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Combustion-derived black carbon (BC) is an important component of sedimentary carbon pool. Due to different physicochemical properties, determining the source of char and soot is crucial for BC cycling, especially for nonhomologous char and soot in the Tibetan Plateau (TP). This study analyzed the sequestration and source of BC, char, and soot in the Dagze Co (inner TP) sediment core via the content and δ 13 C, revealing the biomass and fossil fuel driving on nonsynchronous char and soot and their response to local anthropogenic activities and atmospheric transmission. The results showed that BC concentration increased from 1.19 ± 0.35 mg g -1 (pre-1956) to 2.03 ± 1.05 mg g -1 (after 1956). The variation of char was similar to BC, while nonhomologous growth was detected in char and soot ( r = 0.29 and p > 0.05). The source apportionment showed that biomass burning for 71.52 ± 10.23% of char and promoted char sequestration. The contribution of fossil fuel combustion to soot (46.67 ± 14.07%) is much higher than char (28.48 ± 10.23%). Redundancy analysis confirmed that local anthropogenic activities significantly influenced BC burial and atmospheric transport from outside TP-regulated BC burial. The contribution of biomass and fossil fuels to nonsynchronous char and soot is conducive to understanding the anthropogenic effect on BC burial in the TP.

The regulation of lacustrine organic carbon (OC) burial by nutrient is an outstanding knowledge gap in the current understanding of lake carbon cycles. In this study, we determined how nutrients quantitatively correspond with OC burial using the parallel factor analysis (PARAFAC) method in Dianchi Lake, southwest China. Factors were classified into three types according to their historical sedimentation characteristics: the background factor (BF), response factor (RF), and contingency factor (CF). The BF represented the original OC input combination in the lake and was insensitive to nutrient changes. The RF represented the OC input combination that was induced or promoted by nutrient changes in the lake. The CF represented short-term discontinuous factors in sedimentary history, which may be related to unique historical events. The results indicate that changes in the total nitrogen (TN) to total phosphorus (TP) ratio correlated with changes in the BF contribution; whereas the quantity of OC was mainly correlated with TN. The >90% of OC buried in sediment was quantitatively simulated by BF and RF; the driving effect of RF on OC burial was approximately 13 times higher than that of BF. It was observed that a 1 mg kg -1 increase in TN led to approximately 2.2 units increase in RF contribution in Dianchi Lake, while the BF was insensitive to changes in TN. Thus, changes in lake nutrients may effectively change the composition and quantity of OC buried in lake sediment., Competing Interests: Declaration of competing interest I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed. We declare that we have no financial and personal relationships with other people of organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature of kind in any product, service or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Black carbon (BC) is a retarder in carbon cycle, and the proportion of char and soot in BC restricts the significance of BC as a sink in carbon cycle. Tracing the sources of char and soot is helpful for in-depth understanding the anthropogenic-driven burial and pattern of BC, and is crucial for regulating emissions of BC and impact of BC on carbon cycle/climate change. This study investigated source-driven BC via the concentration and δ 13 C of BC (char and soot) in a Plateau lake sediment. The burial rate of BC (mean: 6.42 ± 5.09 g m -2 yr -1 ) showed an increasing trend (3.7 times after 1977 compared with before), and the growth rate of char (4.1 times) was faster than soot (2.5 times). The source trace results, showing faster growth of coal combustion ratio in char (increased 21% after 1980 compared with before) than soot (13%), proved that coal combustion promoted faster growth of char in BC. Redundancy analysis confirmed that more low-temperature utilization of coal urged a stronger driving force for char than soot, which caused BC to have lower aromatic content and higher reactivity in organic carbon pool from the past to present, further impact the effects of BC on carbon cycle., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Sediment dissolved organic matter (DOM) in inland waters is commonly affected by environmental changes. However, knowledge about how sediment DOM responds to eutrophication and the associations between sediment DOM and bacterial communities requires further investigation. We selected a sediment core from Dianchi Lake (China) that was dated from 1864 to 2019 by the activity of radionuclides ( 210 Pb and 137 Cs). δ 13 C DOC changes fit well with the historical record that heavy eutrophic status in Dianchi Lake were observed since 1980s. Large amounts of dissolved organic carbon (DOC), chromophoric (CDOM) and fluorescent (FDOM) DOM accumulated at the top of the sediments during the eutrophication period (1982-present). The additional algae sources with a higher degradation rate altered the composition, aromaticity and humification of DOM. After long-term mineralization, the remaining DOM became more and more recalcitrant and kept a relatively stable level at older sediments. A co-occurrence network analysis revealed that Proteobacteria, Chloroflexi, Acidobacteriota, Bacteroidota and Desulfobacterota were the most abundant species at the phylum level and clustered into three primary modules. Different microbes shared unique preferences for niches, causing a heterogeneous bacterial distribution at different depths. We conducted Spearman's correlation and redundancy analysis (RDA) to explore potential interactions between bacterial community and sediment DOM. The richness and diversity of bacterial communities were positively related to DOM content, suggesting abundant DOM can produce more available resources for bacteria. RDA results showed some specific species might modify DOM composition and structure. This study suggests that sediment DOM properties were regulated by source transformation during eutrophication, and emphasizes the importance of microbial role on sediment biogeochemical process., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Black carbon (BC), characterized by high aromaticity and stability, has been recognized as a substantial fraction of the carbon pool in soil and sediment. The effect of BC on the particulate organic carbon (POC) pool in lake water, which is an important medium of carbon transmission and transformation, has not been thoroughly studied. The investigations of BC composition and distribution, POC, polycyclic aromatic hydrocarbons (PAHs), and stable carbon and nitrogen isotopes were conducted in a eutrophic urban lake, Taihu Lake, which is the third largest freshwater lake in China. The results indicate that the BC is composed of 55 ± 12% char and 45 ± 12% soot and accounted for 12 ± 6% of POC (the maximum value is 31%). The comparatively high levels of BC and char are distributed in the northern Taihu Lake, especially in Meiliang Bay (0.72 ± 0.38 mg L -1 and 0.45 ± 0.24 mg L -1 ). The distribution of soot presents a declining trend from the lakeshore to the central lake, particularly in the northern, western, and southern lakes. Source apportionment results from positive matrix factorization of PAHs suggest that consumption of fossil fuel (79 ± 20%) is the dominant source of BC, which agrees with the low ratio of char/soot (1.41 ± 0.71) and relatively depleted δ 13 C. The covariation of BC and PAHs and terrestrial dissolved organic carbon indicate that the effect of terrestrial input significantly regulates the distribution of BC in Taihu Lake, which is reflected in the high BC value along the lakeshore., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

The continuous flux of organic carbon (OC) from terrestrial ecosystems into inland water is an important component of the global carbon cycle. The buried OC pool in inland water sediments is considerable, and black carbon (BC) is a significant contributor to this OC pool because of the continuous growth in BC emissions. Therefore, determining the effect of BC on total OC burial and variations in the structure of BC during the burial process will contribute significantly to our understanding of lacustrine carbon cycling. This study investigated BC burial and its structural variations in response to anthropogenic drivers using four dated sedimentary cores from a deep plateau lake in China. The BC burial rate rose from 0.96 ± 0.64 g·m -2 ·y -1 (mean of sedimentary cores pre-1960s) to 4.83 ± 1.25 g·m -2 ·y -1 (after 2000), which is a 5.48 ± 2.12-fold rise. The increase of char was similar to those of BC. The growth rate of soot was 7.20 ± 4.30 times, which is higher than that of BC and char, increasing from 0.12 ± 0.08 to 0.64 ± 0.23 g·m -2 ·y -1 . There was a decreasing trend in the ratio of char and soot at a mean rate of 62.8 ± 6.46% (excluding core 3) in relation to increased fossil fuel consumption. The contribution of BC to OC burial showed a significant increasing trend from the past to the present, particularly in cores 3 and 4, and the mean contribution of the four cores was 11.78 ± 2.84%. Source tracer results from positive matrix factorization confirmed that the substantial use of fossil fuels has promoted BC burial and altered the BC structure. This has resulted in BC with a higher aromatic content in the lake sediment, which exhibits reduced reactivity and increased stability. The strong correlation between BC and allochthonous total OC indicates that the input pathways of the buried BC in this plateau lake sediment were terrestrial surface processes and not atmospheric deposition.

Estimation of phytoplankton biomass (noted as phytoplankton carbon, C phyto ) and evaluation of phytoplankton physiology is central to the estimation of primary productivity and the carbon cycle. This issue has been widely considered in oceans but not in inland water. Here, we develop experiential and semi-analytical models, which validated by independent in situ measurement data, respectively, to derive C phyto and phytoplankton absorption coefficient at 675 nm (a ph (675)) from MODIS. The effects of nutrients and temperature on the seasonal variation of phytoplankton physiology were assessed through a novel proxy of C phyto to a ph (675) ratio (C phyto /a ph (675)) over the Lake Taihu, the third largest lake in China. Significant seasonal climatological cycles of C phyto , a ph (675) and C phyto /a ph (675) were observed in Lake Taihu, especially in Meiliang Bay and Zhushan Bay, where algal blooms occur frequently. The highest C phyto and a ph (675) values were observed in summer due to the growth of phytoplankton biomass and chlorophyll-a concentration. Lower values were observed in winter and spring, which are characterized by relatively high total nitrogen levels and low irradiance, owing to the low temperature astricts the algae growth. However, the C phyto /a ph (675) shows an opposite trend compared to C phyto and a ph (675), which have high values in winter and low values in summer. The analysis of C phyto , a ph (675) and C phyto /a ph (675) with total phosphorus (TP) levels and temperature indicates that TP are the main positive driver of the increase in C phyto and a ph (675) and negatively regulate C phyto /a ph (675). Warming promotes an increase in C phyto and a ph (675) and restricts C phyto /a ph (675) in summer. Biomass and nutrient levels are the primary drivers of the decrease of C phyto /a ph (675) in such a typical eutrophic lake. The results present some new findings compared to previous oceanic studies and expand our knowledge in the study of phytoplankton biomass and physiology in eutrophic lakes., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Two sediment cores were collected from Dianchi Lake, a plateau lake in Southwest China, to study the temporal trends and to investigate the sources of sedimentary deposited polycyclic aromatic hydrocarbon. The ΣPAH16 concentration in the two sediment cores ranged from 172.5 to 2244.8 ng/g and from 211.4 to 1777.8 ng/g, with mean values of 1106.2 and 865.1 ng/g, respectively. Three temporal trends for the ΣPAH16 concentration and the composition of PAHs in Dianchi Lake all showed three typical changing stages: (1) slight changes in deeper segments before the 1950s; (2) a rapid increase in PAH concentrations between the 1960s and 1990s; and (3) a slight reduction from the 1990s onward. These trends differ from those observed in developed countries due to differences in the timing of industrialization and urbanization processes. According to the results of the molecular ratios and principal component analysis, the PAH deposition was dominated by coal combustion, wood combustion, and vehicle emissions before and after the 1960s, respectively.

Organic carbon (OC) buried in lake sediment is an important component of the global carbon cycle. The impact of eutrophication on OC burial in lakes should be addressed due to worldwide lake eutrophication. Fourteen 210 Pb- and 137 Cs-dated sediment cores taken in Dianchi Lake (China) in August 2006 (seven cores) and July 2014 (seven cores) were analyzed to evaluate the response of the organic carbon accumulation rate (OCAR) to eutrophication and algal blooms over the past hundred years. The mean value of OCAR before eutrophication occurred in 1979, 16.62±7.53 (mean value±standard deviation), increased to 54.33±27.29gm -2 yr -1 after eutrophication. It further increased to 61.98±28.94gm -2 yr -1 after algal blooms occurred (1989). The accumulation rate of organic nitrogen (ONAR) is coupled with OCAR. The high loss rate of OC and organic nitrogen (ON) leads to a long-term burial efficiency of only 10% and 5% of OC and ON. However, this efficiency can still lead to an increase in OCAR by a factor of 4.55 during algal blooms in Dianchi Lake. Dianchi Lake stored 1.26±0.32 Tg carbon and 0.071±0.018 Tg nitrogen, including 0.94±0.23 Tg OC and 0.32±0.14 Tg inorganic carbon, 0.066±0.018 Tg ON, 0.002±0.001 Tg nitrate nitrogen (NO 3 -N) and 0.003±0.001 Tg ammonium nitrogen (NH 4 -N) between 1900 and 2012., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Microcystins (MCs) produced by cyanobacteria pose a serious threat to public health. Intelligence on MCs distributions in freshwater is therefore critical for environmental agencies, water authorities, and public health organizations. We developed and validated an empirical model to quantify MCs in Lake Taihu during cyanobacterial bloom periods using the atmospherically Rayleigh-corrected moderate resolution imaging spectroradiometer (MODIS-Aqua) (Rrc) products and in situ data by means of chlorophyll a concentrations (Chla). First, robust relationships were constructed between MCs and Chla (r = 0.91; p < 0.001; t-test) and between Chla and a spectral index derived from Rrc (r = -0.86; p < 0.05; t-test). Then, a regional algorithm to analyze MCs in Lake Taihu was constructed by combining the two relationships. The model was validated and then applied to an 11-year series of MODIS-Aqua data to investigate the spatial and temporal distributions of MCs. MCs in the lake were markedly variable both spatially and temporally. Cyanobacterial bloom scums, temperature, wind, and light conditions probably affected the temporal and spatial distribution of MCs in Lake Taihu. The findings demonstrate that remote sensing reconnaissance in conjunction with in situ monitoring can greatly aid MCs assessment in freshwater.

Humans have had a significant impact on the terrestrial pedosphere through activities such as agriculture and urbanization. The effects of human activities on land use and the related environmental changes were investigated through point and areal studies surrounding Meiliang Bay, which is an open area of extreme eutrophication in Taihu Lake, China. This study used remote sensing and environmental-tracer profiles [total nitrogen (TN), total phosphorus (TP), total organic carbon (TOC), grain size, and geochemical parameters] to determine the causes of changes in land use and the associated environmental parameters. The results of LUCCs (Land use/cover changes) indicate that over the past three decades, total farmland decreased by 862.49 km2, with an annual decrement rate of 28.75 km2/year, and total urbanized land increased by 859.71 km2, with an annual growth rate of 28.66 km2/year. The geochemical results indicate that the trophic state of Taihu Lake was persistently intensifying and that the TN, TP, and TOC concentrations increased twofold, threefold, and twofold, respectively, from 1949 to 2010. The sources of TN, TP, and TOC were highly similar after 1975. However, before 1974, TN and TP originated from different sources than TOC. The grassland and woodland around the lake retain nutrients and sand from the land of study area. The increase in urbanized land and tertiary industries significantly increased the sediment concentrations of TN, TP, and TOC after 1980.

Human activities contribute highly to water eutrophication. In this study, the relationship between human activities and water eutrophication in Dianchi Lake in China was characterized using a combination of satellite imaging, sedimentary physicochemical and meteorological data analyses. The heavy eutrophic status and algal bloom in Dianchi Lake were first observed by satellite in 1977 and 1989, respectively. The C/N ratio, an indicator of organic sources in sediments, also showed that the planktonic organic source in the sediment significantly increased beginning in 1989. The land use cover in the Dianchi basin showed that both farm lands and forests, but particularly farmlands, were reduced during the period from 1974 to 2009. The urbanized land area increased from 1974 to 2009, particularly after 2000. The effects of human activities on water eutrophication were expressed by land use cover, population, gross domestic product (GDP; separated into primary, secondary and tertiary industries) and wastewater discharge. For land use cover, farm and urbanized lands were the main sources of water nutrients; forest contributed slightly to these nutrients. For GDP, primary (correlation coefficient=0.94, P<0.001) and tertiary (correlation coefficient=0.95, P<0.001) industries were highly correlated with total nitrogen. Secondary (correlation coefficient=0.95, P<0.001) and tertiary (correlation coefficient=0.96, P<0.001) industries were highly correlated with total phosphorus. The algal bloom area was significantly correlated with wastewater discharge (correlation coefficient=0.78, P<0.005) (except industrial wastewater), which was primarily led by the non-agricultural population, from 2000 to 2009. This study suggests that the protection of water environments requires a comprehensive protection policy in addition to a unilateral protection policy., (Copyright © 2014 Elsevier B.V. All rights reserved.)

An analysis of the correlation between polycyclic aromatic hydrocarbons (PAHs) and economic parameters demonstrates that the total population, gross domestic product, coal consumption, petroleum, temperature, and day consumption significantly affect PAH concentrations in Dianchi Lake, Yunnan province, China. An artificial neural network (ANN) model was developed to predict the trend in PAH concentrations in the sediments of Dianchi Lake over the next 10 years based on current indicators of economic development. The ANN model estimated the concentration of PAHs from 1980 to 2014. The model was evaluated using available observations for the historical trends; concentrations of PAHs in the sediments of Dianchi Lake are calculated to be at 2128.1 ng/g in 2025 and are expected to decline up to 1044.3 ng/g by 2030. These concentrations are considered relatively high because of their impacts on the health of people and aquatic organisms and the development of surrounding industries. We show the importance of the socioeconomic and climate factors in increasing the pollution levels. Our results could support the local government to formulate effective measures to reduce the pollution levels in the lake. [ABSTRACT FROM AUTHOR]

Increased organic carbon and nitrogen accumulation rates (OCAR and ONAR) in lake sediment significantly regulate the global carbon cycle. However, the reasons for and contributors to the increased OCAR and ONAR are unclear. Seven sediment cores, collected in July 2014 from Dianchi Lake, China, were used to evaluate the effects of autochthonous and allochthonous sources on OCAR and ONAR. The results indicate that OCAR and ONAR increased by factors of 4.33 and 7.34 over the past hundred years (1900–2000), particularly after algal blooms began to occur frequently (beginning in the 1980s). Dianchi stored 0.467 ± 0.0055 Tg (mean value ± standard deviation) organic carbon (OC) and 0.033 ± 0.0004 Tg organic nitrogen (ON) after 1986, which is almost equal to the total storage of OC and ON from 1900 to 1985 (OC, 0.468 ± 0.0022 Tg; ON, 0.032 ± 0.0002 Tg). OCAR and ONAR increases were due to increasing autochthonous production and allochthonous loading. Examination of the increased OCAR, which was estimated from a newly developed multi-source mixing model, suggests that > 90% of increased OCAR was caused by allochthonous sources (such as intensified cultivation, land-use cover change, etc.) in southern and eastern Dianchi and that > 70% of the increased OCAR was due to autochthonous sources in western and northern Dianchi Lake. The significant spatial and temporal variation in the contributors to increased OCAR indicates complicated migration and transformation of OC in inland lakes. Land use cover change around Dianchi and the occurrence of algal blooms regulate the contributions of allochthonous and autochthonous sources to the increased OC and ON. [ABSTRACT FROM AUTHOR]

An integrated system including eutrophication simulation and algal bloom predication was established and applied in Lake Taihu. The mechanic models, which are based on a three-dimensional numerical model coupling hydrodynamic module, pollutant transport module, suspended sediment resuspension module, wind wave module and algal bloom predication module, can simulate not only the single function such as current pattern, nutrient distribution, sediment concentration and wave height field, but also the interaction among them. Using these models, some complex phenomenon such as endogenous pollution can be simulated. In the study, some key technologies are introduced to make the simulation reasonable and feasible. First, a new method for predicting algal bloom is introduced. Differing from the traditional phytoplankton dynamic model, judging the algal bloom through biomass or chlorophyll concentration, the method quantitatively estimates algal growth state switching among bloom, collapse and steady based on algal stability criterion in water column. Second, the sediment resuspension is estimated using calibrated critical shear stresses derived from remote sensing data and the wave-enhanced bottom stress during the windy days. Third, a user-friendly operation environment based on plug-in development and geographic information system is developed. The tests demonstrated that the system could both simulate the reactive solutes such as BOD and DO and the suspended sediment correctly under interaction of gravity and turbulent diffusion. The simulation results demonstrated that the system could obtain reasonable and reliable simulated results in Lake Taihu. [ABSTRACT FROM AUTHOR]

Land target-based atmospheric correction methods over inland waters are an effective alternative method for when the standard or improved atmospheric correction method that is based on water is invalid. The type of aerosol model that is selected and land dark target that is distinguished are two key factors that could determine the atmospheric correction’s accuracy. We present an improved land target-based iterative method [denoted as dense dark vegetation (DDV)-WC] for the atmospheric correction of Moderate Resolution Imaging Spectroradiometer (MODIS) over Lake Taihu. The improvements include the use of aerosol models in Optical Properties of Aerosols and Clouds (OPAC), which fully consider the impact of relative humidity (RH) on aerosol properties, and a rigorous land dark target selection rule (DDV-WC-Selection). The DDV-WC algorithm includes three steps: the selection of DDV pixels from lakeshore areas, the retrieval of the aerosol optical thickness (AOT) at 550 nm (AOT550) for the selected DDV pixels, and the retrieval of the remote sensing reflectance (Rrs) over water. The AOT550 inversion accuracy of different aerosol models in OPAC is tested in Lake Taihu. The “continental clean model” has the best performance compared with five other aerosol models for every month of the year. Therefore, the aerosol model of the DDV-WC algorithm is fixed to the “continental clean model” in Lake Taihu because of its stable and excellent performance. When using the “continental clean model” to perform these retrievals, the DDV pixels selection (DDV-WC-Selection) in the DDV-WC method (\text MRE(\text Mean\;\text Relative\;\text Error) = 14.63\% ) is more reliable than the DDV pixels selection (DDV-Selection) in the MODIS V5.2 method (\text MRE = 17.24\% ). The atmospheric correction of MODIS indicates that this approach produces superior performance in the green and red bands but poor results in the near-infrared (NIR) and blue bands. The MRE is only 13.15% at 645 nm. These findings illustrate that the DDV-WC algorithm is a practical MODIS atmospheric correction method for inland turbid waters. [ABSTRACT FROM PUBLISHER]

Treatment of eutrophication and algal blooms in large shallow lakes has been constrained by the endogenetic release of nutrients from sediment resuspension despite the well-controlled input of nutrients from surrounding land in Taihu Lake, China. In this study, we propose a method to simplify regionally empirical algorithms for total suspended matter (${{C}_{{\rm{TSM}}}}$), chlorophyll-a (${{C}_{{\rm{Chl\hbox{-}a}}}}$), and phosphorus concentration (${{C}_{\rm{p}}}$) for the Geostationary Ocean Color Imager (GOCI) for Taihu Lake. The results show that the GOCI-derived ${{C}_{{\rm{TSM}}}}$, ${{C}_{{\rm{Chl\hbox{-}a}}}}$, and ${{C}_{\rm{p}}}$ match well with the in situ measured data. Based on the validated synchronous and timely observation data (${{C}_{{\rm{TSM}}}}$, ${{C}_{{\rm{Chl\hbox{-}a}}}}$ and ${{C}_{\rm{p}}}$) from GOCI level-1b data, the interaction of phosphorus with algae and suspended sediment were analyzed from August 6 to August 9, 2013. The results indicate that phosphorus released from sediment resuspension is significant in Taihu Lake, with a high spatial–temporal variation. The effect of the restriction of nutrients (nitrogen and phosphorus) on algal growth varied temporally and spatially with the hydrodynamic characteristics of the lake. In most cases, restricted phosphorus in the lake is the primary reason for the algal growth, especially in open regions such as Center Lake (CL), Northwestern Lake (NWL), and Southwestern Lake (SWL). Both nitrogen and phosphorus are the restrictive factors to algal growth in Meiliang and Gonghu Bays. More attention should thus be focused on the endogenetic release of phosphorus in Taihu Lake when treating eutrophication and algal blooms in addition to controlling the input of nitrogen and phosphorus from surrounding land. [ABSTRACT FROM PUBLISHER]

Pigment C-phycocyanin (C-PC) is a useful indicator for the presence of cyanobacteria in inland waters, which has been well known as a phytoplankton group with many negative effects on human, animal, and aquatic ecosystem health. In recent years, the remote detection of the C-PC concentrations for inland waters has received much attention. However, their accurate quantification by means of remote sensing is still a challenge due to the significant bio-optical complexity of turbid inland waters. In this paper, three typical turbid inland lakes in China were investigated through in situ observed data sets containing optical and water quality parameters. By using a recently proposed TD680 optical classification method, all collected samples were first classified into three types. For each type of water, we determined specific spectral sensitive regions for the pigment C-PC. Then, we developed three type-specific support vector regression (SVR) algorithms and an aggregated SVR algorithm. The performances of these algorithms were evaluated through the validation data sets. The results show that the type-specific algorithms generally have significantly improved performance over the aggregated SVR algorithm. Their assessment errors [mean absolute percentage error (MAPE) and root-mean-square error ( rmse)] were as follows: 1) MAPE = 15.6\% and rmse =\break 30.6\ \mg\cdot \m^-3 for Type 1 water; 2) MAPE = 47.1\% and rmse = 61.5\ \mg\cdot\m^-3 for Type 2 water; and 3) MAPE =\break 26.4\% and rmse = 19.1\ \mg\cdot\ m^-3 for Type 3 water. The findings in this paper demonstrate that a prior water classification is needed for the development of accurate C-PC retrieval algorithms. This paper provides a valid strategy for improving C-PC estimation accuracy and enhancing algorithm commonality for optically complex turbid waters. [ABSTRACT FROM AUTHOR]

For optically complex turbid productive waters, the optical behavior of suspended particles is the keynote of characterizing the unordered variations of inherent optical properties (IOPs). Multiple bio-optical measurements and sampling of optically active substances were performed in Lake Taihu, Lake Chaohu, and Lake Dianchi, and Three Gorges reservoir of China, in 2008, 2009, and 2010. On the basis of obtaining adequate observation data, we developed an improved and robust water classification approach, by which complex water conditions were divided into three types, i.e., Type 1 (Normalized Trough Depth at 675 nm, hereafter NTD675, ≥0.092), Type 2 (0 ＜ NTD675 ＜ 0.092), and Type 3 (NTD675 ≤ 0). Furthermore, the specific inherent optical quantities for suspended particles, including the specific absorption coefficient of non-algal particles (a*nap), the specific absorption coefficient of phytoplankton (a*ph), and the specific scattering coefficient of the suspended particles (b*p), were determined for the three classified types of waters. The validation results showed that our proposed values for these specific inherent optical quantities presented relatively high predictive accuracies, with most mean absolute percentage errors (MAPE) near 30%, and more importantly, performed much better than that of non-classified waters. Additionally, relative contributions of phytoplankton and non-algal particles to the total particulate absorption and scattering, as well as the spectra, were also analyzed, and the differences among the three classified types of waters were clarified. Overall, the results obtained in this study provide us with new knowledge for understanding complex varied inherent optical properties of highly turbid productive waters. [ABSTRACT FROM AUTHOR]

This study develops a novel support vector regression (SVR) model for retrieving the specific cyanobacterial pigment C-phycocyanin (C-PC) concentrations in cyanobacteria-dominated large turbid lakes of China. Lake Taihu, Lake Chaohu, and Lake Dianchi in China were our study areas. Five field cruises were carried out to collect data sets of optical and water quality parameters. To retrieve the C-PC, three types of reflectance forms, including single band, band ratio, and three-band-combination, were compared. The band ratio was the best candidate to serve for algorithm development. On this basis, two types of models, including linear models and a SVR model, were originally established. The previous typical algorithms were also examined. The obtained results showed that the best-performing model was the SVR model. By our validation data set, the proposed SVR model also presented accurate prediction results, with the lowest errors among all methods. The novelty of the SVR model compared to the previous ones lies in the inclusion of band ratios that are located outside of the main pigment absorption peaks but hold information on inflection points, curvature, etc., into empirical optimization. The implications of these findings indicates the potential applicability of the SVR models in lakes of the similar type. [ABSTRACT FROM AUTHOR]

Water spectral absorption characteristics of eutrophic lakes are largely different from those of ocean and coastal waters. We therefore studied them with the aim of establishing an analytical model for inland water colour, to be used in remote sensing. Field measurements were carried out on 16 and 17 August 2006 (summer), 5 and 6 November 2006 (winter), and 29 and 30 March 2007 (spring) at 15 stations in northern Lake Taihu (China). Chromophoric dissolved organic matter (CDOM) absorption coefficients (a CDOM) are higher in summer than in spring and winter, with the ratios of a CDOM in spring, summer and winter being approximately 1 : 4.0 : 1.2 at both UV-C and UV-B. The spectral slope S CDOM values change with wavelength and season, and covary with CDOM concentration, as shown by regression analysis. For the CDOM absorption spectrum in the wavelength region 500-700 nm (important for water colour remote sensing), a linear method simulates better than an exponential method. Seasonal variations in non-algal particulate (NAP) absorption (a NAP) at blue, green and red wavelengths show better consistency, in the order winter>spring>summer. The average S NAP is 0.0065±0.0009 nm-1, which is lower than that in other types of waters. Phytoplankton absorption (a ph) peak height changes with the season, with the pattern summer>winter>spring, and phytoplankton absorption spectra can be expressed with high accuracy by a quadratic model. CDOM absorption contributions in the three seasons are low compared to phytoplankton and NAP. [ABSTRACT FROM AUTHOR]

[Display omitted] • Combining R rs (λ) and δ13 C POC can track and monitor the source of POC. • The source of POC in surface water of Taihu Lake in summer is mainly endogenous POC. • Endogenous POC has a certain physiological relationship with water temperature and nutrient (TN, TP), while terrestrial POC was closely related to wind speed. • The a ph (λ) and a nph (λ) can effectively reflect the source of POC. The sources of particulate organic carbon (POC) determine its conversion, thereby playing an important role in the carbon cycle of lakes. Accurate estimation of the sources and dynamic characteristics of POC is important for understanding the migration and transformation of organic carbon. However, the synchronous observation of POC sources with large areas through remote sensing is still challenging because of the complex composition of POC and the optical conditions of inland lakes. In this study, a three-band (1/ R rs (689)–1/ R rs (717)) × R rs (697)) empirical algorithm of POC sources was constructed based on remote sensing reflectance (R rs (λ)) and the proportion of endogenous POC estimated from the field-measured stable isotope (δ13 C POC) values. The validation and calibration results of the three-band algorithm showed robust performance, with MAPE and RMSE of estimated values and measured values of 10% and 0.07, respectively. The three-band algorithm had good simulation results for the Ocean and Land Color Instrument (OLCI), Moderate Imaging Spectroradiometer (MODIS), Geostationary Ocean Color Imager (GOCI), and Medium Resolution Imaging Spectrometer (MERIS) spectra. The POC sources estimated by the three-band algorithm suggest that the endogenous POC of Taihu Lake in August showed a decreasing trend from 2006 to 2019. The variation in terrestrial POC was slow and stable for both annual and monthly variations. The analysis of POC sources with total phosphorus (TP), total nitrogen (TN), water temperature, and wind speed indicated that terrestrial POC was closely related to wind speed (r = 0.33, P < 0.001), while endogenous POC was significantly associated with TP (r = 0.6, P < 0.001), TN (r = 0.56, P < 0.001), and water temperature (r = 0.49, P < 0.001). The use of remote sensing algorithms to evaluate POC from different sources is convenient and effective; furthermore, it helps to better understand the carbon cycle in lacustrine ecosystems. [ABSTRACT FROM AUTHOR]

• Lake CO 2 and CH 4 concentrations varied with seasons and mixing regimes on a regional scale. • System productivity directly affected the carbon concentrations in the dry season and stratified sites. • Direct impacts of lake morphology and geoclimatic factors were found except for the dry season. • The effects of season and mixing regime should be considered in estimating lake carbon emissions. Due to the large spatiotemporal variability in the processes controlling carbon emissions from lakes, estimates of global lake carbon emission remain uncertain. Identifying the most reliable predictors of CO 2 and CH 4 concentrations across different hydrological features can enhance the accuracy of carbon emission estimates locally and globally. Here, we used data from 71 lakes in Southwest China varying in surface area (0.01‒702.4 km2), mean depth (< 1‒89.6 m), and climate to analyze differences in CO 2 and CH 4 concentrations and their driving mechanisms between the dry and rainy seasons and between different mixing regimes. The results showed that the average concentrations of CO 2 and CH 4 in the rainy season were 23.9 ± 18.8 μmol L−1 and 2.5 ± 4.9 μmol L−1, respectively, which were significantly higher than in the dry season (10.5 ± 10.3 μmol L−1 and 1.8 ± 4.2 μmol L−1, respectively). The average concentrations of CO 2 and CH 4 at the vertically mixed sites were 24.1 ± 21.8 μmol L−1 and 2.6 ± 5.4 μmol L−1, being higher than those at the stratified sites (14.8 ± 13.4 μmol L−1 and 1.7 ± 3.5 μmol L−1, respectively). Moreover, the environmental factors were divided into four categories, i.e., system productivity (represented by the contents of total nitrogen, total phosphorus, chlorophyll a and dissolved organic matter), physicochemical factors (water temperature, Secchi disk depth, dissolved oxygen and pH value), lake morphology (lake area, water depth and drainage ratio), and geoclimatic factors (altitude, wind speed, precipitation and land-use intensity). In addition to the regression and variance partitioning analyses between the concentrations of CO 2 and CH 4 and environmental factors, the cascading effects of environmental factors on CO 2 and CH 4 concentrations were further elucidated under four distinct hydrological scenarios, indicating the different driving mechanisms between the scenarios. Lake morphology and geoclimatic factors were the main direct drivers of the carbon concentrations during the rainy season, while they indirectly affected the carbon concentrations via influencing physicochemical factors and further system productivity during the dry season; although lake morphology and geoclimatic factors directly contributed to the carbon concentrations at the vertically mixed and stratified sites, the direct effect of system productivity was only observed at the stratified sites. Our results emphasize that, when estimating carbon emissions from lakes at broad spatial scales, it is essential to consider the influence of precipitation-related seasons and lake mixing regimes. [Display omitted] [ABSTRACT FROM AUTHOR]

Accurate estimation of phytoplankton chlorophyll a (Chla) concentration from remotely sensed data is particularly challenging in turbid, productive waters. The objectives of this study are to validate the applicability of a semi-analytical three-band algorithm in estimating Chla concentration in the highly turbid, widely variable waters of Taihu Lake, China, and to improve the algorithm using a proposed four-band algorithm. The improved algorithm is expressed as [Rrs(λ 1)−1 −Rrs(λ 2)−1][Rrs(λ 4)−1 −Rrs(λ 3)−1]−1. The two semi-analytical algorithms are calibrated and evaluated against two independent datasets collected from 2007 and 2005 in Taihu Lake. Strong linear relationships were established between measured Chla concentration and that derived from the three-band algorithm of [Rrs−1(660)−Rrs−1(692)]Rrs(740) and the four-band algorithm of [Rrs−1(662)−Rrs−1(693)][Rrs−1(740)−Rrs−1(705)]−1. The first algorithm accounts for 87% and 80% variation in Chla concentration in the 2007 and 2005 datasets, respectively. The second algorithm accounts for 97% of variability in Chla concentration for the 2007 dataset and 87% of variation in the 2005 dataset. The three-band algorithm has a mean relative error (MRE) of 43.9% and 34.7% for the 2007 and 2005 datasets. The corresponding figures for the four-band algorithm are 26.7% and 28.4%. This study demonstrates the potential of the four-band model in estimating Chla even in highly turbid case 2 waters. [Copyright &y& Elsevier]