Diurnal to annual changes in latent, sensible heat, and CO₂ fluxes over a Laurentian Great Lake: A case study in Western Lake Erie

Abstract

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 to 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 À2 d À1 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 À2 d À1 in Year 1 and Year 2, respectively. As a result, the Bowen ratio was <1 during most of the 2 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 À19.0 ± 5.4 and À40.2 ± 13.3 g C m À2 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 À2 yr À1 in Year 1 and Year 2, respectively. The CO 2 flux (F CO2 ). Similar to LE, F CO2 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 F CO2 and chlorophyll a on monthly fluxes. Three gap-filling methods, including marginal distribution sampling, mean diurnal variation, and monthly mean, were quantitatively assessed, yielding an uncertainty of 4%, 6%, and 10% in LE, H, and F CO2 , respectively.

The monsoon effect on energy and carbon exchange processes over a highland lake in southwest of China

“…According to the quality control procedure presented above, around 34% of the Hs, 46% of LE and 45% of CO 2 fluxes are removed and the retained data are analyzed in our study. Although the gap ratio is large, it's similar to other studies over 5 lakes (Nordbo et al, 2012;Goldbach and Kuttler, 2015;Shao et al, 2015). A long large gap between August and November of exists in 2014 due to malfunction.…”

“…The rain is also observed to be closely correlated with CO 2 flux on daily scale during pre-monsoon period and on monthly scale during post-monsoon period. It has been reported that the more rain could bring more nutrients in to the water body, which will promote the photosynthesis of the phytoplankton (Shao et al, 2015). As only minor 25 rain falls during these two periods, the role of rain becomes more important.…”

“…The uptake of CO 2 flux may be caused by the phytoplankton due to the eutrophication of Lake Erhai. It has also been reported that the shallow lake is more affected 20 by the rich phytoplankton (Huotari et al, 2011;Shao et al, 2015).…”

“…During monsoon period, the main drivers for monthly CO 2 flux (i.e., Rn, Ta and U) are in good accordance with LE. The relationship between Ts and CO 2 flux is likely to be attributed to that the variation of Ts could 20 influence the solubility in the water (Shao et al, 2015). The △e also has a large effect on monthly CO 2 flux during both pre-monsoon and post-monsoon periods.…”

“…There are 304 million lakes globally and they are of significant importance in determining local weather and climate through complex physical, biochemical and biological interactions (Cole et al, 2001;Downing et al, 2006;Shao et al, 2015). Because of the substantial differences in underlying surface characteristics between lake surface and its surrounding land surface (i.e., albedo, roughness, 5 heat capacity) (O'Donnell et al, 2010), the carbon and energy exchange processes over lakes are expected to produce a different way in responding to climate change.…”