Dept. Civil & Environmental Engineering Michigan Technological University Houghton, MI 49931

Summary

        Carbon budgets are useful for tracing material and energy flows through ecosystems, for understanding linkages between adjacent systems, and for understanding trophic interactions. This paper highlights the dearth of data for Lake Superior, key gaps in data, and interesting hypotheses suggested by existing data. This paper does not address all aspects of carbon cycling but focuses on three questions: 1) What are the relative importance of the benthic and pelagic food webs? 2) What are the sources and pathways of carbon flow in near- and offshore areas of the lake? And 3) What is the regional significance of the CO₂ flux across the lake surface? To answer these questions, the literature on production and biomass of each trophic level is briefly summarized, extant data on stable isotope ratios (d¹³C, d¹⁵N) in food web compartments are compiled, measurements by the U.S. EPA relating to pCO₂ in the lake are utilized to estimate gas emissions, and CO₂ measurements from a tall tower are examined to evaluate the regional effect of the lake. Existing data do not provide final answers to the questions above, but are adequate to pose the following hypotheses. The data suggest that only about 15% of net primary production is funneled into the benthic food web. The microbial loop reduces fish production in the lake by shunting carbon through longer food chains. Two distinct food webs are discernible with reported stable isotope ratios; additional measurements will be needed to determine the contribution of temporal and spatial variability to the observed patterns. The putative nearshore food web is more enriched in ¹³C, but existing data suggest that benthic algae are not the source of the heavy carbon. The carbon source for the putative offshore food web is not as clear, but it may be autotrophs living in the deep chlorophyll maximum or intermediate nepheloid layer (INL). Organic carbon in the benthic nepheloid layer (BNL) appears to be derived from the INL in offshore waters. Seasonal measurements indicate that the lake is highly supersaturated with respect to atmospheric CO₂ in spring and less supersaturated in summer. Emissions from the lake cause elevated CO₂ concentrations in a broad region, and may be a significant component of the regional carbon budget.