With funding from the Hydrology Program of the National Science Foundation (GEO), IES researchers Dr. Nina Caraco, Dr. Gene Likens, Mr. Don Buso and Dr. Jon Cole, along with Dr. Tom Winter from the United States Geological Survey (Denver) are investigating the sources of excess CO₂ in lakes. The surface waters of lakes are in constant, turbulent, contact with the overlying atmosphere. This contact allows gases dissolved in the water to equilibrate with those in the atmosphere. Despite this gas exchange, certain gases in the surface waters of lakes are consistently out of equilibrium with the atmosphere. The partial pressure of carbon dioxide, for example, tends to be several-fold higher in most lakes than one would expect from equilibrium with the atmosphere. Because CO₂ is an end-product of biological respiration, many researchers have assumed that this CO₂ supersaturation in lakes is the result of metabolism. That is, these researchers assume that respiration exceeds photosynthesis in the water body. For this to occur the lake's metabolism must be subsidized by organic C from the terrestrial watershed. Other processes, however, can also lead to carbon dioxide supersaturation. These include: the input of highly supersaturated ground water; or the input of reduced chemical species such as iron or sulfur, the oxidation of which generates acidity and converts bicarbonate to carbon dioxide. The research here is designed to determine which processes are most important in causing elevated CO₂ in lakes.

The researchers are determining the role of hydrologic inputs as a cause for elevated CO₂ in lake water by studying Mirror Lake, a very-well studied lake which is part of the Hubbard Brook Ecosystem Study. The preliminary CO₂ budget for this lake suggests that the loss of CO₂ to the atmosphere is several-fold larger than can be accounted for by the sum of all known processes. The major goal of this proposal is to balance this CO₂ mass balance. The researchers hypothesize that significant direct or indirect inputs of CO₂ have been underestimated because the role of hydrologic events, and spatial variation in ground water inputs have not yet been incorporated into the input calculations. The researchers will create a spatially explicit hydrologic model to determine the inputs of CO₂ , DOC, and alkalinity-consuming redox constituents. This work is possible at Mirror Lake because of an ongoing collaboration with hydrologists at USGS and an extensive series of continuously monitored wells in the Mirror Lake watershed, and continuously monitored inputs and outputs of surface waters and surface water chemistry.

Mirror Lake PCO₂

Figure Legend: The partial pressure of CO₂ in the surface waters (solid circles) and overlying atmosphere (open circles) at Mirror Lake, New Hampshire. Note that the partial pressure in the water is nearly always greater than that of the atmosphere. This means that the lake is a net source of CO₂ to the atmosphere. This export of CO₂ must be balanced by an input. The possible inputs include an excess of respiration over primary production (net heterotrophy) subsidized by an input of terrestrial organic C; an injection of CO₂ in incoming ground water; or the by consumption of alkalinity in the lake causing a shift from bicarbonate to CO₂.