Articles | Volume 9, issue 9
Biogeosciences, 9, 3663–3678, 2012
Biogeosciences, 9, 3663–3678, 2012

Research article 25 Sep 2012

Research article | 25 Sep 2012

An isotopic (Δ14C, δ13C, and δ15N) investigation of the composition of particulate organic matter and zooplankton food sources in Lake Superior and across a size-gradient of aquatic systems

P. K. Zigah1,2, E. C. Minor3, J. P. Werne3, and S. Leigh McCallister4 P. K. Zigah et al.
  • 1Large Lakes Observatory and Water Resources Science Program, University of Minnesota, Minnesota, USA
  • 2Department of Surface Waters, Swiss Federal Institute of Aquatic Science and Technology (Eawag), Kastanienbaum, Switzerland
  • 3Large Lakes Observatory and Department of Chemistry and Biochemistry, University of Minnesota-Duluth, Minnesota, USA
  • 4Department of Biology, Virginia Commonwealth University, Richmond, Virginia, USA

Abstract. 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 support zooplankton production in aquatic ecosystems. Here we apply natural abundance radiocarbon (Δ14C) and stable isotope (δ13C, δ15N) analyses 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 Δ14C and δ13C values show that the average lake-wide 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. 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.

Final-revised paper