Patterns of Allochthonous Resource Availability and Use by Aquatic Consumers in Lakes

Aquatic ecosystems are hotspots of decomposition and a globally significant source of carbon dioxide to the atmosphere. Carbon fixed in the terrestrial biosphere (allochthonous) supplements the carbon fixed in aquatic ecosystems (autochthonous), forming the organic matter (OM) available to aquatic consumers. Recent studies have re-emphasized the potential importance of allochthonous inputs to lake food webs and their dynamics. Specifically, the variation among lake ecosystems in the amount of allochthonous inputs and importance of those resources to aquatic consumers is not well understood.
The aim of this dissertation is to quantify the patterns of allochthonous resource availability and use among and within lakes. Using hydrogen and carbon stable isotopes, I found that the organic matter in 39 north temperate lakes is dominated by allochthonous material. Additionally, within a lake, the composition of organic matter does not vary vertically in the water column. I demonstrated using oxygen and argon saturations that metalimnetic oxygen maxima are not solely the result of biological production or indicative of increased autochthonous resource availability for consumers compared to surface waters. This finding was confirmed with a whole-lake metalimnetic 13C addition in Peter Lake which was also used to track metalimnetic resource use by zooplankton. As hypothesized, metalimnetic resource use was low for Peter Lake consumers with allochthonous resources being the most important. This is consistent with the high but variable allochthonous resource use by zooplankton quantified using hydrogen stable isotopes in the same lakes that allochthonous resource availability was measured. Allochthonous resource use increases with allochthonous resource availability in these lakes and is highest in small lakes. Finally, I evaluate three key issues regarding the use of hydrogen stable isotopes in mixing models; 1) correction for environmental water in consumer tissues, 2) consideration of differential fractionation among biochemical constituents, and 3) consideration of differential H-exchange among samples and standards. Despite these complicating factors, analysis of consumer resource use is possible using whole organisms’ δ2H.
My dissertation quantifies the variability of allochthonous resource availability and use by zooplankton in lakes and related it to ecosystem characteristics. Based on my findings and given the numerical dominance of small lakes globally, allochthonous resources are likely important for consumers in most lakes. Allochthonous resource use in aquatic food webs indicates interdependence and connectivity between ecosystems. Therefore, being able to predict the allochthonous resource availability and use in a food web is necessary for ecosystem management, restoration, and identifying lakes that could be susceptible to watershed, regional, or global change.