Comparing the diel cycles of dissolved organic matter fluorescence in a clear-water and two dark-water Wisconsin lakes: potential insights into lake metabolism

• Published in: Canadian journal of fisheries and aquatic sciences 2016-01, Vol.73 (1), p.65-75
• Main Authors: Watras, Carl J, Morrison, Kenneth A, Lottig, Noah R, Kratz, Timothy K
• Format: Article
• Language: English
• Subjects: Aquatic ecosystems; Carbon; Carbon cycle (Biogeochemistry); Comparative analysis; Dissolved organic matter; Diurnal cycles (Earth sciences); Environmental aspects; Fluorescence; Hypolimnion; Lakes; Metabolism; Organic carbon; Watersheds
• ISSN: 0706-652X; 1205-7533
• DOI: 10.1139/cjfas-2015-0172

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 O 2 or CO 2 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 ). We conclude 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.