ALBERT

Description: Quantifying the role that freshwater ecosystems play in the global carbon cycle requires accurate measurement and scaling of dissolved organic carbon (DOC) removal in river networks. We reviewed reach-scale measurements of DOC uptake from experimental additions of simple organic compounds or leachates to inform development of aquatic DOC models that operate at the river network, regional, or continental scale. Median DOC uptake velocity ( v f ) across all measurements was 2.28 mm min -1 . Measurements using simple compound additions resulted in faster v f (2.94 mm min -1 ) than additions of leachates (1.11 mm min -1 ). We also reviewed published data of DOC bioavailability for ambient stream water and leaf leachate DOC from laboratory experiments. We used these data to calculate and apply a correction factor to leaf leachate uptake velocity to estimate ambient stream water DOC uptake rates at the reach-scale. Using this approach, we estimated a median ambient stream DOC v f of 0.26 mm min -1 . Applying these DOC v f values (0.26, 1.11, 2.28, and 2.94 mm min -1 ) in a river network inverse model in seven watersheds revealed that our estimated ambient DOC v f value is plausible at the network scale and 27 to 45% of DOC input was removed. Applying the median measured simple compound or leachate v f in whole river networks would require unjustifiably high terrestrial DOC inputs to match observed DOC concentrations at the basin mouth. To improve the understanding and importance of DOC uptake in fluvial systems, we recommend using a multi-scale approach coupling laboratory assays, with reach-scale measurements, and modeling.

Description: To understand the carbon and energy exchange between the lake surface and the atmosphere, direct measurements of latent, sensible heat and CO 2 fluxes were taken using the eddy covariance (EC) technique in western Lake Erie during October 2011-September 2013. We found that the latent heat flux ( LE ) had a marked one-peak seasonal change in both years that differed from the diurnal course and lacked a sinusoidal dynamic common in terrestrial ecosystems. Daily mean LE was 4.8±0.1 and 4.3±0.2 MJ m -2 d -1 in Year 1 and Year 2, respectively. The sensible heat flux ( H ) remained much lower than the LE , with a daily mean of 0.9±0.1 and 1.1±0.1 MJ m -2 d -1 in Year 1 and Year 2, respectively. As a result, the Bowen ratio was 〈1 during most of the two-year period, with the lowest summer value at 0.14. The vapor pressure deficit explained 35% of the variation in half-hourly LE , while the temperature difference between the water surface and air explained 65% of the variation in half-hourly H . Western Lake Erie acted as a small carbon sink holding -19.0±5.4 and -40.2±13.3 g C m -2 in the first and second summers (May-September) but as an annual source of 77.7±18.6 and 49.5±17.9 g C m -2 yr -1 in Year 1 and Year 2, respectively. The CO 2 flux ( ) rate varied from -0.45 g C m -2 d -1 to 0.98 g C m -2 d -1 . Similar to LE , had noticeable diurnal changes during the months that had high chlorophyll a months, but not during other months. A significantly negative correlation ( P 〈0.05) was found between and chlorophyll a on monthly fluxes. Three gap-filling methods, including marginal distribution sampling (MDS), mean diurnal variation (MDV) and monthly mean, were quantitatively assessed, yielding an uncertainty of 4%, 6% and 10% in LE , H and , respectively.