[1] Peatlands deform elastically during precipitation cycles by small (±3 cm) oscillations in surface elevation. In contrast, we used a Global Positioning System network to measure larger oscillations that exceeded 20 cm over periods of 4-12 hours during two seasonal droughts at a bog and fen site in northern Minnesota. The second summer drought also triggered 19 depressuring cycles in an overpressured stratum under the bog site. The synchronicity between the largest surface deformations and the depressuring cycles indicates that both phenomena are produced by the episodic release of large volumes of gas from deep semi-elastic compartments confined by dense wood layers. We calculate that the three largest surface deformations were associated with the release of 136 g CH 4 m À2 , which exceeds by an order of magnitude the annual average chamber fluxes measured at this site. Ebullition of gas from the deep peat may therefore be a large and previously unrecognized source of radiocarbon depleted methane emissions from northern peatlands.

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We describe a conceptual model, the wetland continuum, which allows wetland managers, scientists, and ecologists to consider simultaneously the influence of climate and hydrologic setting on wetland biological communities. Although multidimensional, the wetland continuum is most easily represented as a two-dimensional gradient, with ground water and atmospheric water constituting the horizontal and vertical axes, respectively. By locating the position of a wetland on both axes of the continuum, the potential biological expression of the wetland can be predicted at any point in time. The model provides a framework useful in the organization and interpretation of biological data from wetlands by incorporating the dynamic changes these systems undergo as a result of normal climatic variation rather than placing them into static categories common to many wetland classification systems. While we developed this model from the literature available for depressional wetlands in the prairie pothole region of North America, we believe the concept has application to wetlands in many other geographic locations.

Ground water exchange affects the ecology of surface water by sustaining stream base flow and moderating water-level fluctuations of ground water-fed lakes. It also provides stable-temperature habitats and supplies nutrients and inorganic ions. Ground water input of nutrients can even determine the trophic status of lakes and the distribution of macrophytes. In streams the mixing of ground water and surface water in shallow channel and bankside sediments creates a unique environment called the hyporheic zone, an important component of the lotic ecosystem. Localized areas of high ground water discharge in streams provide thermal refugia for fish. Ground water also provides moisture to riparian vegetation, which in turn supplies organic matter to streams and enhances bank resistance to erosion. As hydrologists and ecologists interact to understand the impact of ground water on aquatic ecology, a new research field called "ecohydrology" is emerging.

[1] Hydraulic head was overpressured at middepth in a 4.2-m thick raised bog in the Glacial Lake Agassiz peatlands of northern Minnesota, and fluctuated in response to atmospheric pressure. Barometric efficiency (BE), determined by calculating ratios of change in hydraulic head to change in atmospheric pressure, ranged from 0.05 to 0.15 during July through November of both 1997 and 1998. The overpressuring and a BE response were caused by free-phase gas contained primarily in the center of the peat column between two or more semielastic, semiconfining layers of more competent peat. Two methods were used to determine the volume of gas bubbles contained in the peat, one using the degree of overpressuring in the middepth of the peat, and the other relating BE to specific yield of the shallow peat. The volume of gas calculated from the overpressuring method averaged 9%, assuming that the gas was distributed over a 2-m thick overpressured interval. The volume of gas using the BE method averaged 13%. Temporal changes in overpressuring and in BE indicate that the volume of gaseous-phase gas also changed with time, most likely because of rapid degassing (ebullition) that allowed sudden loss of gas to the atmosphere. Estimates of gas released during the largest ebullition events ranged from 0.3 to 0.7 mol m À2 d À1 . These ebullition events may contribute a significant source of methane and carbon dioxide to the atmosphere that has so far largely gone unmeasured by gas-flux chambers or tower-mounted sensors.

Eleven equations for calculating evaporation were compared with evaporation determined by the energy budget method for Williams Lake, Minnesota. Data were obtained from instruments on a raft, on land near the lake, and at a weather station 60 km south of the lake. The comparisons were based on monthly values for the open-water periods of 5 years, a total of 22 months. A modified DeBruin-Keijman, Priestley-Taylor, and a modified Penman equation resulted in monthly evaporation values that agreed most closely with energy budget values. To use these equations, net radiation, air temperature, wind speed, and relative humidity need to be measured near the lake. In addition, thermal surveys need to be made to determine change in heat stored in the lake. If data from distant climate stations are the only data available, and they include solar radiation, the Jensen-Haise and Makkink equations resulted in monthly evaporation values that agreed reasonably well with energy budget values. Jensen-Haise, cm d -• Makkink, cm d -• Mass transfer, cm d -• Papadakis, cm month -• Penman,* cal cm-2 d-• ß (eo -ea)] PET = (a/(a-1))1.141(7/(s + 7)) ß [(3.6 + 2.5(U3))(e0 -ea) ] PET = [SVP/(0.95SVP + 0.637)] '(Qn -Qx) PET = [0.55(D/12)2(SVD/100)]2.54 PET = {[((0.014Ta) -0.50)(Qs)]0.000673 }2.54 PET: [0.61(s/(s + 7))(Qs/L)] -0.012 cm d -• DeBruin [1978] DeBruin and Keijman [1979] Hamon [1961] McGuinness and Bordne [1972] McGuinness and Bordne [1972] Harbeck et al. [1958] McGuinness and Bordne [1972] Jensen et al. [ 1974] Stewart and Rouse [1976] McGuinness and Bordne [1972] PET = 0.5625[eomax-(e0min -2)] PET: (s/(s + 7))(Q,-Qx) + (7/(s + 7))[(15.36(0.5 + 0.01U2)) '(eo -ea)] PET = a(s/(s + 7))[(Qn -Qx)/L] PET = {[(0.0082Ta) -0.19] -(Q•/1500)}2.54 PET, for periods of 10 days or greater PET, daily PET, daily PET for periods greater than 5 days (Nebraska) PET, monthly (Holland) Evaporation, depending on calibration of N PET, monthly PET, for periods greater than 10 days PET for periods of 10 days or greater PET, monthly (Florida) Here, a = 1.26 is the Priestley-Taylor empirically derived constant, dimensionless; s/(s + 7) and 7/(s + 7) are parameters derived from the slope of the saturated vapor pressure-temperature curve at the mean air temperature; 7 is the psychrometric constant; Qn is net radiation (in calories per square centimeter per day); Q• is solar radiation (in calories per square centimeter per day); Qx is change in heat stored in the water body (in calories per square centimeter per day); U2 and U3 is wind speed at 2 or 3 m respectively, above surface (in meters per second); e 0 is saturated vapor pressure (in millibars); e a is vapor pressure at temperature and relative humidity of the air (in millibars); SVP is saturated vapor pressure at mean air temperature (in millibars per degree kelvin); SVD is saturated vapor density at mean air temperature (in grams per cubic meter); T a is air temperature, in degrees Fahrenheit for the Jensen-Haise and Stephens-Stewart equations; L is the latent heat of vaporization (in calories per gram)...

[1] Freshwater lakes are an important component of the global carbon cycle through both organic carbon (OC) sequestration and carbon dioxide (CO 2 ) emission. Most lakes have a net annual loss of CO 2 to the atmosphere and substantial current evidence suggests that biologic mineralization of allochthonous OC maintains this flux. Because net CO 2 flux to the atmosphere implies net mineralization of OC within the lake ecosystem, it is also commonly assumed that net annual CO 2 emission indicates negative net ecosystem production (NEP). We explored the relationship between atmospheric CO 2 emission and NEP in two lakes known to have contrasting hydrologic characteristics and net CO 2 emission. We calculated NEP for calendar year 2004 using whole-lake OC and inorganic carbon (IC) budgets, NEP OC and NEP IC , respectively, and compared the resulting values to measured annual CO 2 flux from the lakes. In both lakes, NEP OC and NEP IC were positive, indicating net autotrophy. Therefore CO 2 emission from these lakes was apparently not supported by mineralization of allochthonous organic material. In both lakes, hydrologic CO 2 inputs, as well as CO 2 evolved from net calcite precipitation, could account for the net CO 2 emission. NEP calculated from diel CO 2 measurements was also affected by hydrologic inputs of CO 2 . These results indicate that CO 2 emission and positive NEP may coincide in lakes, especially in carbonate terrain, and that all potential geologic, biogeochemical, and hydrologic sources of CO 2 need to be accounted for when using CO 2 concentrations to infer lake NEP.