Diel cycles in the fluorescence of dissolved organic matter in dystrophic Wisconsin seepage lakes: Implications for carbon turnoverby C. J. Watras, K. A. Morrison, J. T. Crawford, C. P. McDonald, S. K. Oliver, P. C. Hanson

Limnol. Oceanogr.

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Year
2015
DOI
10.1002/lno.10026
Subject
Aquatic Science / Oceanography

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Diel cycles in the fluorescence of dissolved organic matter in dystrophic Wisconsin seepage lakes: Implications for carbon turnover

C. J. Watras,*1,2 K. A. Morrison,1,2 J. T. Crawford,2 C. P. McDonald,3 S. K. Oliver,2 P. C. Hanson2 1Wisconsin Department of Natural Resources, Fishery and Aquatic Science Section, University of Wisconsin Trout Lake

Station, Boulder Junction, Wisconsin 2Center for Limnology, University of Wisconsin-Madison, Madison, Wisconsin 3Wisconsin Department of Natural Resources, Fishery and Aquatic Science Section, Madison, Wisconsin

Abstract

We monitored the dynamics of chromophoric dissolved organic matter (CDOM) fluorescence in two Wisconsin bog lakes over timescales ranging from hours to months. Peatland-derived dissolved organic matter (DOM) was the major solute in both bog lakes, and diel cycles were dominant features of the CDOM fluorescence time series. Two distinct diel cycles that differed in amplitude and timing were observed: one in oxic epilimnia and a second in anoxic, hypolimnetic waters. These cycles were not attributable to instrumental artifacts (i.e., daily oscillations of temperature, ambient light, or battery voltage), hydrologic forcing, or the effects of inner filtering, pH, or redox conditions. High light extinction coefficients, especially in the ultraviolet region ( 10 m21 to 30 m21), suggest that DOM photolysis was negligible at the depths of the CDOM fluorescence probes in these dark-water lakes (dissolved carbon concentration: 10 mg C L21 to 20 mg C L21). The diel cycles were apparently governed primarily by biological activities that mediate DOM production (release) and destruction (uptake). Rates of carbon turnover derived from properties of the epilimnetic CDOM fluorescence cycle (0.28 mg C L21 d21) were similar to rates of net ecosystem production based on daily CO2 dynamics (0.32 mg C L21 d21). It appears that a small, secondary pool of labile organic carbon turns over at relatively high rates in these bog lakes, consistent with the two-compartment view of DOM stability.

Dissolved organic matter (DOM) is an important constituent of aquatic ecosystems given its role in the aquatic carbon cycle, light attenuation, metal complexation, acid–base chemistry, and microbial metabolism. Until recently, research into the quantity and quality of DOM has typically required discrete sample collection and laboratory analyses, thus limiting the temporal and spatial scale of many studies.

The development of in situ optical probes that measure the absorbance or fluorescence of chromophoric dissolved organic matter (CDOM fluorescence) has enabled this component of DOM to be measured at high frequency and high resolution opening promising new avenues of research on a fundamental property of lakes and rivers. Using fluorescence probes, several investigators have observed diel CDOM cycles in freshwater environments. These cycles have been attributed to such factors as the diel entrainment of hypolimnetic waters (Gibson et al. 2001), a combination of photochemical and biologically mediated processes (Spencer et al. 2007; Saraceno et al. 2009), and the daily cycling of external hydrologic loads (Pellerin et al. 2012). Potential interactions between multiple processes currently limit interpretations of

CDOM fluorescence patterns in lakes and other freshwater environments.

Diel cycles are commonly observed in freshwaters due to biological and physical rhythms that are related, either directly or indirectly, to the daily cycle of solar radiation.

Specific examples include diel oscillations in pH, nitrate and other N species, metals, dissolved gases, chlorophyll a (Chl a), and stable isotopes (Nimick et al. 2011) as well as diel vertical migrations (DVM) of plankton. Many of these cycles involve or influence DOM and CDOM fluorescence. Diel cycles of CDOM fluorescence may indicate fluctuations in

DOM quantity, as might result from cyclical hydrologic inputs or the biological release and uptake of DOM. Alternatively, they may result from fluctuations in the optical properties of a relatively constant pool of DOM molecules as ambient conditions change (e.g., fluorescence quench).

Although, most in situ CDOM probes cannot discriminate unequivocally between changes in DOM quantity vs. changes in DOM quality, calibration against water samples of known dissolved organic carbon (DOC) concentration can be used to quantify high frequency changes in DOM and relate them to ecosystem processes in individual lakes or rivers. For example, Spencer et al. (2012) showed that DOC*Correspondence: cjwatras@wisc.edu 482

LIMNOLOGY and

OCEANOGRAPHY Limnol. Oceanogr. 60, 2015, 482–496VC 2015 Association for the Sciences of Limnology and Oceanography doi: 10.1002/lno.10026 concentrations in many U.S. rivers exhibit strong linear relationships with CDOM absorbance (r20.7), thus allowing high resolution estimates of river-specific DOM export. It is also possible to extract information on the chemical properties of DOM by measuring absorbance and/or fluorescence across multiple wavelengths, thereby assessing DOM quality on the basis of molecular weight, aromaticity, and/or origin (McKnight et al. 2001; Helms et al. 2008; Carter et al. 2012).

Although these analyses are typically done in the laboratory, the deployment of multichannel in situ CDOM sensors has been advanced by Spencer et al. (2007) and Sandford et al. (2010). Given the high sensitivity of fluorescence sensors, they can potentially quantify small changes in DOM concentration that are within the measurement error of standard wet oxidation techniques (Aiken 1992).

Here, we describe the use of single channel, in situ fluorescence sensors to detect and disentangle factors potentially driving the temporal variability of DOM in dystrophic seepage lakes over timescales ranging from hours to months. We focus on seepage lakes to minimize the effect of variable hydrologic loads; and we focus on peatland-dominated lakes to emphasize the terrigenous fraction of DOM. By combining high frequency CDOM fluorescence data collected at multiple depths with ancillary limnological data and targeted field and laboratory experiments, we describe two different diel CDOM cycles and investigate relationships with ambient light intensity, temperature, pH, redox, inner filtering, and lake metabolism. We compare diel CDOM oscillations in oxic epilimnia to those in an aphotic, anoxic hypolimnion, and we investigate mechanisms that could potentially account for their differences. Finally, we explore implications with respect to internal DOM processing in peatland-dominated lakes.