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The boom in dam construction and continuous river‐lake exchange has had a profound impact on the transmission and transformation of riverine dissolved inorganic carbon (DIC). An in‐depth understanding of the change mechanisms of DIC concentrations and sources driven by dam operation and lake recharge is crucial for regulating greenhouse gas emissions and evaluating the impact of DIC on the global carbon cycle. This study investigated dam‐ and lakes‐driven DIC via the concentration and δ13C of DIC, combined with anions, cations, δD and δ18O in the main stream of the Yangtze River. DIC showed a decreasing trend from upper reach (2,262.31 ± 113.69 μmol kg−1) to lower reach (1,771.61 ± 89.36 μmol kg−1). Carbonate dissolution proportion (from 36.45% to 28.44%) and atmospheric CO2 proportion (from 37.51% to 22.94%) of DIC decreased from upper reach to lower reach, whereas soil CO2 proportion of DIC (from 26.01% to 48.63%) increased. The control of dam operation on DIC biogeochemical process was revealed from different time scales. From the perspective of short‐term seasonal changes (from 2020 to 2021), the mineralization of organic matter in the dry season strengthened CO2 degassing and calcite precipitation, reducing the DIC and increasing the proportion of soil CO2. Meanwhile, longer periods of runoff retention provided sufficient time for water–rock reactions in the wet season and increased the DIC and carbonate dissolution source in the reservoir area. On a long‐term scale (from 2009 to 2021), a decrease in pH driven by sediment mineralization contributed to an annual increase in DIC in the reservoir. The flow of lakes mixed into the mainstream was revealed by the enrichment of δ18O, and river‐communicating recharge decreased the DIC and carbonate dissolution source. We show that dam operation and lake inflow change DIC concentrations and sources and therefore need to be considered in the transmission and transformation processes of DIC in the river‐ocean continuum. [ABSTRACT FROM AUTHOR]

Purpose: Large additions of organic carbon (OC) have been introduced into the carbon cycle of lakes from algae during the process of lake eutrophication. The impact of eutrophication on OC burial and CO2 fixing has been widely studied; however, there is still a gap regarding the impact of excess OC from algae on sedimentary OC mineralization. In this study, we aim to fill this gap by analyzing in situ measurements. Materials and methods: Three pairs of sediment cores collected from a plateau lake (Dianchi Lake) in 2006 and 2014 were used to estimate the accumulation loss rate (ALR) of OC (and thus the total mineralization rate) in the sediment. n-Alkanes, measured from the same sedimentary cores, were used to identify the source of OC. An OC mineralization experiment in a laboratory was used to confirm the enhanced effect of excess phytoplankton carbon on ALR and reveal the potential influence of microorganisms. Results and discussion: The results indicate that the sedimentary core (core 3), with high excess OC from algae (located in an algal bloom area), possessed a higher ALR (85.66%) and a higher attenuation coefficient (0.078), indicating the low burial efficiency and short mineralization duration of OC. Sedimentary core 1, controlled by terrestrial OC, had a relatively lower ALR (64.60%) and lower attenuation coefficient (0.029), indicating a high burial efficiency and long period of OC mineralization. The mineralization of OC in core 2 was impacted by terrestrial and endogenous OC, with an ALR of 72.00% and attenuation coefficient of 0.064, which is between that of cores 1 and 3. Excess OC from algae corresponded to an increase in ALR by 32.60% when comparing core 1 to core 3. The increased ALR and attenuation coefficient could be caused by excess OC from algae. Conclusions: Sedimentary OC mineralization indicates that the ALR with dominantly allochthonous OC (64.60%) is much lower than that controlled by autochthonous OC (85.66%). Excess OC from phytoplankton increases the mineralization of OC: not only via increased ALR but also increased mineralization speed. The laboratory experiment on the mixture of algae and sediment suggested that excess phytoplankton OC increased the emission of CO2 by 20–70% (mineralization rate). [ABSTRACT FROM AUTHOR]

Abstract 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. Graphical abstract Image 1 Highlights • Developed the retrieval model of C phyto and a ph (675). • Established an novel proxy (C phyto : a ph (675)) in high turbid inland water to monitor the phytoplankton physiology. • Revealed the regulation effects of temperature and nutrient to C phyto and C phyto : a ph (675). [ABSTRACT FROM AUTHOR]

We examined the relationship between, and variations in, particulate organic carbon (POC) and particulate organic nitrogen (PON) based on previously acquired ocean and inland water data. The latitudinal dependency of POC/PON is significant between 20 and 90° N but weak in low-latitude areas and in the Southern Hemisphere. The mean values of POC/PON in the Southern Hemisphere and Northern Hemisphere were 7:40±3:83 and 7:80±3:92, respectively. High values of PON/POC appeared between 80-90 (12:2±7:5) and 70-80° N (9:4±6:4), while relatively low POC/PON was found from 20 (6:6±2:8) to 40° N (6:7±2:7). The latitudinal variation of POC/PON in the Northern Hemisphere is much stronger than in the Southern Hemisphere due to the influence of more terrestrial organic matter. Higher POC and PON could be expected in coastal waters. POC/PON growth ranged from 6:89±2:38 to 7:59±4:22 in the Northern Hemisphere, with an increasing rate of 0.0024 km from the coastal to open ocean. Variations of POC/PON in lake water also showed a similar latitude-variation tendency of POC/PON with ocean water but were significantly regulated by the lakes' morphology, trophic state and climate. Small lakes and high-latitude lakes prefer relatively high POC/PON, and large lakes and low-latitude lakes tend to prefer low POC/PON. The coupling relationship between POC and PON in oceans is much stronger than in inland waters. Variations in POC, PON and POC/PON in inland waters should receive more attention due to the implications of these values for the global carbon and nitrogen cycles and the indeterminacy of the relationship between POC and PON. [ABSTRACT FROM AUTHOR]

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 ± 27.29 g m − 2 yr − 1 after eutrophication. It further increased to 61.98 ± 28.94 g m − 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. [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]

Field measurements of dissolved organic carbon (DOC) concentration and remote-sensing reflectance were conducted to develop a regional, empirical red-blue algorithm to retrieve surface DOC from Geostationary Ocean Color Imager (GOCI) data for Lake Taihu, China. The auxiliary data (in-situ observations of the optical properties and water quality, buoy measurements of hydrodynamic data and water chemical parameters) were used to investigate the spatial and temporal variations in DOC. GOCI was shown to be capable of successfully obtaining hourly variations in DOC, with a root mean square error percentage (RMSP) of 17.29% (RMSE = 0.69 mg/L) for the match-up data. The GOCI-derived DOC in Lake Taihu confirms that the highest DOC concentration is in northwest Lake Taihu, followed by Meiliang Bay, Gonghu Bay and northeast Lake Taihu. Hourly DOC variation is significant and presents a different trend for each lake segment due to the variety of influencing factors. Discharge of DOC from surrounding rivers is an important factor to the variation of DOC in northeast Lake Taihu. However, organic products of algae will be the primary contributor to DOC when algal bloom occurred. During the period of algal bloom, high DOC levels in Lake Taihu can lead to hypoxia when coupled with high temperatures and low disturbance. [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]

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 δ13C 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. [Display omitted] • BC in sediment core witnessed the advancement of regional urbanization and industrialization. • Faster burial of char than soot was discovered in BC of Dianchi sediment core. • Coal combustion facilitated faster burial of char than soot in BC. • Source of char and soot were estimated via stable carbon isotopic signature. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) is a ubiquitous group of organic compounds in rainwater that plays vital roles in the biogeochemical cycle. However, little is known about its chemical composition, optical characterization, potential sources, and controlling mechanisms in peri-urban atmospheric rainwater. One-year rainfall samples were collected between September 2018 and August 2019 in a Nanjing suburb. Stoichiometric, ultraviolet–visible (UV–Vis) absorption, and three-dimensional fluorescence excitation-emission matrix (EEM) with parallel factor analysis (PARAFAC) analyses were utilized to characterize DOM. Several proxies for DOM concentration, including dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and absorption coefficient (a254), exhibited power-function relationships with the rainfall amount (r2 = 0.33–0.55, p < 0.001). Particularly, the DOC and a254 value showed significant seasonal variability, with lower value in two wet seasons (winter and summer) than in the dry seasons (autumn and spring). The annual wet depositional (WD) flux of DOC in the Xianlin (XL) site was calculated as 1.68 g C m−2 yr−1, agreeing with the range of global WD flux values. Two terrestrial humic-like components (C1–C2), one microbial humic-like component (C3), and one tryptophan-like component (C4) were identified by EEM-PARAFAC in rainwater DOM. The results of the specific absorption indices (SUVA 254, S R) and PARAFAC component analyses revealed that summer DOM samples had lower fluorescence intensities, aromatic contents, and molecular sizes. Principal component analysis (PCA) based on DOM indices showed that rainwater DOM had predominantly terrestrial humic origins, while this characteristic became slightly weaker in summer. The quantity and quality of rainwater DOM were closely related to meteorological variables (i.e., air mass, rainfall amount, and solar radiation) as well as anthropogenic activities. This study could improve the understanding of the quantity and quality of rainwater DOM on carbon budgets and biogeochemical cycling from some new insights of optical proxies about different fates of rainwater DOM under complex controlling mechanisms. Image 1 • Concentration and chemical structure of rainwater DOM showed seasonal variations. • Climatic factors and human activities were dominant effects on rainwater DOM. • Rainwater DOM components were mainly generated from humic substances. • Wet inorganic N in atmosphere was closely coupled with rainwater DOM. [ABSTRACT FROM AUTHOR]