Assessing the Composition of Particulate Organic Matter and Zooplankton Food Sources in Lake Superior and Across a Size-Gradient of Aquatic Systems

Food webs in aquatic systems can be supported both by carbon from recent local primary productivity and by carbon subsidies, such as material from terrestrial ecosystems or past in situ primary productivity. The importance of these subsidies to respiration and biomass production remains a topic of debate. While some studies have reported that terrigenous organic carbon supports disproportionately high zooplankton production, others have suggested that phytoplankton preferentially supports zooplankton production in aquatic ecosystems. Quantification of zooplankton food sources using ambient stable carbon isotopic signatures is difficult because of the inherent difficulty in directly measuring the
13C of phytoplankton, and the narrow and overlapping range of phytoplankton and terrigenous organic matter
13C signatures, especially in freshwater systems. The objectives of this study were (1) to use isotopic composition to assess the food sources of mesozooplankton in Lake Superior, and (2) to investigate food sources of zooplankton in a suite of other aquatic systems. Natural abundance radiocarbon (
14C) and stable isotope (
13C,
15N) analyses were applied to show that zooplankton in Lake Superior selectively incorporate recently-fixed, locally-produced (autochthonous) organic carbon even though other carbon sources are readily available. Estimates from Bayesian isotopic modeling based on
14C and
13C values show that the average lakewide median contributions of recent in-lake primary production and terrestrial, sedimentary, and bacterial organic carbon to the bulk POM in Lake Superior were 58%, 5%, 33%, and 3%, respectively. However, isotopic modeling estimates also show that recent in situ production contributed a disproportionately large amount (median, 91%) of the carbon in mesozooplankton biomass in Lake Superior. Although terrigenous organic carbon and old organic carbon from resuspended sediments were significant portions (median, 38%) of the available basal food resources, these contributed only a small amount to mesozooplankton biomass (median, 3% from sedimentary organic carbon and 3% from terrigenous organic carbon). The isotopic investigation shows that intermediate trophic-level mesozooplankton in Lake Superior prefer to incorporate fresh autochthonous food, despite the availability of other organic carbon sources, and that upper trophic levels are likely not supported by terrestrial and/or resuspended-sediment OC subsidies to the carbon cycle. Comparison of zooplankton food sources based on their radiocarbon composition showed that terrigenous organic carbon was relatively more important in rivers and small lakes, and the proportion of terrestrially-derived material used by zooplankton correlated with the hydrologic residence time and the ratio of basin area to water surface area. Further research should focus on the catabolic metabolism of mesozooplankton and both anabolic and catabolic metabolism in the microbial loop to further our understanding of such subsidies in the carbon cycle and energy transfer.