Mingliang Liu

The contribution of phytoplankton degradation to chromophoric dissolved organic matter (CDOM) in eutrophic shallow lakes: Field and experimental evidence

Eight field campaigns in the eutrophic, shallow, Lake Taihu in the summers from 2005 to 2007, and a phytoplankton degradation experiment of 33 days, were carried out to determine the contribution of phytoplankton degradation to CDOM. Significant and positive correlations were found between the CDOM absorption coefficient at 355 nm [a(CDOM)(355)], normalized fluorescence emission (QSU) at 450 nm from excitation at 355 nm [F(n)(355)], and the chlorophyll a (Chla) concentration for all eight field campaigns, which indicates that the decomposition and degradation of phytoplankton is an important source of CDOM. In the degradation experiment, the CDOM absorption coefficient increased as phytoplankton broke down during the first 12 days, showing the production of CDOM from phytoplankton. After 12 days, a(CDOM)(355) had increased from the initial value 0.41+/-0.03 m(-1) to 1.37+/-0.03 m(-1) (a 234% increase), and the Chla concentration decreased from the initial value of 349.1+/-11.2 microg/L to 30.4+/-13.2 microg/L (a 91.3% decrease). The mean daily production rate of CDOM from phytoplankton was 0.08 m(-1) for a(CDOM)(355). Parallel Factor Analysis (PARAFAC) was used to assess CDOM composition from EEM spectra, and four components were identified: a terrestrial-like humic component, two marine-like humic components, and a protein-like component. The rapid increase in marine-like humic fluorophores (C3 and C4) during the degradation experiment suggests that in situ production of CDOM plays an important role in the dynamics of CDOM. The field campaigns and experimental data in the present study show that phytoplankton can be one of the important CDOM producers in eutrophic shallow lakes.

A study of absorption characteristics of chromophoric dissolved organic matter and particles in Lake Taihu, China

Absorptions by non-phytoplankton particles and phytoplankton, and chromophoric dissolved organic matter (CDOM) were measured at 50 sites in large, shallow, Lake Taihu in winter and summer 2006 to study their seasonal and spatial variations, and their relative contributions to total absorption. The CDOM absorption was significantly higher in winter than in summer, due to degradation and release of fixed carbon in phytoplankton and submerged aquatic vegetation (SAV). The hyperbolic model was used to model the spectral absorption of CDOM, and the mean spectral slope of 6.38xa0nm−1 was obtained. At most sites, the spectral absorption of non-phytoplankton particles was similar to that of the total particles, demonstrating that the absorption of the total particles is dominated by the absorption of non-phytoplankton particles. In summer, phytoplankton absorption increased markedly, due to frequent algal blooms especially in Meiliang Bay. In winter, the significant increase in non-phytoplankton particle absorption resulted from the increase of inorganic particulate matter caused by sediment resuspension. Strong linear relationships were found between ad(440) and total suspended matter (TSM), organic suspended matter (OSM), and inorganic suspended matter (ISM). Strong linear relationships were also found between aph(440), aph(675) and chlorophyll a (Chl-a) concentration. The total relative contributions of non-phytoplankton particles over the range of photosynthetically active radiation (PAR) (400–700xa0nm) were 48.4 and 79.9% in summer and winter respectively. Non-phytoplankton particle absorption dominated the total absorption, especially in winter, in Lake Taihu, due to frequent sediment resuspension in the large shallow lake as a result of strong windy conditions. The results indicate that strong absorption by CDOM and non-phytoplankton particles at the blue wavelength has an impact on the spectral availability, and acts as a selection factor for the composition of the phytoplankton community, with cyanobacteria being the dominate species in Lake Taihu.

Photochemical degradation of chromophoric-dissolved organic matter exposed to simulated UV-B and natural solar radiation

Photochemical degradation of chromophoric-dissolved organic matter (CDOM) by UV-B radiation decreases CDOM absorption in the UV region and fluorescence intensity, and alters CDOM composition. CDOM absorption, fluorescence, and the spectral slope indicating the CDOM composition were studied using 0.22-μm-filtered samples of Meiliang Bay water from Lake Taihu that were exposed to short-term (0–12xa0h) simulated UV-B radiation and long-term (0–12 days) natural solar radiation in summer. CDOM absorption coefficient and fluorescence decreased with increasing exposure time, which relates to the amounts of absorbed light energy. The decreases of CDOM absorption and normalized fluorescence corresponded to first order kinetics reactions. Different decreases of CDOM absorption and fluorescence at different wavelengths suggested that the composition of CDOM changed when it absorbed ultraviolet radiation. Photochemical degradation increased the spectral slope during 275–295xa0nm region (S275–295) but decreased the spectral slope during 275–295xa0nm region (S350–400). The slope ratio SR (S275–295:S350–400) increased in the photochemical process, which could be used as an indicator of photobleaching and composition change of CDOM. Our results show that photochemical degradation is important in the cycling of CDOM, which indicated change in the composition of CDOM.

Modeling Remote-Sensing Reflectance and Retrieving Chlorophyll-a Concentration in Extremely Turbid Case-2 Waters (Lake Taihu, China)

Accurate assessment of concentration of chlorophyll a (Chla) and correct identification of algal blooms by remote sensing have previously been a great challenge in the optically complex Case-2 waters. In this paper, we used a large biooptical data set to model the remote-sensing reflectance in an extremely turbid and biologically productive Lake Taihu in China. The conceptual three-band model [R_rs ^-1 (lambda₁) - R_rs ^-1 (lambda₂₎] times R_rs ( lambda₃) (where R_rs represents remote-sensing reflectance just above the water surface) to retrieve Chla concentration was calibrated and validated, and a detailed assessment of its accuracy was obtained. Water samples were collected for four seasons from 2006 to 2007 at 50 sites, covering different ecosystem types, and contained three very variable optically active substances (tripton 7.9-281.7 mg middot L^-1, Chla 4.0-448.9 mug middot L^-1, and chromophoric dissolved organic matter [a_CDOM(440)] 0.27-2.36 m^-1). Secchi disk transparency ranged from 8 to 85 cm. The retrieval accuracies (r²) of the optimal three-band model and the related band-ratio method were 0.94 and 0.92, while the root mean-square errors (RMSE) and relative errors (RE) were 15.1 mug middot L^-1 (37.3% accounting for the mean value) and 18.0 mug middot L^-1, and 44.4% and 60.2%, respectively. Applications of the three-band model using MERIS central bands [R_rs ^-1(681) - R_rs ^-1(709)] times R_rs(754) also allowed accurate estimation of Chla, with r², RMSE, and RE of 0.92, 17.0 mug middot L^-1, and 48.1%, respectively. The establishment of a simple and robust biooptical model with high retrieval accuracy and known error budgets will help the rapid, accurate, and real-time assessment of algal blooms using <i>in</i> <i>situ</i> and satellite remote-sensing techniques.

Dissolved oxygen stratification and response to thermal structure and long-term climate change in a large and deep subtropical reservoir (Lake Qiandaohu, China).

From January 2010 to March 2014, detailed depth profiles of water temperature, dissolved oxygen (DO), and chromophoric dissolved organic matter (CDOM) were collected at three sites in Lake Qiandaohu, a large, deep subtropical reservoir in China. Additionally, we assessed the changes in DO stratification over the past 61 years (1953-2013) based on our empirical models and long-term air temperature and transparency data. The DO concentration never fell below 2 mg/L, the critical value for anoxia, and the DO depth profiles were closely linked to the water temperature depth profiles. In the stable stratification period in summer and autumn, the significant increase in CDOM in the metalimnion explained the decrease in DO due to the oxygen consumed by CDOM. Well-developed oxygen stratification was detected at the three sites in spring, summer and autumn and was associated with thermal stratification. Oxycline depth was significantly negatively correlated with daily air temperature and thermocline thickness but significantly positively correlated with thermocline depth during the stratification weakness period (July-February). However, there were no significant correlations among these parameters during the stratification formation period (March-June). The increase of 1.67 °C in yearly average daily air temperature between 1980 and 2013 and the decrease of 0.78 m in Secchi disk depth caused a decrease of 1.65 m and 2.78 m in oxycline depth, respectively, facilitating oxygen stratification and decreasing water quality. Therefore, climate warming has had a substantial effect on water quality through changing the DO regime in Lake Qiandaohu.

Inflow rate-driven changes in the composition and dynamics of chromophoric dissolved organic matter in a large drinking water lake

Drinking water lakes are threatened globally and therefore in need of protection. To date, few studies have been carried out to investigate how the composition and dynamics of chromophoric dissolved organic matter (CDOM) in drinking water lakes are influenced by inflow rate. Such CDOM can lead to unpleasant taste and odor of the water and produce undesirable disinfection byproducts during drinking water treatment. We studied the drinking water Lake Qiandao, China, and found that the concentrations of suspended particulate matter (SPM) in the lake increased significantly with inflow rate (pxa0<xa00.001). Similarly, close relationships between inflow rate and the CDOM absorption coefficient at 350xa0nm a(350) and with terrestrial humic-like fluorescence C3 and a negative relationship between inflow rate and the first principal component (PC1) scores, which, in turn, were negatively related to the concentrations and relative molecular size of CDOM (pxa0<xa00.001), i.e. the concentration and molecular size of CDOM entering the lake increased proportionately with inflow rate. Furthermore, stable isotopes (δD and δ(18)O) were depleted in the upstream river mouth relative to downstream remaining lake regions, substantiating that riverine CDOM entering the lake was probably driven by inflow rate. This was further underpinned by remarkably higher mean chlorophyll-a and in situ measured terrestrial CDOM fluorescence (365/480xa0nm) and apparent oxygen utilization (AOU), and notably lower mean PC1 and CDOM spectral slope (S275-295) recorded in the upstream river mouth than in the downstream main lake area. Strong negative correlations between inflow rate and a(250):a(365), S275-295, and the spectral slope ratio (SR) implied that CDOM input to the lake in rainy period was dominated by larger organic molecules with a more humic-like character. Rainy period, especially rainstorm events, therefore poses a risk to drinking water safety and requires higher removal efficiency of CDOM during drinking water treatment processes.

Modelling the spectral absorption of tripton using exponential and hyperbolic models

The optical properties of tripton, and the correlation between the absorption coefficient and tripton concentration, were investigated based on a large dataset of 727 samples, collected from different regions of shallow Lake Taihu, in China, from July 2004 to April 2007. Four models describing tripton absorption spectra in the visible domain (400–700 nm) were examined to find the optimal model. The conventional single exponential model performed the poorest. Statistically, the most ‘useful’ model was the hyperbolic model; judged by the root mean square error (RMSE) and the magnitude of the F statistic from an analysis of variance, this was found to give a closer fit for the measured absorption (70.1% reduction in RMSE and 449.7% increase in F value) than did the simple exponential model method. The mean S d value derived for the hyperbolic model was 5.98 ± 0.30 nm–1, with a very small coefficient of variation of 5.0%. Significant linear correlations were found between a d(440) and total suspended matter, inorganic suspended matter and tripton concentrations, but with the minimal RMSE of a d(440) estimation (0.815 m–1) for tripton. The results demonstrate that it is possible to develop regional models for the estimation of tripton absorption coefficient from the concentration.

Thermal stratification dynamics in a large and deep subtropical reservoir revealed by high-frequency buoy data

We measure the thermal stratification dynamics in Lake Qiandaohu, China, a deep subtropical reservoir, to better understand the mixing mechanism and its response to lake warming. A high-frequency monitoring buoy dataset from February 2016 to October 2017 is used to evaluate variations in the water temperature profile, Schmidt stability (SS) and thermocline parameters, such as the thermocline depth (TD), bottom depth (TB), thickness (TT), and strength (TS), and elucidate the potential effects of thermal stratification on the lakes ecosystem. High-frequency observation data demonstrate that the lakes thermal-stratification cycle can be divided into three stages: formation, stationary and weakening periods. Consequently, a significant positive correlation between the TB and TT during the formation period and a significant negative correlation between the TD and TT are found during the stationary and weakening periods. Additionally, strong positive correlations exist among the TS, TT and SS for all the data. Our data indicated that an increase in the air temperature caused the surface water temperature, TT, TS and SS to increase. Furthermore, thermal stratification affected the vertical distribution of dissolved oxygen and expanded the area of the hypoxic-anoxic zone. The incomplete mixing of the water from December 2016 to February 2017 because of the high air temperature, which was 2.49u202f°C higher than the mean air temperature of 1966-2015 (6.44u202f°C), created the hypoxia hypolimnion from March to May 2017. Under the background of global warming, the thermal stratification of Lake Qiandaohu will likely intensify and further significantly affect the lakes ecosystem.

Extreme weather event may induce Microcystis blooms in the Qiantang River, Southeast China

A severe cyanobacterial bloom in the mainstem of a large Chinese river was first reported from China. The Qiantang River is the longest river in the Zhejiang province, southeast China. It provides drinking water supply to ~u200916 million people, including Hangzhou city. Fifteen sites along the Qiantang River (including upper, middle (Fuchunjiang Reservoir), and lower reaches and tributaries) were sampled between August 13 and September 9, 2016 to conduct a preliminary examination of the outbreak of Microcystis blooms. Laboratory investigation revealed that Microcystis spp. are dominant in the Fuchunjiang Reservoir (an overflow reservoir on the mainstem of the Qiantang River) with an extremely high cell density of 2.3u2009×u2009108xa0cells/L, leading to a severe bloom in the mainstem of the Qiantang River. Investigations of the meteorological, hydrological, and nutrient characteristics associated with the bloom indicated that extremely dry (6.8xa0mm rainfall from August 13 to September 9, 2016) and hot (32 consecutive days of temperatures >u200930xa0°C from July 20 to August 31, 2016) weather might be the key factors triggering the bloom. Additionally, the extremely low flow of the tributary, Lanjiang River (142u2009±u200956xa0m3/s from August 13 to September 9), and its high nutrient background, favored the bloom. While nutrient reductions are important, the most immediate and effective management approach might be to implement appropriate minimal flow conditions to mitigate the blooms.