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The Northern Highland Lake District of Wisconsin contains thousands of kettle lakes that were formed during the last period of continental glaciation, roughly 10,000 years BP. It is a hummocky region of pitted outwash with low topographic relief (~40m) that spans several river basins, including headwaters of the Wisconsin, Chippewa (Mississippi) and Presque Isle rivers (Superior). The region is sparsely populated, covered with mixed hardwood/coniferous forest and underlain by 30 to 60 m-thick deposits of glacial drift (Attig, 1985). Due to the low relief, poorly integrated surface flow and the variable drift composition, wetlands are abundant (Webster et al., 2006).

Historically, northern wetlands have served as a large sink for carbon, storing vast amounts in deep organic peat. Sphagnum mosses and other wetland plants remove CO2 from the atmosphere during photosynthesis and convert it to biomass. Due to the low temperature and water-logged condition of wetland soils, decomposition of plant biomass is slow and large amounts of peat accumulate. Although global estimates of carbon storage are highly variable, some studies indicate that northern wetlands alone may hold ~500 gigatonnes of carbon, an amount comparable to the total carbon pool in the atmosphere (~700 gigatonnes) (Mitra et al., 2005).

The position of the water table is critical to peat preservation and carbon storage. When the water table is high, rates of peat decomposition are low – and vice versa. This is because the concentration of oxygen is much lower in water than in air. When the water table drops, air penetrates the peat and rates of decomposition accelerate. An increase in temperature can further accelerate decomposition, and if the water table then suddenly rises during an intense storm, the labile decomposition products can be mobilized and subsequently returned to the atmosphere as gaseous carbon.

Field data indicate that the amount of dissolved organic carbon in boreal lakes and streams has increased during the last decade, possibly due to increased peat decomposition and mobilization (Evans et al., 2006). A modeling study of Canadian wetlands indicated that a warming of 4 oC in northern bogs would cause a 40% carbon release from shallow peat and an 86% release from deep peat (Ise et al., 2008). More recent empirical studies indicate that carbon release rates from warming peatlands may be even higher than these model estimates (Dorrepaal et al, 2009).The timing and magnitude of net carbon export from wetlands remains challenging to quantify because we lack the data to sufficiently calibrate predictive models that couple carbon and hydrology.

In this project, wireless sensors have been embedded in two small wetland catchments to monitor the interaction between water table fluctuations, temperature and carbon export as weather and climate change. The two study catchments are located within the Trout Lake watershed, which has been intensively studied since 1982 as part of the NSF-LTER northern temperate lakes program (Magnuson et al., 2006; http://lter.limnology.wisc.edu/). The watershed area is 130 km2. It comprises 115 lakes and ponds with a total surface area of ~30 km2. Wetlands constitute ~7% of the terrestrial surface. Across the watershed, groundwater elevations vary from about 514 masl to 492 masl, with Trout Lake being the terminal discharge point.

The Crystal Bog sub-catchment (CB) is situated at a relatively high elevation near the top of the Trout Lake watershed It comprises a weakly minerotrophic fen (7 ha) surrounding a small bog pond (0.5 ha) that has no channelized inflow or outflow. The sub-catchment overlies deep outwash and glacial till typical of the region. The peatland surrounding the bog pond varies in thickness from 2m to 10m, and groundwater discharge has been estimated to constitute 3% to 17% of the catchment’s annual water budget (Kratz and Medland 1989). Direct precipitation is the dominant hydrologic input to the sub-catchment as a whole. Water quality characteristics of the CB pond include: pH = 5.2 ± 0.2, ANC = 12 ± 19 μeq L-1, DOC = 8.9 ± 0.9 mg L-1; SO4 = 0.5 ± 0.2 mg L-1, conductivity = 10.5 ± 2.3 μS cm-1 (mean ± SD:

The Trout Bog sub-catchment (TB) lies at a relatively low elevation near the bottom of the Trout Lake watershed. This sub-catchment also comprises a moderately minerotrophic fen surrounding a small bog pond. The peatland vegetation and hydrology are not yet well characterized, but the bog pond is distinctly more acidic and more darkly stained than Crystal Bog. As with CB, the TB pond is small has no tributary or distributary streams. Water quality characteristics of the TB pond include: pH = 4.8 ± 0.2, ANC = -7 ± 32 μeq L-1, DOC = 22 ± 2.8 mg L-1; SO4 = 0.9 ± 0.4 mg L-1, conductivity =23 μS cm-1 (mean ± SD).

In addition to being long term research sites within the NSF NTL-LTER program, both Crystal Bog and Trout Bog are also currently being studied as part of the GLEON program (http://www.gleon.org/).




Characteristics of the two primary study sites in northcentral Wisconsin.
Surface Area (ha) Pond Morphometry
Study Site Peatland Pond Max Depth (m) Mean Depth (m) Volume (1000 m3)
Crystal Bog 7.0 0.5 2.5 1.7 8.5
Trout Bog 4.5 1.1 7.9 5.6 61.6




References Attig J.W. 1985. Pleistocene Geology of Vilas County, Wisconsin. Wisconsin Geologic and Natural History Survey, 50, 32p.

Dorrepaal, E. et al. 2009. Carbon respiration from subsurface peat accelerated by climate warming in the subarctic. Nature 460: 616-619.

Evans, C.D. et al. 2006. Alternative explanations for rising dissolved organic carbon export from organic soils. Global Change Biology 12:2044-2053.

Ise, T. et al. 2008. High sensitivity of peat decomposition to climate change through water-table feedback. Nature Geoscience 1, 763-766

Kratz T.K. and Medland V.L. 1989. Relationship of landscape position and groundwater input in northern Wisconsin kettle-hole peatlands. In: Sharitz R.R. and Gibbons J.W., editors. Freshwater Wetlands and Wildlife . p. 1141-1151.

Magnuson J.J., Kratz T.K., Benson B.J., eds. 2006.
Mitra, S. et al. 2005. An appraisal of global wetland area and its organic carbon stock. Current Science 88:25-38.

Webster, K.E., Bowser, C.J., Anderson, M.P. and Lenters, J.D. 2006. Understanding the lake-groundwater system: Just follow the water. In Long-Term Dynamics of Lakes in the Landscape. Edited by J.J. Magnuson, G.J. Kenoyer and B.J. Benson. Oxford University Press. pp. 19-48.