Terrestrial support of pelagic consumers in lakes: Results of a multi-lake study

Background/Question/Methods: Lake food webs can be supported by primary production from within the lake, organic matter imported from the watershed, or some mixture of these two sources. Current debates about food web subsidies to lake ecosystems often depend on data from only a few ecosystems, and therefore do not consider the potential variability of subsidies among diverse ecosystems on a landscape. The goal of this work is to quantitatively account for the variation among lake ecosystems in terrestrial (allochthonous) utilization by pelagic consumers. We conducted a 40 lake hydrogen and carbon stable isotope survey, sampling Chaoborus spp. and other zooplankton taxa in order to quantify consumer allochthony and to develop a model that predicts the variation in allochthonous resource use based on availability.

Results/Conclusions: Consumer allochthony varied between 4 and 82% (mean=36%) for Chaoborus spp, and between 1 and 76% for other zooplankton consumers. Our results demonstrate that both the range in consumer allochthony among systems and the degree of allochthonous support within systems is substantial. The degree of allochthonous resource use increased exponentially (r²=0.44, p-value<0.01) with the proportion of allochthonous resources available (i.e. consumers in the most allochthonous-rich lakes use the larges fraction of allochthonous resources). Additionally, lake surface area was inversely correlated consumer allochthony (r² = 0.49) indicating that allochthonous subsidies are more important in smaller, rather than larger systems.  Given these results, it is clear that allochthonous subsidies are important in a large number of lake ecosystems.