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Using Stable Isotope Analysis to Determine Zooplankton Trophic Response to the Biogeochemical Gradient in a Coastal Tributary
Citation:
PETERSON, G. S., T. D. CORRY, AND A. M. COTTER. Using Stable Isotope Analysis to Determine Zooplankton Trophic Response to the Biogeochemical Gradient in a Coastal Tributary. Presented at Coastal and Estuarine Research Federation 20th Biennial Conference, Portland, OR, November 01 - 05, 2009.
Impact/Purpose:
The goal of our research is to identify energy inputs that support lower food web production in a coastal tributary using the biogeochemical gradient that arises from the mixing of river and Great Lake water.
Description:
The goal of our research is to identify energy inputs that support lower food web production in a coastal tributary using the biogeochemical gradient that arises from the mixing of river and Great Lake water. We characterized the food web along the lower 35 km of the St. Louis River, MN, a coastal tributary that flows into Lake Superior, by measuring the carbon and nitrogen stable isotope ratios (13C, 15N) of the major components of the ecosystem: primary producers, suspended particulates, benthic macroinvertebrates, and zooplankton. Conservative tracers revealed increasing dilution of river water by Lake Superior with increasing proximity to Lake Superior. There was a large isotopic gradient in 13C along the river, with increasingly enriched values toward Lake Superior. The zooplankton gradient spanned a ca. 6‰ range from the upper river to the most Lake-influenced station, Duluth inlet (-30.2‰ to -23.7‰, respectively) as did zoobenthos (-30.9‰ and -24.7‰, respectively), which were isotopically similar. There was a broad 15N range among zooplankton taxa (0.5‰ to 9.5‰). The data were consistent with reliance on isotopically light organic matter sources by both groups, likely autochthonous phytoplankton and allochthonous terrestrial-derived riverine organic matter, though a few species captured at Duluth inlet had isotopic signatures consistent with reliance on enriched Lake Superior primary producers. The observation that the zooplankton 13C gradient is similar to the expected conservative mixing gradient for dissolved inorganic carbon lends support to the interpretation that food webs are localized and persist long enough to be detected with stable isotope techniques.