Influence of plankton metabolism and mixing depth on CO2 dynamics in an Amazon floodplain lake

Lake metabolic processes and their effects on the carbonate weathering CO2 sink: Insights from diel variations in the hydrochemistry of a typical karst lake in SW China

The precipitation of carbonate minerals does not invariably result in CO₂ emission to the atmosphere, because dissolved inorganic carbon (DIC) can be partially utilized by terrestrial aquatic phototrophs, thus generating an autochthonous organic carbon (AOC) sink. However, little is known about the potential effects of this mechanism on carbon cycles in DIC-rich lakes, mainly due to the lack of detailed documentation of the related processes, which limits our ability to accurately evaluate and predict the magnitude of this carbon sink. We conducted field observations in Fuxian Lake, a large and representative karst lake in the Yunnan-Guizhou Plateau, SW China. Continuous diel monitoring was conducted to quantitatively assess the coupled relationship between lake metabolism and DIC cycling and its influence on the carbonate weathering-related CO₂ sink. We found that the diel physicochemical variations and isotopic characteristics were mainly controlled by the metabolism of aquatic phototrophs, evidenced by a significant relationship between net ecosystem production and diel DIC cycling, and demonstrating the significance of DIC fertilization in supporting high primary production in karst lakes. The data showed that a reduction in photosynthesis occurred in the afternoon of almost every day, which can be explained by the lower CO₂/O₂ ratio that increased the potential for the photorespiration of aquatic plants, thus reducing photosynthesis. We found that a net autotrophic ecosystem prevailed in Fuxian Lake, suggesting that the lake functions more as a sink than a source of atmospheric CO₂. Considering carbonate weathering, the estimated AOC sink amounted to 650-704 t C km^-2 yr^-1, demonstrating both the potentially significant role of metabolism in lacustrine carbon cycling and the potential of the combination of photosynthesis and carbonate weathering for carbon sequestration. Our findings may help to quantitatively estimate the future impact of lake metabolism on carbon cycling, with implications for formulating management policies needed to regulate the magnitude of this carbon sink.

Precipitation Mediates the Distribution but Not the Taxonomic Composition of Phytoplankton Communities in a Tributary of Three Gorges Reservoir

Precipitation is a driver of changes in the spatiotemporal distribution of phytoplankton communities. The ecological consequence of precipitation is important, but the underlying processes are not clear. Here we conducted an immediate prior- and after-event short-interval investigation in the Three Gorges Reservoir region, to test whether the short-term changes in the phytoplankton communities and functional groups could be predicted based on the precipitation level. We found that precipitation of moderate and high levels immediately changed the phytoplankton distribution and altered functional groups. According to structural equation model, the vertical velocity (λ = −0.81), light availability (Zeu/Zmix, λ = 0.47) and relative water column stability (RWCS, λ = 0.38) were important parameters for phytoplankton distribution during the precipitation event. Water quality did not directly affect phytoplankton distribution (λ = −0.11) and effects of precipitation on the water quality only lasted 1–2 days. The phytoplankton community was redistributed with some tolerance functional groups appearance, such as groups F, Lo, M and groups M, MP, TB, W1 appeared during- and after- precipitation event, respectively. We also found that mixing rather than flushing was the driving force for the decrease of phytoplankton biomass. Our study provided valuable data for reservoir regulation and evidence for predictions of phytoplankton during the precipitation events under different climate change scenarios.

Urban rivers are hotspots of riverine greenhouse gas (N2O, CH4, CO2) emissions in the mixed-landscape chaohu lake basin

Growing evidence shows that riverine networks surrounding urban landscapes may be hotspots of riverine greenhouse gas (GHG) emissions. This study strengthens the evidence by investigating the spatial variability of diffusive GHG (N₂O, CH₄, CO₂) emissions from river reaches that drain from different types of landscapes (i.e., urban, agricultural, mixed, and forest landscapes), in the Chaohu Lake basin of eastern China. Our results showed that almost all the rivers were oversaturated with dissolved GHGs. Urban rivers were identified as emission hotspots, with mean fluxes of 470 μmol m^-2d^-1 for N₂O, 7 mmol m^-2d^-1 for CH₄, and 900 mmol m^-2d^-1 for CO₂, corresponding to ~14, seven, and two times of those from the non-urban rivers in the Chaohu Lake basin, respectively. Factors related to the high N₂O and CH₄ emissions in urban rivers included large nutrient supply and hypoxic environments. The factors affecting CO₂ were similar in all the rivers, which were temperature-dependent with suitable environments that allowed rapid decomposition of organic matter. Overall, this study highlights that better recognition of the influence that river networks have on global warming is required-particularly when it comes to urban rivers, as urban land cover and populations will continue to expand in the future. Management measures should incorporate regional hotspots to more efficiently mitigate GHG emissions.

Tracing solute sources and carbon dynamics under various hydrological conditions in a karst river in southwestern China

Understanding the mechanisms that lead various hydrological conditions to influence solute and carbon dynamics in karst rivers is a crucial issue. In this study, high-frequency sampling and analyses of water chemistry and ẟ¹³C_DIC were conducted from October 2013 to September 2014 in a typical karst river, the Beipan River in southwestern China. The major ions (such as Ca²⁺, Mg²⁺, HCO₃^-, K⁺, SO₄^2-, Na⁺, and Cl^-) in the river are mainly from the weathering of carbonates and silicates and present temporal hydrological variabilities. Sr and U are mainly derived from carbonate weathering and show chemostatic behaviors responding to increasing discharge, similar to carbonate-sourced ions Ca²⁺, Mg²⁺, and HCO₃^-. Silicate weathering is the primary source of Al and Li, which show significant dilution effects similar to those of Na⁺ responding to high discharge. Meanwhile, most dissolved trace elements (such as Zn, Cu, Ba, Sb, Mn, Mo, and Pb) are strongly impacted by anthropogenic overprints and also exhibit a significant seasonal variability, which may be related with mining activities in the investigated area. A simultaneous increase of ẟ¹³C_DIC and decrease in ∆DIC contents and pCO₂ values suggests that photosynthesis is the primary control on riverine DIC variability during the high-flow season. Besides, the pCO₂ values display significant chemostatic behaviors owing to the influx of biological CO₂, which is produced by microbiological activities and ecological processes, and enhanced by monsoonal climatic conditions. A two-dimensional endmember mixing model demonstrates that carbonate weathering (averaging 62%) along with biological carbon (averaging 38%) are main sources to the riverine DIC, with temporal variability. Consequently, these results show that carbonate weathering and involved plant photosynthesis are the dominant processes controlling the riverine DIC contents under high discharge and temperature conditions. This work provides insight into the crucial influence of hydrological variability on solute sources and carbon dynamics under monsoonal climate for the karst rivers.

Spatial heterogeneity and hydrological fluctuations drive bacterioplankton community composition in an Amazon floodplain system

Amazonian floodplains form complex hydrological networks that play relevant roles in global biogeochemical cycles, and bacterial degradation of the organic matter in these systems is key for regional carbon budget. The Amazon undergoes extreme seasonal variations in water level, which produces changes in landscape and diversifies sources of organic inputs into floodplain systems. Although these changes should affect bacterioplankton community composition (BCC), little is known about which factors drive spatial and temporal patterns of bacterioplankton in these Amazonian floodplains. We used high-throughput sequencing (Illumina MiSeq) of the V3-V4 region of the 16S rRNA gene to investigate spatial and temporal patterns of BCC of two size fractions, and their correlation with environmental variables in an Amazon floodplain lake (Lago Grande do Curuai). We found a high degree of novelty in bacterioplankton, as more than half of operational taxonomic units (OTUs) could not be classified at genus level. Spatial habitat heterogeneity and the flood pulse were the main factors shaping free-living (FL) BCC. The gradient of organic matter from transition zone-lake-Amazon River was the main driver for particle-attached (PA) BCC. The BCC reflected the complexity of the system, with more variation in space than in time, although both factors were important drivers of the BCC in this Amazon floodplain system.

Effects of seasonality, trophic state and landscape properties on CO2 saturation in low-latitude lakes and reservoirs

The role of tropical lakes and reservoirs in the global carbon cycle has received increasing attention in the past decade, but our understanding of its variability is still limited. The metabolism of tropical systems may differ profoundly from temperate systems due to the higher temperatures and wider variations in precipitation. Here, we investigated the spatial and temporal patterns of the variability in the partial pressure of carbon dioxide (pCO₂) and its drivers in a set of 102 low-latitude lakes and reservoirs that encompass wide gradients of precipitation, productivity and landscape properties (lake area, perimeter-to-area ratio, catchment size, catchment area-to-lake area ratio, and types of catchment land use). We used multiple regressions and structural equation modeling (SEM) to determine the direct and indirect effects of the main in-lake variables and landscape properties on the water pCO₂ variance. We found that these systems were mostly supersaturated with CO₂ (92% spatially and 72% seasonally) regardless of their trophic status and landscape properties. The pCO₂ values (9-40,020 μatm) were within the range found in tropical ecosystems, and higher (p < 0.005) than pCO₂ values recorded from high-latitude ecosystems. Water volume had a negative effect on the trophic state (r = -0.63), which mediated a positive indirect effect on pCO₂ (r = 0.4), representing an important negative feedback in the context of climate change-driven reduction in precipitation. Our results demonstrated that precipitation drives the pCO₂ seasonal variability, with significantly higher pCO₂ during the rainy season (F = 16.67; p < 0.001), due to two potential main mechanisms: (1) phytoplankton dilution and (2) increasing inputs of terrestrial CO₂ from the catchment. We conclude that at low latitudes, precipitation is a major climatic driver of pCO₂ variability by influencing volume variations and linking lentic ecosystems to their catchments.

Cyanobacteria dynamics in a small tropical reservoir: Understanding spatio-temporal variability and influence of environmental variables

Anthropogenic disturbances within or near aquatic ecosystems often contribute to eutrophication events. Cyanobacteria are a key group responsible for environmental problems associated with eutrophication processes. Interest is growing in estimating the threat of cyanobacteria in tropical Africa, however, there is still a lack of understanding regarding temporal drivers of cyanobacteria dynamics in natural aquatic ecosystems given the paucity of relevant fundamental research in this area. To better understand cyanobacteria dynamics, potential drivers of cyanobacteria dynamics were investigated in a model tropical reservoir system, whereby phytoplankton communities and water quality parameters were sampled during the tropical hot-wet, cool-dry and hot-dry seasons. Fifteen cyanobacteria taxa were recorded over the study period. Microcystis spp. and Cylindrospermopsis spp., known cyanotoxins producers, were the most prevalent bloom-forming taxa found in the study, with overall Cyanobacteria relative abundances being greatest during the cool-dry season. This was likely driven by decreased river inflows and increased reservoir mixing during the cool-dry period. Combinations of macrophyte cover, dissolved oxygen levels, water transparency, reactive phosphorus, water depth and chemical oxygen demand were found to significantly affect cyanobacteria community structure. The study highlights that under climate change forecasts (for much of tropical arid Africa), potentially harmful and problematic algal species may proliferate. Management options, therefore, need to be explored to maintain water quality and potable availability to mitigate against indirect harmful effects of environmental changes on ecosystems and human communities that utilise their services.

pCO2 Dynamics of Stratified Reservoir in Temperate Zone and CO2 Pulse Emissions During Turnover Events

This study explores the dynamic changes in the partial pressure of CO2 (pCO2) with depth, and the temporal variations of CO2 net atmospheric flux (NAF) in a stratified reservoir. A total of 16 field campaigns were conducted from the summer stratification to fall turnover period in 2017. A random forest (RF) model was developed to estimate the pCO2 using concurrently measured water quality variables. The results showed that the vertical distribution of pCO2 and associated temporal variations of the NAF are closely related to the stratification strength of the reservoir. The reservoir surface pCO2 was supersaturated (1542 µatm) in summer (July 11), but this decreased to undersaturation as algae grew. Meanwhile, dissolved CO2 continuously accumulated below the reservoir mixed-layer due to the thermal stratification barrier and organic-rich floodwater intrusion. Vertical mixing began instantly as the stratification strength began to weaken in mid-October, and the surface pCO2 increased sharply up to 1934 µatm. Consequently, the NAF drastically increased to 3235 mg−CO2 m−2·day−1, which implies that the NAF changes seasonally and large CO2 pulsing occurs during the turnover events. The results provide valuable information about pCO2 variability and physical mixing processes, as well as carbon budget estimation in stratified reservoirs, and offer an improved understanding of these phenomena.