Underestimated methane production triggered by phytoplankton succession in river-reservoir systems: Evidence from a microcosm study

Authors

Haolian Xu, Hong Li, Zhenzhen Tang, Yi Liu Guo Li, Qiang He

The impoundment of dammed rivers accelerates phytoplankton succession from river-dominated to lake-dominated species. Little is known about the role of phytoplankton succession in methane (CH₄) production. In this study, we performed a 61-day microcosm investigation to simulate the collapse processes of Cyclotella meneghiniana (river-dominated algae) and Chlorella pyrenoidosa and Microcystis aeruginosa (lake-dominated algae). The results suggested that different methanogenic conditions were induced by the collapse of river-and lake-dominated algae. The rapid settlement of C. meneghiniana induced aerobic conditions in the water that inhibited anaerobic CH₄ production and intensified CH₄ oxidation as a result of an increase in pmoA. However, the decomposition of C. pyrenoidosa and M. aeruginosa depleted dissolved oxygen and provided abundant labile organic matter, which jointly elevated mcrA and the mcrA/pmoA ratio. Under this condition, anaerobic CH₄ production was the dominant pathway for the mineralization of algae-derived carbon. Finally, the CH₄ produced per unit of particulate total carbon (identified as the carbon content of the algal biomass) by C. pyrenoidosa and M. aeruginosa was 16.29-fold and 8.56-fold higher, respectively, than that produced by C. meneghiniana. These observations provided evidence that lake-dominated algae played a more vital role in CH₄ production than river-dominated algae when algal succession occurred. This discovery might be a new and vital, yet largely underestimated CH₄ emission pathway in river-reservoir systems, that should be considered when evaluating the effect of hydraulic projects on greenhouse gas emissions.

Variable Oxygen Levels Lead to Variable Stoichiometry of Benthic Nutrient Fluxes in a Hypertrophic Estuary

Authors

Marco Bartoli, Sara Benelli, Marta Lauro, Monia Magri, Irma Vybernaite-Lubiene & Jolita Petkuviene

Harmful blooms of cyanobacteria may extend over long time spans due to self-sustaining mechanisms. We hypothesized that settled blooms may increase redox-dependent P release and unbalance the stoichiometry of benthic nutrient regeneration (NH₄⁺:SiO₂:PO₄³⁻ ratios). We tested this hypothesis in the hypertrophic Curonian Lagoon, the largest in Europe. During summer, at peak chlorophyll and water temperatures, sediment cores were collected over 19 stations representing all the lagoon sedimentary environments. Sediment organic content, granulometry, aerobic respiration, and oxic and anoxic fluxes of dissolved inorganic nutrients and metals—Fe²⁺ and Mn²⁺—were measured. Loads and stoichiometry of regenerated nutrients were compared with those from the watershed. Analyzed sediments had elevated oxygen demand (−1.90 to −5.66 mmol O₂ m⁻² h⁻¹), generally uncoupled to their variable organic matter content (1–23%) and median particle size (30–300 μm). Under oxic conditions, summer internal recycling equaled (SiO₂) or exceeded, by a factor of ~66 and ~ 2, external loads of NH₄⁺ and PO₄³⁻, respectively. Transient anoxia produced a general decrease of NH₄⁺ and SiO₂ regeneration, likely due to decreased macrofauna activity or inefficient mineralization, whereas it doubled average PO₄³⁻ fluxes. In sandy, well-flushed areas, anoxia had a minor effect on PO₄³⁻, but stimulated a large production of Mn²⁺. Muddy sediments in lagoon areas with slow water renewal displayed large redox-dependent PO₄³⁻ mobility, coupled to Fe²⁺ release. Settled algal blooms and hypoxic conditions might unbalance benthic regeneration stoichiometry and sustain blooms. The sedimentary pool of Mn⁴⁺ may represent a natural buffer preventing iron reduction and PO₄³⁻ mobility.

Grain size tunes microbial community assembly and nitrogen transformation activity under frequent hyporheic exchange: A column experiment

Authors

Yi Li, Jinxin Zhu, Longfei Wang, Yu Gao, Wenlong Zhang, Huanjun Zhang, Lihua Niu

Hyporheic zones (HZ) are hotspots for biogeochemical reactions where groundwater and surface water mix. River dam buildings and other hydrologic controls alter the sediment grain size distribution and modify the downstream hyporheic exchange, with cascading effects on geochemical and microbial processes in river corridors. In this lab-scale column experiment, the N transformations in HZ filled with sediments in different grain sizes were investigated with a focus on understanding the interplay among variational hydraulic connectivity, microbial community structure, functional potential under frequent groundwater−surface water exchange. Porosity was identified as the main driver determining bacterial community assembly in HZ sediments. Significant microbial zonation was observed along the columns and the degree of co-occurrence of bacterial communities in the Fine column was lower than that in the Coarse and Mix columns. The Coarse column allowed for almost 2.47 times the exchange flux relative to the Fine column, and generates the fastest DO consumption rate (−6.52 μg O₂/L·s). The enrichment of nitrifiers, i.e., Cytophagaceae and Bacillaceae and nitrification functional genes, i.e., amoA_AOA and amoA_AOB revealed the higher nitrification potential in column filled with coarse sediments. In comparison, the highest NH₄⁺ production rates (2.4 × 10⁻³ μg N/L·s) took place in Fine column. The higher abundancies of denitrifiers such as Comamonadaceae and Lysobacter and enrichment of functional genes of nirK and nirS interestingly suggested the elevated denitrification potential in Fine column in a more anaerobic environment. The results implied that variations in microbial functional potential and associated nitrogen transformation may occur in size-fractioned HZ to dynamic hyporheic exchange, which added new knowledge to the underlying biogeochemical and ecological processes in regulated river corridors.

Water column changes under ice during different winters in a mid-latitude Mediterranean high mountain lake

Authors

Ignacio Granados, Manuel Toro, Santiago Giralt, Antonio Camacho & Carlos Montes

Winter limnology has been developed mainly in northern-temperate or subarctic lakes, while mid-latitude Mediterranean high mountain lakes have been less studied during the ice-covered season. We present a five-year study in one of these lower latitude high mountain lakes, conducted with multiprobes located at two different depths recording high-frequency data. We have studied highly contrasted winters regarding temperature and snowfall. The results of a principal component analysis and a subsequent hierarchical clustering show that bottom oxygen depletion under ice is strongly influenced by winter cover thickness and transparency. A thick and opaque winter cover (dark mode) leads to an intense oxygen depletion until anoxia, while a thin and clear winter cover (clear mode) allows higher dissolved oxygen saturation due to oxygen production by photosynthesis. Under unusual conditions of cold and dry climate, only a weak decrease in bottom dissolved oxygen was observed. The extent of dissolved oxygen depletion and its relationship with meteorological parameters and climate patterns are further investigated with generalized additive models. Winter cover thickness and phenology are driven by the joint effect of the North Atlantic Oscillation and East Atlantic climatic patterns. Ice phenology could also influence the following ice-free period given the correlation of lake heat content with the ice-off date and the ice cover length. We propose some features jointly occurring in Lake Cimera as a model for mid-latitude Mediterranean high mountain lakes, including (yet not exclusive) the occurrence of very thick winter covers, the prevalence of dark mode, the relatively high and similar through the years oxygen depletion rates, and high water temperatures during the ice-free periods. It is unclear how the ongoing climate change will affect the prevalence of dark over clear modes due to the expected antagonistic effects of warmer temperatures and lower snowfalls.

Wide-spread inconsistency in estimation of lake mixed depth impacts interpretation of limnological processes

Authors

Emma Gray, Eleanor B. Mackay, J. Alex Elliott, Andrew M. Folkard, Ian D. Jones

The mixed layer, or epilimnion, is a physical concept referring to an isothermal layer at the surface of a water body. This concept is ubiquitous within limnology, is fundamental to our understanding of chemical and ecological processes, and is an important metric for water body monitoring, assessment and management. Despite its importance as a metric, many different approaches to approximating mixed depth currently exist. Using data from field campaigns in a small meso-eutrophic lake in the UK in 2016 and 2017 we tested whether different definitions of mixed depth resulted in comparable estimates and whether variables other than temperature could be assumed to be mixed within the layer. Different methods resulted in very different estimates for the mixed depth and ecologically important variables were not necessarily homogenously spread through the epilimnion. Furthermore, calculation of simple ecologically relevant metrics based on mixed depth showed that these metrics were highly dependent on the definition of mixed depth used. The results demonstrate that an idealised concept of a well-defined fully mixed layer is not necessarily appropriate. The widespread use of multiple definitions for mixed depth impairs the comparability of different studies while associated uncertainty over the most appropriate definition limits the confirmability of studies utilising the mixed depths.

 

On the calculation of lake metabolic rates: Diel O2 and 18/16O technique

Authors

Frank Peeters, Hilmar Hofmann, Jorge Encinas Fernández

Metabolic transformations have a major impact on the development of primary producers in aquatic systems and thus affect the dynamics of the entire aquatic food web. Furthermore, metabolic transformations contribute to the carbon budget and thereby influence CO₂ emissions from aquatic systems. Several techniques have been developed that aim at an easy assessment of metabolic rates over long time periods or in many systems. The ^18/16O technique, which utilizes the isotopic fractionation between ¹⁸O and ¹⁶O isotopes due to metabolic transformations, is receiving increasing popularity in studies comparing the metabolism in many different lakes and served as basis for the conclusions that production increases with increasing atmospheric CO₂ and that surprisingly little terrestrial carbon is recycled in lakes of the arid circumpolar landscape. However, we demonstrate here that the steady state assumptions underlying the ^18/16O technique cause large uncertainties in the estimated metabolic rates. This conclusion is based on a sensitivity analysis using a numerical model of dissolved oxygen, DO, and of dissolved ¹⁸O, ¹⁸O_DO, but is also confirmed by published metabolic rates estimated from the ^18/16O and the diel O₂ techniques. Metabolic rates obtained from the ^18/16O technique appear unsuited for correlation analyses between lakes but may provide reasonable estimates in systems with low and long-term stable production. In addition we illustrate that the combination of few ¹⁸O measurements with the diel O₂ technique and an inverse fitting procedure can improve estimates of metabolic rates and in particular of respiration rates.

Predicting biochemical oxygen demand in European freshwater bodies.

Authors

Olga Vigiak, Bruna Grizzetti, Angel Udias-Moinelo, Michela Zanni, Chiara Dorati, Fayçal Bouraoui, Alberto Pistocchi

Biochemical Oxygen Demand (BOD) is an indicator of organic pollution in freshwater bodies correlated to microbiological contamination. High BODconcentrations reduce oxygen availability, degrade aquatic habitats and biodiversity, and impair water use. High BOD loadings to freshwater systems are mainly coming from anthropogenic sources, comprising domestic and livestock waste, industrial emissions, and combined sewer overflows. We developed a conceptual model (GREEN⁺_BOD) to assess mean annual current organic pollution (BOD fluxes) across Europe. The model was informed with the latest available European datasets of domestic and industrial emissions, population and livestock densities. Model parameters were calibrated using 2008–2012 mean annual BOD concentrations measured in 2157 European monitoring stations, and validated with other 1134 stations. The most sensitive model parameters were abatement of BOD by secondary treatment and the BOD decay exponent of travel time. The mean BOD concentrations measured in monitored stations was 2.10 mg O₂/L and predicted concentrations were 2.54 mg O₂/L; the 90th percentile of monitored BOD concentration was 3.51 mg O₂/L while the predicted one was 4.76 mg O₂/L. The model could correctly classify reaches for BOD concentrations classes, from high to poor quality, in 69% of cases. High overestimations (incorrect classification by 2 or more classes) were 2% and large underestimations were 5% of cases. Across Europe about 12% of freshwater network was estimated to be failing good quality due to excessive BOD concentrations (>5 mg O₂/L). Dominant sources of BOD to freshwaters and seas were point sources and emissions from intensive livestock systems. Comparison with previous assessments confirms a decline of BOD pollution since the introduction of EU legislation regulating water pollution.

Distribution of Humic Fluorescent Dissolved Organic Matter in Lake Shihwa: the Role of the Redox Condition

Authors

Jeonghyun Kim, Tae-Hoon Kim

Hypoxia has occurred worldwide in coastal and marginal oceans. The redox condition has recently garnered major interest as a favorable condition for delivering sedimentary organic matter to the water column. In this study, we measured the fluorescence of fluorescent dissolved organic matter (FDOM) in brackish Lake Shihwa, Korea, in which hypoxic conditions are typically observed during summer. Especially, fluorescent intensities of the humic FDOM (FDOM_H) were relatively high during summer, with a relatively lower dissolved oxygen (DO) level, and exponentially increased with decreasing DO concentrations. The results indicated that the production of FDOM_H is associated with anaerobic processes. It was further supported by a significantly positive relationship between FDOM_H and ammonium. Based on the relatively low values of redox potential (up to 60.0 mV) and high concentration of phosphate with the low DO level, this enrichment of FDOM_H seems to be linked to the input of anoxic porewater. Using a simple schematic diagram, the contribution of FDOM_H from reducing environments is comparable to that from stream water, which is known to be a major source in coastal regions. This study highlights that the redox condition is a key factor contributing to the production of FDOM_H in coastal marine environments.

Nitrogen and Phosphorus Exchanges Across the Sediment‐Water Interface in a Bay of Lake Chaohu

Authors

Liu, Cheng; Gu, Xiaozhi; Chen, Kaining; Fan, Chengxin; Zhang, Lei; Huang, Wei

A year-long field investigation was carried out in the most heavily polluted bay of Lake Chaohu to assess the temporal exchanges of ammonium nitrogen () and soluble reactive phosphorus (SRP) across the sediment‐water interface (SWI) and to provide remediation advises. Results showed that the monthly average fluxes of and SRP were 31.38 and 6.98 mg m^‐2 d^‐1, respectively, both of which were higher than those in many other hyper-eutrophic lakes around the world. The exchanges of and SRP were both closed related to the oxygen penetration. Low oxygen penetration depth and generally negative oxygen uptake rates provoked the dissolution of redox sensitive phosphorus and labile in the sediment and increased the fluxes. In addition, the generally higher fluxes during late spring to autumn should be noted during the reduction of internal loadings, when applicable techniques should be implemented accordingly to achieve better reduction effects.

Water temperature dynamics and the prevalence of daytime stratification in small temperate shallow lakes

Authors

Kenneth Thorø Martinsen, Mikkel René Andersen, Kaj Sand-Jensen

Small lakes are understudied compared to medium- and large-sized lakes, but have recently received increased attention due to their abundance and importance for global scale biogeochemical cycles. They have close terrestrial contact, extensive environmental variability, and support high biodiversity among them. Temporal and spatial variability of water temperature, oxygen, and stratification–mixing dynamics were examined during a year in nine small Danish lakes. We found that diel mean surface water temperatures were similar among lakes while the diel range decreased with increasing water depth. Vertical temperature stratification occurred on 47% of the days during the entire year and 64% of summer days, usually with daytime stratification and nocturnal convective mixing. The probability of daytime stratification increased with higher incident irradiance, higher air temperature, and lower wind speed. During spring, daytime stratification caused differences in oxygen saturation between surface and bottom waters. These findings offer new insights on the high variability of water temperature and oxygen in time and space in small temperate shallow lakes. The variable water temperature and the regular stratification–mixing processes will have a pronounced influence on biogeochemical cycles. Also, these features are expected to affect the performance and evolutionary process of organisms associated with small lakes.