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Madison, WI, United States

Watras C.J.,Fishery and Aquatic Science Section | Watras C.J.,University of Wisconsin - Madison | Morrison K.A.,Fishery and Aquatic Science Section | Lottig N.R.,University of Wisconsin - Madison | Kratz T.K.,University of Wisconsin - Madison
Canadian Journal of Fisheries and Aquatic Sciences | Year: 2015

The cycling of organic carbon is fundamental to aquatic ecosystems, reflecting processes that extend from terrestrial watersheds to fish. Here, we use embedded fluorescence sensors that sample at high frequency to investigate the daily dynamics of a proxy for the major pool of organic carbon (chromophoric dissolved organic matter, CDOM) in a clear-water Wisconsin lake (~3 mg C·L-1). We compare the diel CDOM cycle in this lake with cycles observed previously in two dark-water lakes (10 to 20 mg C·L-1). Despite differences in DOM quality and quantity, diel fluorescence cycles were evident in the epilimnia and hypolimnia of all three lakes. The amplitude differed among lakes, but the timing of the diel cycles was similar, with increases in fluorescence during nighttime and decreases during daylight (except in the aphotic hypolimnion of the darkest lake). The amplitude of the diel cycle increased with increasing DOM concentration, and estimates of DOM turnover based on the magnitude of oscillation ranged from 0.28 mg C·L-1·day-1 in the darkest lake to 0.14 mg C·L-1·day-1 in the clear lake. Independent estimates of free water metabolism based on the daily dynamics of O2 or CO2 were in general agreement, ranging from 0.32 to 0.06 mg C·L-1·day-1. Although absolute rates of turnover varied directly with DOM concentration, relative rates were highest in clear waters (~5%·day-1).Weconclude that these daily oscillations may be a common property of lakes and that they may provide insights into internal DOM processing over short time scales. © 2015, National Research Council of Canada. All rights reserved.

Watras C.J.,Fishery and Aquatic Science Section | Watras C.J.,University of Wisconsin - Madison | Morrison K.A.,Fishery and Aquatic Science Section | Morrison K.A.,University of Wisconsin - Madison | And 2 more authors.
Journal of Hydrology | Year: 2016

Evaporation (E) dominates the loss of water from many small lakes, and the balance between precipitation and evaporation (P–E) often governs water levels. In this study, evaporation rates were estimated for three small Wisconsin lakes over several years using 30-min data from floating evaporation pans (E-pans). Measured E was then compared to the output of mass transfer models driven by local conditions over daily time scales. The three lakes were chosen to span a range of dissolved organic carbon (DOC) concentrations (3–20 mg L−1), a solute that imparts a dark, tea-stain color which absorbs solar energy and limits light penetration. Since the lakes were otherwise similar, we hypothesized that a DOC-mediated increase in surface water temperature would translate directly to higher rates of evaporation thereby informing climate response models. Our results confirmed a DOC effect on surface water temperature, but that effect did not translate to enhanced evaporation. Instead the opposite was observed: evaporation rates decreased as DOC increased. Ancillary data and prior studies suggest two explanatory mechanisms: (1) disproportionately greater radiant energy outflux from high DOC lakes, and (2) the combined effect of wind speed (W) and the vapor pressure gradient (es − ez), whose product [W(es − ez)] was lowest on the high DOC lake, despite very low wind speeds (<1.5 m s−1) and steep forested uplands surrounding all three lakes. Agreement between measured (E-pan) and modeled evaporation rates was reasonably good, based on linear regression results (r2: 0.6–0.7; slope: 0.5–0.7, for the best model). Rankings based on E were similar whether determined by measured or modeled criteria (high DOC < low DOC). Across the 3 lakes and 4 years, E averaged ∼3 mm d−1 (C.V. 9%), but statistically significant differences between lakes resulted in substantial differences in cumulative E that were consistent from year to year. Daily water budgets for these lakes show that inputs were dominated by P and outputs by E; and our findings indicate that subtle changes in the variables that drive E can have measurable effects on water levels by shifting the balance between P and E. © 2016 Elsevier B.V.

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