Terrestrial dissolved organic matter inputs accompanied by dissolved oxygen depletion and declining pH exacerbate CO2 emissions from a major Chinese reservoir.

Terrestrial inputs and subsequent degradation of dissolved organic matter (DOM) in lake ecosystems can result in rapid depletion of dissolved oxygen (DO). Inputs of terrestrial DOM including organic acids can also lead to decreases in pH. However, to date, few studies have investigated the linkages between terrestrial DOM inputs, DO and pH levels in the water column, and carbon dioxide (CO2) emissions from lake ecosystems. Based on monthly field sampling campaigns across 100 sites in Lake Qiandao, a major man-made drinking water reservoir in China, from May 2020 to April 2021, we estimated an annual CO2 efflux (FCO2) of 37.2 ± 29.0 gC m-2 yr-1, corresponding to 0.02 ± 0.02 TgC yr-1 from this lake. FCO2 increased significantly with decreasing DO, chlorophyll-a (Chl-a) and δ2H-H2O, while FCO2 increased with increasing specific UV absorbance (SUVA254) and a terrestrial humic-like component (C2). We found that DO concentration and pH declined with increasing terrestrial DOM inputs, i.e. increased SUVA254 and terrestrial humic-like C2 levels. Vertical profile sampling revealed that the partial pressure of CO2 (pCO2) increased with increasing terrestrial DOM fluorescence (FDOM), while DO, pH, and δ13C-CO2 declined with increasing terrestrial FDOM. These results highlight the importance of terrestrial DOM inputs in altering physico-chemical environments and fueling CO2 emissions from this lake and potentially other aquatic ecosystems.

Potential of Mie-Fluorescence-Raman Lidar to Profile Chlorophyll a Concentration in Inland Waters.

Vertical distribution of phytoplankton is crucial for assessing the trophic status and primary production in inland waters. However, there is sparse information about phytoplankton vertical distribution due to the lack of sufficient measurements. Here, we report, to the best of our knowledge, the first Mie-fluorescence-Raman lidar (MFRL) measurements of continuous chlorophyll a (Chl-a) profiles as well as their parametrization in inland water. The lidar-measured Chl-a during several experiments showed good agreement with the in situ data. A case study verified that MFRL had the potential to profile the Chl-a concentration. The results revealed that the maintenance of subsurface chlorophyll maxima (SCM) was influenced by light and nutrient inputs. Furthermore, inspired by the observations from MFRL, an SCM model built upon surface Chl-a concentration and euphotic layer depth was proposed with root mean square relative difference of 16.5% compared to MFRL observations, providing the possibility to map 3D Chl-a distribution in aquatic ecosystems by integrated active-passive remote sensing technology. Profiling and modeling Chl-a concentration with MFRL are expected to be of paramount importance for monitoring inland water ecosystems and environments.

[Evaluation of ecosystem services of the Thousand-Island Lake, Zhejiang, China.] / 千岛湖生态系统服务价值评估.

We quantitatively evaluated Thousand-Island Lake's aquatic ecosystem service value and analyzed its developing trend at the whole lake scale. We built an index system to evaluate the ecosystem service of Thousand-Island Lake, which included 16 indicators of 10 categories. The results showed that the total value of ecosystem services of Thousand-Island Lake was 49.3 billion and 57.2 billion yuan in 2007 and 2017, respectively. The highest value of Thousand-Island Lake's ecosystem service was its regulating function, which played an absolutely dominant role. Ranked according to their values, different functions of the final ecosystem services were in a descending order of floodwater regulation, surface water regulation, carbon sequestration (reduction), oxygen generation, tourism, water purification, fishing, energy saving through water source heat pump, water for hydropower, agriculture, forestry and animal husbandry uses of water resources, ecological and public use of water resources, water source for industrial use, volume of freight transportation, water source for city resident use, water circulation cooling, and passenger transportation. Thousand-Island Lake provided a total ecosystem services value of 86.0 million and 99.8 million yuan·km-2, which was 47.6 and 17.3 times as the GDP per area in Chun'an County in 2007 and 2017.