Isotopic variability in lake littoral organisms presents a challenge for food web studies

Abstract

Most studies of lake ecosystems, including those that use stable isotope analysis (SIA), have traditionally concentrated on the pelagic system (Beaudoin et al. 2001, Vander Zanden & Vadeboncoeur 2002). Hence, temporal, spatial, and interspecies variability in isotopic signatures of pelagic food web components (especially zooplankton) have been the focus of recent studies (e.g., Matthews & Mazumder 2003, 2007). Recent SIA studies have also clearly indicated, however, that benthic production and the littoral areas of lakes may contribute more than previously thought to the whole lake ecosystem energy budget (Hecky & Hesslein 1995, Vander Zanden & Vadeboncoeur 2002, Vander Zanden et al. 2006). Studies of lake food webs based on SIA often assume single isotope values to represent baseline values for pelagic and littoral food webs. These values may then be used as “end members” in isotope mixing models, such as to assess trophic positions or relative contributions from different energy sources. But such single values ignore temporal, spatial or especially inter-specific variation in stable isotope values for these primary consumers, as shown for pelagic consumers by Matthews & Mazumder (2003). Such variability may have considerable impacts on the outputs from mixing models and hence on the inferences drawn about food web structure. Lake littoral habitats have received much less attention in this respect, despite being important feeding areas for many fish species and therefore contributing significantly to the observed stable isotope signatures at higher trophic levels. Littoral habitats are also inherently more heterogeneous than pelagic habitats, and primary production from phytoplankton, periphyton, and macrophytes, combined with allochthonous sources of energy, potentially result in complex and variable isotopic signatures in littoral primary consumers (Syvaranta et al. 2006). We present stable isotope data for littoral invertebrate consumer populations (benthic invertebrates and zooplankton) from Lake Jyvasjarvi in central Finland and show how variation in isotope signatures between different littoral invertebrate consumers can be reflected at higher trophic levels.