynthetic phytoplankton. In spring and summer, delta C-13 of both adult and juvenile Daphnia increased by similar to 10 parts per thousand, indicating that Daphnia utilized autochthonous carbon. However, this C-13 labeling of Daphnia was not so obvious during the autumn period, when their delta C-13 generally decreased. According to the IsoSource model outputs based on both delta C-13 and delta N-15 values, Daphnia utilized all four potential food source types during spring, summer, and autumn, but in different proportions. The possible contribution of phytoplankton to Daphnia diet was substantial (25-71%) in all seasons. The possible contributions of the bacterial food sources were more variable. The possible contribution of GSB was minor (0-20%) at all times and negligible in autumn. The possible contribution of HB was higher but very variable. Methanotrophic bacteria always made a significant contribution to Daphnia diet and were likely the single most important food source in autumn. Since both HB and MOB in this high-DOC lake probably depend largely on allochthonous organic carbon, our results highlight the seasonal variability in the potential importance of ecosystem subsidies in lake food webs.