Development of subsurface chlorophyll maximum layer and its contribution to the primary productivity of water column in a large subtropical reservoir

Effectively mitigated eutrophication risk by strong biological carbon pump (BCP) effect in karst reservoirs

Karst reservoirs can significantly enhance the effect of biological carbon pump (BCP), a crucial process for carbon sequestration, water purification, and eutrophication mitigation. However, the effects of BCP on the fate of carbon (C), nitrogen (N), and phosphorus (P) and its role in regulating eutrophication within river-reservoir systems, remains insufficiently understood, particularly across different geological settings. We investigated the Hongfeng Reservoir (HFR), a typical karst reservoir, analyzing water chemistry, nutrient concentrations, and stable isotopes of dissolved inorganic carbon (δ13CDIC) and nitrate (δ15N-NO3-) to uncover the underlying mechanisms governing the migration of biogenic elements and the process of eutrophication. Our findings reveal a strong BCP effect in the reservoirs that leads to substantial CO2 and HCO3- uptake via phytoplankton photosynthesis during the warm-wet season, resulting in decreased dissolved inorganic carbon (DIC) concentrations and increased pH in the epilimnion. The δ13CDIC (-4.0 ± 0.5 ‰) values in the epilimnion relatively increased in response to phytoplankton photosynthesis that preferentially absorbs the lighter isotope of 12C. Compared with the inflow, the δ15N-NO3- (7.4 ± 0.2 ‰) in the epilimnion of the reservoir is significantly depleted, with the water predominantly aerobic or oxygen-supersaturated. This suggests that nitrification is the dominant process during the warm-wet season. The high NO3- concentrations (44.3 ± 10.1 μmol/L) indicate a sufficient N supply for biological uptake. The strong BCP effects in the epilimnion convert substantial amounts of DCO2 and nutrients into autochthonous organic matter. The resulting increase in pH further reduces the availability of DCO2. Furthermore, BCP-induced calcium carbonate precipitation enhances P removal through co-precipitation, thereby accelerating nutrient depletion and carbon sequestration, which collectively contribute to the mitigation of eutrophication risks. To assess the broader applicability of these findings, we analyzed data from 129 lakes and reservoirs globally. Our results show that karst reservoirs, with their strong BCP effect, exhibit an average Carlson trophic status index (CTSI) 9.8 % lower than non-karst reservoirs, indicating a reduced risk of eutrophication. These insights offer valuable implications for the management of water resources in karstic reservoirs globally.

Periodically asymmetric responses of deep chlorophyll maximum to light and thermocline in a clear monomictic lake: Insights from monthly and diel scale observations

Deep chlorophyll maximum (DCM), a chlorophyll peak in the water column, has important implications for biogeochemical cycles, energy flow and water surface algal blooms in deep lakes. However, how an observed periodically asymmetric DCM response to environmental variables remains unclear, limiting our in-depth understanding and effective eco-environmental management of deep lakes. Based on both monthly field investigations in 2021 and diel continuous observations in 2021-2023 in clear, monomictic Lake Fuxian, Southwest China, the temporal dynamics and drivers of DCM were examined and periodic features of DCM were found, with a formation period (FP, February-July) and a weakening period (WP, August-December). On the monthly scale, although DCM dynamics were partly attributed to thermocline structures, the role of light penetration depths varied with period. In the FP, the influence of light on DCM was direct, i.e., increased depth and thickness but decreased magnitude. Differently, the influence of light mainly occurred by affecting thermocline structures in the WP, where water quality was another important driver. On the diel scale, light was a major reason for a thicker and lower (magnitude) DCM during day than at night, and the response of DCM to environmental factors between the FP and WP differed also more during day. This periodically asymmetric response of daytime DCM not only being caused by light but possibly also related to other physical factors such as lake surface water temperature, wind speed and precipitation. Bayesian network modelling suggested that water darkening and stratification intensification may promote a shallower, thinner and larger (magnitude) DCM in both FP and WP, but achieving such changes in DCM requires different light and thermocline thresholds. Our findings provide new information valuable for modelling DCM and for predicting the related surface algal blooms in deep lakes under climate change and eutrophication.

Divergent contributions of microbes and plants to soil organic carbon in the drawdown area of a large reservoir: Impacts of periodic flooding and drying

The distribution patterns and accumulation mechanisms of plant and microbial residues, along with their potential contributions to soil organic carbon (SOC), remain subjects of considerable debate, particularly within drawdown areas affected by reservoir operation. In this study, surface soil samples (0-10 cm) were collected from three different elevations within the drawdown area of the Three Gorges Reservoir. Amino sugars and lignin phenols served as biomarkers for microbial residues and plant-derived materials, respectively. The results revealed that with increasing duration of flooding, the content of amino sugars increased from 0.26 mg g-1 to 0.64 mg g-1, whereas the content of lignin phenols decreased from 204.09 mg kg-1 to 37.93 mg kg-1. Moreover, as the duration of flooding increased, the contribution of microbial necromass carbon (MNC) to SOC rose from 29% to 47%, while the contribution of plant-derived carbon to SOC gradually declined. Plants biomass and iron minerals influenced the accumulation of lignin phenols, whereas amino sugars were affected by plants biomass, microbial biomass carbon and nitrogen, and clay minerals. The periodic flooding and drying events induced alterations in carbon inputs and environmental characteristics within the drawdown area, resulting in fluctuations in the contributions of plants and MNC to SOC in this region. The findings of this study highlight the critical role played by both plant- and microbial-derived carbon in the retention and turnover of SOC within the terrestrial-aquatic transition zone.

Environmental and Climatic Drivers of Phytoplankton Communities in Central Asia

Artificial water bodies in Central Asia offer unique environments in which to study plankton diversity influenced by topographic barriers. However, the complexity of these ecosystems and limited comprehensive studies in the region challenge our understanding. In this study, we systematically investigated the water environment parameters and phytoplankton community structure by surveying 14 artificial waters on the southern side of the Altai Mountains and the northern and southern sides of the Tianshan Mountains in the Xinjiang region. The survey covered physical and nutrient indicators, and the results showed noticeable spatial differences between waters in different regions. The temperature, dissolved oxygen, total nitrogen, and total phosphorus of artificial water in the southern Altai Mountains vary greatly. In contrast, the waters in the northern Tianshan Mountains have more consistent physical indicators. The results of phytoplankton identification showed that the phytoplankton communities in different regions are somewhat different, with diatom species being the dominant taxon. The cluster analysis and the non-metric multidimensional scaling (NMDS) results also confirmed the variability of the phytoplankton communities in the areas. The variance partitioning analysis (VPA) results showed that climatic and environmental factors can explain some of the variability of the observed data. Nevertheless, the residual values indicated the presence of other unmeasured factors or the influence of stochasticity. This study provides a scientific basis for regional water resource management and environmental protection.

Diel and seasonal mixing patterns and water quality dynamics in a multipurpose tropical semiarid reservoir

Eutrophication has become a recurrent concern in reservoirs worldwide. This problem is intensified in tropical semiarid regions, where the reservoirs have high seasonal and annual variability of water level and volume. Therefore, an extensive understanding of the diel variation of water quality key-parameters can help improve management of such reservoirs. This study focuses on Castanhão reservoir with the largest multipurpose dam in the Brazilian semiarid. Its main water uses are irrigation, fish farming, and human supply. The reservoir faced a decline in water quality due to a prolonged drought period. While previous research has predominantly emphasized the seasonal dynamics of thermal and chemical stratification, our investigation provides diel assessments of multiple water quality parameters, including nutrient concentrations and phytoplankton abundance. Our primary objective is to compare seasonal and diel variations in stratification and nutrient distribution within the reservoir. Key findings reveal a diel cycle of thermal stratification, primarily during dry season, driven by higher wind speeds. This is corroborated by a significant negative correlation between wind speed and the relative water column stability index. In contrast, during the rainy season, the reservoir experiences continuous thermal stratification due to inflowing water being warmer than the reservoir's water temperature. Notably, a significant negative correlation between total phosphorus and chlorophyll-a, along with a two-fold increase of this nutrient throughout the day during the rainy season, underscores the influence of the phytoplankton community dynamics on the diel nutrient variation. Chemical stratification of dissolved oxygen occurred during dry and rainy seasons, indicating that even during the dry season, where there is no significant inflow, the internal nutrient loading can also significantly impact the water quality of a reservoir. This study advances the understanding of diel water quality dynamics in tropical semiarid reservoirs, shedding light on both climatic and anthropogenic influences on water resources.

Geochemistry and the optics of geospatial analysis as a preposition of water quality on a macroscale

The water treatment depends exclusively on the identification of residues containing toxic chemical elements accumulated in NPs (nanoparticles), and ultrafine particles sourced from waste piles located at old, abandoned sulfuric acid factories containing phosphogypsum requires global attention. The general objective of this study is to quantify and analyze the hazardous chemical elements present in the leachate of waste from deactivated sulfuric acid factories, coupled in NPs and ultrafine particles, in the port region of the city of Imbituba, Santa Catarina, Brazil. Samples were collected in 2020, 2021, and 2022. Corresponding images from the Sentinel-3B OLCI satellite, taken in the same general vicinity, detected the levels of absorption coefficient of Detritus and Gelbstoff (ADG443_NN) in 443 m-1, chlorophyll-a (CHL_NN (m-3)), and total suspended matter (TSM_NN (g m-3) at 72 points on the marine coast of the port region. The results of inductively coupled plasma atomic-emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS) demonstrate that the leaching occurring in waste piles at the port area of Imbituba was the likely source of hazardous chemical elements (e.g., Mg, Sr, Nd, and Pr) in the environment. These leachates were formed due to the presence of coal pyrite and Fe-acid sulfates in said waste piles. The mobility of hazardous chemical elements potentiates changes in the marine ecosystem, in relation to ADG443_NN (m-1), CHL_NN (m-3), and TSM NN (g m-3), with values greater than 20 g m-3 found in 2021 and 2022. This indicated changes in the natural conditions of the marine ecosystem up to 30 km from the coast in the Atlantic Ocean, justifying public initiatives for water treatment on a global scale.

Thermocline stratification favors phytoplankton spatial overlap and species diversity in a subtropical deep reservoir

Spatial segregation of species along opposing resource gradients is a major research topic in ecology as it enables the coexistence and maintenance of high diversity. Thermocline stratification causes heterogeneous resource distribution, however, the effect of thermocline stratification on phytoplankton dynamic in-depth profiles is still unclear. To understand the underlying mechanism behind the effects of thermocline stratification on phytoplankton species diversity in stratified lakes, the monthly changes in thermocline parameters, deep chlorophyll maxima (DCM) parameters, spatial overlap (SO) among major phytoplankton taxonomic groups, and phytoplankton species diversity were evaluated in Lake Qiandaohu in the Zhejiang Province of China from April 2017 to December 2018. Thermocline depth (TD) was significantly negatively related to thermocline strength (TS). The monthly air temperature was the main driver behind the thermocline and the seasonal thermal-stratification cycle was divided into two stratification stages. Significant linear relationships were observed between the DCM parameters (depth, concentration, and thickness) and TD during the strong stratification period. TD was significantly positively related to phytoplankton species diversity and the SO between Cryptophyta and Chlorophyta as well as between Cryptophyta and Bacillariophyta during weak stratification periods. Significant positive correlations were observed between SO and Shannon diversity during both periods. Structural equation modeling (SEM) showed that air temperature significantly decreased TD and increased species diversity by increasing SO during the strong stratification period. Strong stratification under warming favored the formation of shallower and thinner DCMs, leading to phytoplankton coexistence and maintenance of high species diversity through a preclusion of dominance. This study characterized the temporal dynamics of phytoplankton dynamic in-depth profiles in response to strong stratification from warming.