Organic matter transfer in Lake Superior's food web: Insights from bulk and molecular stable isotope and radiocarbon analyses

Abstract

A suite of isotopic methods were combined to provide a comprehensive investigation of organic matter transfer and consumer‐resource links in a large lake food web. We applied compound specific isotope analysis (CSIA) of nitrogen within amino acids of organisms, a relatively new method and one not yet widely applied to large lake systems, to determine the trophic positions of several dominant species. Comparison of this CSIA trophic designation to those calculated by traditional bulk stable carbon and nitrogen isotope ratios, as well as gut content analyses, revealed this CSIA method to be the most representative of known trophic links in Lake Superior. Limnocalanus macrurus, an omnivorous copepod, was found to occupy a trophic position higher than would have been predicted based on gut analyses, and one full trophic position above primary consumer Daphnia. Radiocarbon analysis of bulk zooplankton and fish tissue was employed as a modern carbon tracer to elucidate the relative ages of carbon incorporated into, and propagated through, the food web. Comparison of animal tissue, non‐living carbon pools (DOC, DIC), and POC radiocarbon content indicate the food web is supported predominately by newly synthesized autochthonous material. The uniquely depleted radiocarbon signature of the benthic amphipod Diporeia, attributed to consumption of aged sedimentary or allochthonous material, was not reflected in its known predators, indicating little overall incorporation of that aged carbon source into the pelagic food web. This multi‐method isotopic investigation extends our understanding of food web structure and organic matter flow in this large lake.

Historical niche partitioning and long‐term trophic shifts in Laurentian Great Lakes deepwater coregonines

“…The historical gut content Mysis from Lake Michigan had a trophic position of 2.9, while mean trophic position of Lake Superior Mysis reported in Kruger et al. () was 2.7. We did not estimate trophic position of present‐day Lake Michigan Mysis, but if similar to that of Lake Superior, it suggests that Mysis trophic position has not changed much.…”

“…Another possibility that could decrease the trophic position of deepwater coregonines is a shortening of the zooplankton food web, leading to a reduction in the trophic position of invertebrates such as Mysis that composed a large proportion of prey fish diets. The historical gut content Mysis from Lake Michigan had a trophic position of 2.9, while mean trophic position of Lake Superior Mysis reported in Kruger et al (2016) was 2.7. We did Fig.…”

“…Kruger et al. () reported trophic position of Diporeia to be 2.4 and Mysis to be 2.7. Given this difference, the trophic compression in Lake Michigan relative to Lake Superior could be the result of the sharp decline in Diporeia in Lake Michigan (Nalepa et al.…”

“…We also analyze present-day forage fishes in these lakes to examine how trophic niche partitioning has changed over the past century. Few studies have used amino acid-specific stable nitrogen isotope analysis to study freshwater food webs (Ogawa et al 2013, Kruger et al 2016. Our work highlights the application of this approach for understanding how food web interactions and trophic niche partitioning have changed over time.…”

“…, Kruger et al. ). Our work highlights the application of this approach for understanding how food web interactions and trophic niche partitioning have changed over time.…”