PARENT SESSION
Contributed Oral Session 10: Aquatic Trophic Systems I
Monday, August 8, 8:00 AM - 11:30 AM, Meeting Room 519 B, Level 5, Palais des congrès de Montréal

Long-term variation in food web structure in a north temperate lake.

Solomon, Christopher^*,1, Vander Zanden, M. Jake¹, Carpenter, Stephen¹, ¹ University of Wisconsin - Madison, Madison, WI, USA

ABSTRACT- The causes and consequences of variation in food web structure are of fundamental importance in understanding how ecosystems work and how they may respond to external perturbations. Unfortunately, many efforts to understand these phenomena are limited to relatively short temporal scales and thus may overlook important patterns that are manifested only over many years. We take advantage of an unusually complete long-term record to examine variation in food web structure in a north temperate lake. We measured stable isotope ratios (¹³C and ¹⁵N) in archived scale samples of seven fish species over a 23-year period. We also measured ¹³C and ¹⁵N of benthic and pelagic food web end members (macroinvertebrates and zooplankton, respectively). We observed both short-term stochastic variation and long-term trends in end member isotopic signatures, indicating that retrospective food web studies may reach erroneous conclusions about food web change when they fail to consider variation in end members. Mixing models indicated that many fish species (including 'pelagic' top predators) rely heavily on both pelagic and benthic secondary production. Furthermore, the relative importance to fish of energy from these two compartments changed substantially over time, as did fish trophic position. Changes in the relative availability of benthic and pelagic food sources may be important drivers of observed trends in consumers.

Key words: food web, benthic-pelagic coupling, stable isotope, trophic structure