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Limnology and Oceanography







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Recent studies of Lake Superior, the Earth's largest freshwater lake by surface area, describe it as net heterotrophic (primary production < community respiration), making it a net source of carbon dioxide (CO2) to the atmosphere. This conclusion is largely based on measurements made between 1998 and 2001. We present a long‐term (1968–2016) analysis of ice‐free (April–November) surface oxygen (O2) saturation data collected by monitoring agencies. These data indicate that Lake Superior's surface waters are typically supersaturated with dissolved O2 from May to September (May–September mean is 103.5% ± 0.6%; pooled mean from April, October, and November is 97.6% ± 1.1%, standard error of the mean). However, these data also support prior studies which describe a state of net heterotrophy from 1998 to 2001. We investigated potential triggers for a transient heterotrophic period and discuss the sources of organic carbon necessary to fuel net heterotrophy in a large oligotrophic lake. We conclude that net heterotrophy likely resulted from an increase in light period and penetration driven by declines in cloud cover, increases in water clarity, and a reduction of winter ice cover following the 1997–1998 El Niño. Together, these could have depleted a pre‐existing pool of dissolved organic carbon (DOC) via photomineralization and/or photochemical degradation. Our results indicate that Lake Superior is typically net autotrophic (calculated annual CO2 influx = ∼ 0.4 Tg C). These results highlight how water clarity and aquatic DOC pools may interact to induce net metabolic shifts in large oligotrophic aquatic ecosystems.

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