Light may have triggered a period of net heterotrophy in Lake Superior

Abstract

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.

A Seasonal to Interannual View of Inorganic and Organic Carbon and pH in Western Lake Superior

“…Subsequent studies suggest that the respiration term in these budgets may have relied too much on higher nearshore respiration measurements (Bennington et al, ; Strom, ) and that annual primary production may also be higher than previously predicted (Sterner, ). Conversely, the estimates in the carbon budgets from the early 2000s may have been impacted by temporal changes in lake physics and biogeochemistry leading to a multiple‐year metabolic shift to net heterotrophy captured within these budgets, as suggested by Brothers and Sibley ().…”

Tree-ring isotopes adjacent to Lake Superior reveal cold winter anomalies for the Great Lakes region of North America

“…Hence, there appears to be strong potential for reconstructing winter temperatures or atmospheric circulation patterns influencing this region using an intra-annual tree-ring sampling and a dual isotope approach. These same climate signals are also highly correlated with winter ice cover for Lake Superior, which raises the possibility of better understanding past ice dynamics that are a major determinant of annual evaporation and associated lake levels for Lake Superior 60 and can also affect the limnological metabolism of this important ecosystem 61 .…”

Of Small Streams and Great Lakes: Integrating Tributaries to Understand the Ecology and Biogeochemistry of Lake Superior

“…), but also may deliver pulses of phosphorus enhancing rates of primary and bacterial production. Remarkably, photolysis of accumulated recalcitrant DOM in response to a period of high light levels following the 1997–1998 El Niño has been linked to a shift of the Lake Superior ecosystem from autotrophic to heterotrophic (Brothers and Sibley ). Thus, long‐term changes in quantity and quality of DOM interact with climate to drive the net metabolism of Lake Superior.…”