The surface waters of lakes are in constant, and sometime 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 propose to determine 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. One graduate and two undergraduate students will receive training at the interface between hydrology and limnology in the program. Additionally, this work will be linked to the larger educational outreach program of the Hubbard Brook Ecosystem Study, an LTER site. The Mirror Lake research will be a new focus of the Science Links. Program for middle school students.
