Drivers of spatial and seasonal variations of CO2 and CH4 fluxes at the sediment water interface in a shallow eutrophic lake.

Shallow eutrophic lakes contribute disproportional to the emissions of CO2 and CH4 from inland waters. The processes that contribute to these fluxes, their environmental controls, and anthropogenic influences, however, are poorly constrained. Here, we studied the spatial variability and seasonal dynamics of CO2 and CH4 fluxes across the sediment-water interface, and their relationships to porewater nutrient concentrations in Lake Ulansuhai, a shallow eutrophic lake located in a semi-arid region in Northern China. The mean concentrations of CO2 and CH4 in porewater were 877.8 ± 31.0 µmol L-1 and 689.2 ± 45.0 µmol L-1, which were more than 50 and 20 times higher than those in the water column, respectively. The sediment was always a source of both gases for the water column. Porewater CO2 and CH4 concentrations and diffusive fluxes across the sediment-water interface showed significant temporal and spatial variations with mean diffusive fluxes of 887.3 ±124.7 µmol m-2 d-1 and 607.1 ± 68.0 µmol m-2 d-1 for CO2 and CH4, respectively. The temporal and spatial variations of CO2 and CH4 concentrations in porewater were associated with corresponding variations in dissolved organic carbon and dissolved nitrogen species. Temperature and dissolved organic carbon in surface porewater were the most important drivers of temporal variations in diffusive fluxes, whereas dissolved organic carbon and nitrogen were the main drivers of their spatial variations. Diffusive fluxes generally increased with increasing dissolved organic carbon and nitrogen in the porewater from the inflow to the outflow region of the lake. The estimated fluxes of both gases at the sediment-water interface were one order of magnitude lower than the emissions at the water surface, which were measured in a companion study. This indicates that diffusive fluxes across the sediment-water interface were not the main pathway for CO2 and CH4 emissions to the atmosphere. To improve the mechanistic understanding and predictability of greenhouse gas emissions from shallow lakes, future studies should aim to close the apparent gap in the CO2 and CH4 budget by combining improved flux measurement techniques with process-based modeling.

Eutrophication decreased CO2 but increased CH4 emissions from lake: A case study of a shallow Lake Ulansuhai.

Eutrophic lakes, especially shallow eutrophic lakes, disproportionately contribute to greenhouse gas (GHG) emissions. To investigate the effects of eutrophication on GHG dynamics, we conducted field measurements every three months from January 2019 to October 2019 in Lake Ulansuhai, a shallow eutrophic lake (mean depth of 0.7 m) located in a semi-arid region in Northern China. We found that Lake Ulansuhai was a predominantly source of atmospheric carbon dioxide (CO2); however, it converted to a CO2 sink in July due to eutrophication. It was also a strong source of methane (CH4) with a mean CO2 emission of 35.7 ± 12.1 mmol m-2 d-1 and CH4 emission of 5.9 ± 2.9 mmol m-2 d-1. The CO2 concentrations in most sites and CH4 concentrations in all sites were supersaturated, with the average partial pressure of CO2 (pCO2) being 654±34 µatm and the partial pressure of CH4 (pCH4) being 157±37 µatm. The partial pressures and emissions of the greenhouse gases exhibited substantial seasonal and spatial variations. The correlation analysis between the trophic level index and the partial pressure of the greenhouse gases indicated that eutrophication could significantly decrease the CO2 emissions but increase the CH4 emissions from the lake, resulting in a CH4 and CO2 emission ratio of approximately 2 in terms of global warming potential. Eutrophication decreased the pCO2 in the lake and subsequently increased the pCH4 due to nutrient input, thereby enhancing primary production. The results indicated that shallow eutrophic lakes in arid regions are strong sources of CH4 and that eutrophication could alter the greenhouse gas emission patterns.

Spatiotemporal variability of phytoplankton functional groups in a shallow eutrophic lake from cold, arid regions.

In the shallow eutrophic lakes in cold, arid regions, the phytoplankton functional groups and the factors that drive their spatiotemporal variabilities remain unclear. Samples were collected from Lake Ulansuhai in April, August, and October 2017 (wet season) and January 2018 (dry season). Based on the functional group classification method, 23 phytoplankton functional groups with 5 major ones were identified. During the wet season, high amounts of nutrients, elevated temperatures, and heavy rainfall produced spatiotemporal variabilities in phytoplankton communities, whereas during the dry season, the frozen period was the critical factor that determined the spatiotemporal variabilities in the phytoplankton communities. Through redundancy analyses, total nitrogen and total phosphorus concentrations were observed to directly affect the phytoplankton growth; algal growth affected the chemical oxygen demand, and pH and environmental factors interacted with the phytoplankton growth. These results highlight the complex feedbacks of shallow eutrophic lake ecosystems in arid regions. Group TC (represented by Lyngbya) was correlated with Huangtai algae. In August, a Huangtai algal bloom resulted in a relatively stable water column, which was conducive to group TC growth. Therefore, the presence of certain phytoplankton functional groups can indicate the current lake conditions by identifying the coverage of Huangtai algae, which provides a scientific basis for an early warning of a potential algal bloom.