[1] Carbon (C) cycling in Lake Superior was studied within the Keweenaw Interdisciplinary Transport Experiment in Superior (KITES) project to assess (1) whether the lake is net heterotrophic, (2) sources, sinks and residence time for dissolved organic carbon (DOC), (3) importance of terrigenous organic C subsidies, and (4) factors limiting C flow through bacteria. During 3 years of fieldwork, measurements were made of spatial and temporal distributions of C pools and rates of photosynthesis, community respiration, and bacterial production. Measurements were made of the composition of dissolved organic matter (DOM), rates of DOM photolysis, lability of DOM toward microbial consumption, and river inputs of DOM. All measurements suggest the lake is net heterotrophic. The C:N ratios of DOM suggest that it is primarily of terrigenous origin, but other characteristics (size distribution, UV absorption) point to the presence of autochthonous DOM and to alteration of terrigenous material. The lake mass balance indicates that the residence time ($8 years) of the DOC pool (17 Tg) is short relative to the hydrologic residence time (170 years). The known flux of terrigenous DOC ($1 Tg/yr) is too low to support annual bacterial carbon demand (6-38 Tg/yr), but microbial respiration is the major sink for terrigenous DOC. A rapidly cycling, autochthonous DOC pool must exist. Microbial activity was correlated with temperature, phosphorus availability, and DOC concentration but not with photosynthesis rates. Measurements of respiration ($40 Tg/yr), photosynthesis (2-7 Tg/yr), and bacterial production (0.5-2 Tg/yr) are not all mutually compatible and result in a discrepancy in the organic carbon budget.

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Because of the vast areal extent and high rates of organic matter production of peatlands, such ecosystems have a significant but poorly quantified impact on large numbers of lakes. This study reports the export of dissolved organic carbon (DEC) and acidity from two bogs and one fen in north central North America. Rates of export of Dec are high for all three peatlands (8-40 g m -2 yr-•), and variations are largely dependent on water yield. Dissolved organic matter from all sites exhibits a charge density between 7 and 24/xeq (mg Dec) -l, and rates of acidity export from all three sites are similar (100-200 meq m -2 yr-•). Variation in elemental composition of dissolved organic matter suggests a seasonal change in relative proportions of organic matter fractions and indicates direct incorporation of microbially reduced sulfur into organic matter. Current definitions and measurement techniques for alkalinity are inappropriate for such high Dec waters, and fixed end point titration to pH 4 is suggested as an alternative. organic carbon (DEC) from a bog, 8.6 g m -2 yr-• [McKnight et al., 1985], is high relative to Dec export from either mineral soils or southern swamps [e.g., Mulholland and Kuenzler, 1979; Tate and Meyer, 1983; Naiman, 1982]. Concentrations of Dec typically are higher in midwestern bogs relative to fens [Verry, 1975; Sparling, 1967; Glaser et al., 1981] or to maritime bogs [Gorham et al., 1985], but the relative magnitudes of export from these peatland types are not known. Variability in rates of Dec export among peatlands of a single type has not been assessed, and little is known about seasonal dynamics. Kerekes et al. [1986] observed peak concentrations of Dec in summer in Nova ScotJan streams receiving drainage from peatlands, but Glover and Webb [1979] reported maximum weak acid concentrations in spring in a river in southern Norway. On the basis of the few existing measurements and without knowledge of factors controlling the magnitude of DOM export, we cannot predict the export from a single wetland type much less generalize about other wetland types. Second, lack of knowledge about differences in the nature of DOM from peatlands and other sources renders unclear the extent of characterization that is necessary. Thurman [1985] reported that DOM from bogs typically is composed of a greater fraction as fulvic acids (60%) than DOM from other surface or ground waters. McKnight et al. [ 1985] found DOM in Thoreau's bog to be dominated by hydrophobic acids (60% of Dec, primarily as fulvic acids), while hydrophilic acids accounted for 30% of the Dec. Bourbonniere [ 1987] reported generally similar results from fractionation of bog and fen waters in Quebec and Nova Scotia, but he found that hydrophobic and hydrophilic neutral fractions also were important components of the DOM. Oliver et al. [1983] and Steinberg and Muenster [1985] reported that humic acids from bogs tend to have low carboxyl contents and high phenolic contents. Thurman [1985] reported that humic and fulvic acids from bogs are defici...

Abstract.The mass balance and internal cycle of sulfur within a small forested, Sphagnum bog in northern Minnesota are presented here based on a 4-year record of hydrologic inputs and outputs (precipitation, throughfall, streamflow, upland runoff) and a 3-year measurement of plant growth and sulfur uptake. Concentrations and accumulation rates of inorganic and organic sulfur species were measured in porewater. The bog is a large sink for sulfur, retaining 37% of the total sulfur input. Because of the relatively large export of organic S (21% of inputs), retention efficiency for total-S (organic S + SO:; 37%) is less than that for SOT (58%). There is a dynamic cycle of oxidation and reduction within the bog. Annual oxidation and recycling of S is equal to total inputs in the center of the bog. Plants receive 47% of their uptake requirement from atmospheric deposition, 5% from retranslocation from foliage, and the remainder from sulfur remineralized from peat. Mineralization is most intense in the aerobic zone above the water table. Inorganic sulfur species comprise < 5% of the total sulfur burden within the peat.