Low Dissolved Organic Carbon Research Articles

Application of UVC-LED/H2O2 in wastewater treatments: treatment efficacy on disinfection byproduct precursors and micropollutants

The applications of advanced oxidation processes (AOPs) for controlling microcontaminants are essential to meet the water quality criteria for potable or nonpotable water reuses. The objective of this study is to demonstrate the application of light emitting diode (LED) as a possible light source to substitute traditional low-pressure mercury lamp (LPUV) in UV/H2O2 processes in treating precursors of disinfection byproducts (DBPs) and pharmaceutical and personals care products (PPCPs) in wastewater. The results of this study revealed that UV fluence plays the most crucial role in the efficiency of UV/H2O2. At the same time, the initial concentration of H2O2, dissolved organic carbon (DOC), and turbidity had minimal effects, except that poor efficiency result of UV/H2O2 was observed at a solution with low DOC concentration (2.4 mg L−1). Although the concentrations of organic matter decreased after UV/H2O2 treatment, the concentration of precursors of DBPs increased in the early stage of the photolysis process and decreased after that; moreover, the profiles of precursors for trihalomethanes and haloacetic acids were different. A comparison between LPUV and UVC-LED as light sources revealed that, at a fixed UV fluence input into the UV/H2O2 process, the trends and efficiencies in the degradation of organic matter and DBP precursors were similar. Meanwhile, the photoelectric conversion efficiency of UVC-LED should be improved for future applications in water treatment. Based on the UV/H2O2 treatment results on synthetic PPCPs wastewater solution, this study showed the effectiveness of UV/H2O2 to degrade micro organic contaminants.

Dissolved Organic Radiocarbon in the West Indian Ocean

AbstractWe report marine dissolved organic carbon (DOC) concentrations, and DOC Δ14C and δ13C in seawater collected from the West Indian Ocean during the GO‐SHIP I07N cruise in 2018. We find bomb 14C in DOC from the upper 1,000 m of the water column. There is no significant change in ∆14C of DOC in deep water northward, unlike that of dissolved inorganic carbon (DIC), suggesting that transport of deep water northward is not controlling the 14C age of DOC. Variability of DOC ∆14C, including high values in the deep waters, is more pronounced than in other oceans, suggesting that dissolution of surface derived particulate organic carbon is a source of modern carbon to deep DOC in the West Indian Ocean. Low δ13C are present at two of the five stations studied, suggesting a source of low δ13C DOC, or additional microbial utilization of deep DOC.

Naturally-induced biphasic phytoplankton spring bloom reveals rapid and distinct substrate and bacterial community dynamics.

Phytoplankton spring blooms are typical features in coastal seas and provide heterotrophic bacteria with a rich blend of dissolved substrates. However, they are difficult to study in coastal seas in-situ. Here, we induced a phytoplankton spring bloom and followed its fate for 37 days in four 600 L-mesocosms. To specifically investigate the significance of phytoplankton-born dissolved organic carbon (DOC) we used artificial seawater with low DOC background and inoculated it with a 100 µm-prefiltered plankton community from the North Sea. A biphasic bloom developed, dominated by diatoms and Phaeocystis globosa respectively. In between, bacterial numbers peaked, followed by a peak in virus-like particles, implying that virus infection caused the collapse. Concentrations of dissolved free amino acids exhibited rapid changes, in particular during the diatom bloom and until the peak in bacterial abundance. Dissolved combined amino acids and neutral monosaccharides accumulated continuously, accounting for 22% of DOC as a mean and reaching levels as high as 44%. Bacterial communities were largely dominated by Bacteroidetes, especially the NS3a marine group (family Flavobacteriaceae), but Rhodobacteraceae and Gammaproteobacteria were also prominent members. Our study shows rapid organic matter and community composition dynamics that are hard to trace in natural coastal ecosystems.

Depth Moderates DOC Impact on Cold‐Water Refugia in Small, Northern Temperate Lakes

AbstractWidespread changes in dissolved organic carbon (DOC) concentrations in lakes of the Northern Hemisphere are well documented, and there is a need to understand the ecological implications for changes in cold‐water refugia. DOC controls thermal structure in small, wind‐sheltered lakes, thus influencing the volume and persistence of the hypolimnion and cold‐water habitat. We explored the relationship between the quantity and quality of DOC and the availability of cold‐water habitat over the course of summer stratification in small lakes of varying depths. The ratio of the hypolimnetic volume to total lake volume was calculated to assess the relative changes in cold‐water habitat availability in each lake over time. DOC concentration, maximum lake depth, and the interaction between DOC concentration and maximum lake depth explained 87% of the variability in percent change in hypolimnion volume ratio across studied lakes. Hypolimnion compression was greater in shallow, low DOC lakes, whereas deeper, intermediate DOC lakes had more persistent hypolimnion volumes over the course of summer stratification. These results demonstrate that depth moderates the effect of DOC on the volume and persistence of cold‐water refugia availability in north temperate lakes, advancing our understanding of differing lake sensitivity in a rapidly changing physical and chemical environment.

Chronic Toxicity of Iron to Aquatic Organisms under Variable pH, Hardness, and Dissolved Organic Carbon Conditions.

A series of chronic toxicity tests was conducted exposing three aquatic species to iron (Fe) in laboratory freshwaters. The test organisms included the green algae Raphidocelis subcapitata, the cladoceran Ceriodaphnia dubia, and the fathead minnow Pimephales promelas. They were exposed to Fe (as Fe (III) sulfate) in waters under varying pH (5.9-8.5), hardness (10.3-255 mg/L CaCO3 ), and dissolved organic carbon (DOC; 0.3-10.9 mg/L) conditions. Measured total Fe was used for calculations of biological effect concentrations because dissolved Fe was only a fraction of nominal and did not consistently increase as total Fe increased. This was indicative of the high concentrations of Fe required to elicit a biological response and that Fe species that did not pass through a 0.20- or 0.45-µm filter (dissolved fraction) contributed to Fe toxicity. The concentrations frequently exceeded the solubility limits of Fe(III) under circumneutral pH conditions relevant to most natural surface waters. Chronic toxicity endpoints (10% effect concentrations [EC10s]) ranged from 442 to 9607 µg total Fe/L for R. subcapitata growth, from 383 to 15 947 µg total Fe/L for C. dubia reproduction, and from 192 to 58,308 µg total Fe/L for P. promelas growth. Toxicity to R. subcapitata was variably influenced by all three water quality parameters, but especially DOC. Toxicity to C. dubia was influenced by DOC, less so by hardness, but not by pH. Toxicity to P. promelas was variable, but greatest under low hardness, low pH, and low DOC conditions. These data were used to develop an Fe-specific, bioavailability-based multiple linear regression model as part of a companion publication. Environ Toxicol Chem 2023;00:1-15. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Drivers of stability and transience in composition-functioning links during serial propagation of litter-decomposing microbial communities.

Biotic factors that influence the temporal stability of microbial community functioning are an emerging research focus for the control of natural and engineered systems. The discovery of common features within community ensembles that differ in functional stability over time is a starting point to explore biotic factors. We serially propagated a suite of soil microbial communities through five generations of 28-day microcosm incubations to examine microbial community compositional and functional stability during plant litter decomposition. Using dissolved organic carbon (DOC) abundance as a target function, we hypothesized that microbial diversity, compositional stability, and associated changes in interactions would explain the relative stability of the ecosystem function between generations. Communities with initially high DOC abundance tended to converge towards a "low DOC" phenotype within two generations, but across all microcosms, functional stability between generations was highly variable. By splitting communities into two cohorts based on their relative DOC functional stability, we found that compositional shifts, diversity, and interaction network complexity were associated with the stability of DOC abundance between generations. Further, our results showed that legacy effects were important in determining compositional and functional outcomes, and we identified taxa associated with high DOC abundance. In the context of litter decomposition, achieving functionally stable communities is required to utilize soil microbiomes to increase DOC abundance and long-term terrestrial DOC sequestration as one solution to reduce atmospheric carbon dioxide concentrations. Identifying factors that stabilize function for a community of interest may improve the success of microbiome engineering applications. IMPORTANCE Microbial community functioning can be highly dynamic over time. Identifying and understanding biotic factors that control functional stability is of significant interest for natural and engineered communities alike. Using plant litter-decomposing communities as a model system, this study examined the stability of ecosystem function over time following repeated community transfers. By identifying microbial community features that are associated with stable ecosystem functions, microbial communities can be manipulated in ways that promote the consistency and reliability of the desired function, improving outcomes and increasing the utility of microorganisms.

Comparing Sedimentation, Flotation, and In-Line Pretreatment for Low-Pressure Membrane Fouling Reduction

Feed water pretreatment commonly is required for low-pressure membrane technologies employed in drinking water treatment applications to reduce membrane fouling and create stable operating conditions. Comparatively few studies have investigated coagulation–flocculation–dissolved air flotation (CF-DAF) pretreatment for drinking water applications, and none have compared CF-DAF, coagulation–flocculation–sedimentation (CF-S), and in-line coagulation (CF-IN) pretreatments using the same water. This study compared these three pretreatments for the filtration of a high dissolved organic carbon (DOC), high hydrophobic (HPO) surface water using a hydrophilic polyvinylidene fluoride (h-PVDF) ultrafiltration (UF) fiber. Multiday filtration tests were carried out using an automated bench-scale testing system operated in an outside-in configuration. CF-S and CF-DAF were found to be equally effective at mitigating membrane fouling, although CF-DAF pretreated water had a lower residual DOC and the greatest removal of UV254 absorbent organics. Compared with CF-DAF and CF-S, CF-IN pretreatment resulted in higher levels of total and irreversible fouling regardless of the applied coagulant dose. For all the pretreatments studied, irreversible membrane fouling was found to be strongly dependent on both the hydrophobicity of the feed water [in terms of specific UV absorbance (SUVA)] and the concentration of the 5–10-kDa DOC fraction, suggesting that the HPO humic organics were the principal foulant for this membrane–water combination. CF-IN pretreatment performance also was strongly impacted by the feed water zeta potential, suggesting that the characteristics of the formed flocculant particles are critical to the fouling behavior of the hybrid CF-IN-UF system.

Dissolved organic matter in the northwestern Pacific marginal seas: Insight into the distribution of its optical properties

Dissolved organic carbon (DOC) and the optical properties of dissolved organic matter (chromophoric- and fluorescent dissolved organic matter; CDOM and FDOM) were measured to determine the distributions and drivers of DOM in the northwestern Pacific marginal seas, including the East China Sea, the Yellow Sea, and the East/Japan Sea, in August 2020. In this study, the concentrations of DOC and CDOM/FDOM in surface water showed good correlations with salinity, indicating a predominant contribution from the Changjiang River. However, significantly high concentrations of DOC and FDOM were also observed in the central Yellow Sea region, which seems to be produced mainly from the continental shelf-water and enriched over the water residence time of the Yellow Sea, while the lowest concentrations of DOC and CDOM/FDOM were found in high-salinity waters near the southern sea of Korea. In addition, the East/Japan Sea showed relatively low DOC concentrations and high FDOM values. To distinguish the physical mixing and biogeochemical processes of DOM, we estimated the water mass fractions using an optimum multi-parameter analysis with hydrological and DOM parameters of the major water masses in this region. Our results showed that five primary external drivers of DOM distribution were intrusions of (1) the Changjiang diluted water and (2) the Kuroshio Current water in the surface water, the mixing of (3) the Yellow Sea bottom cold water and (4) the Kuroshio sub-surface water in the bottom layer, and (5) upwelling of the East/Japan Sea water. We conclude that the distribution of DOM is significantly influenced by the mixing of multiple water masses, and the optical signature of DOM can be an effective proxy for tracing the origins and characteristics of DOM in this region.

Diversity, distribution, and expression of opsin genes in freshwater lakes.

Microbial rhodopsins are widely distributed in aquatic environments and may significantly contribute to phototrophy and energy budgets in global oceans. However, the study of freshwater rhodopsins has been largely limited. Here, we explored the diversity, ecological distribution, and expression of opsin genes that encode the apoproteins of type I rhodopsins in humic and clearwater lakes with contrasting physicochemical and optical characteristics. Using metagenomes and metagenome-assembled genomes, we recovered opsin genes from a wide range of taxa, mostly predicted to encode green light-absorbing proton pumps. Viral opsin and novel bacterial opsin clades were recovered. Opsin genes occurred more frequently in taxa from clearwater than from humic water, and opsins in some taxa have nontypical ion-pumping motifs that might be associated with physicochemical conditions of these two freshwater types. Analyses of the surface layer of 33 freshwater systems revealed an inverse correlation between opsin gene abundance and lake dissolved organic carbon (DOC). In humic water with high terrestrial DOC and light-absorbing humic substances, opsin gene abundance was low and dramatically declined within the first few meters, whereas the abundance remained relatively high along the bulk water column in clearwater lakes with low DOC, suggesting opsin gene distribution is influenced by lake optical properties and DOC. Gene expression analysis confirmed the significance of rhodopsin-based phototrophy in clearwater lakes and revealed different diel expressional patterns among major phyla. Overall, our analyses revealed freshwater opsin diversity, distribution and expression patterns, and suggested the significance of rhodopsin-based phototrophy in freshwater energy budgets, especially in clearwater lakes.

Machine‐Learning Reveals Equifinality in Drivers of Stream DOC Concentration at Continental Scales

AbstractResearch at long‐term catchment monitoring sites has generated a great volume, variety, and velocity of data for analysis of stream water chemistry dynamics. To harness the potential of these big data and extract patterns that are indicative of underlying functional relationships, machine learning tools have advantages over traditional statistical methods, and are increasingly being applied for dimension reduction, feature extraction, and trend identification. Still, as examples of complex systems, catchments are characterized by multivariate factor interactions and equifinality that are not easily identified by most machine‐learning methods. Using dissolved organic carbon (DOC) dynamics as an illustration, we applied a new evolutionary algorithm (EA) to extract geologic, topographic, meteorologic, hydrologic, and land use attributes that were correlated to mean stream DOC concentration in forested catchments distributed across the continental United States. The EA reduced dimensionality of our attribute dataset to identify the combination of factors, and their specific value ranges, that interacted to drive membership in High or Low mean DOC clusters. High mean DOC concentrations were associated with two distinct geographic locations of variable climatic and vegetative conditions, indicating equifinality. Our findings underscore the importance of critical zone structure in mediating hydrological and biogeochemical processes to govern DOC dynamics at the catchment scale. This multi‐scale, pattern‐to‐process approach is being applied to refine hypotheses for process‐based modeling of DOC dynamics in forested headwater streams at catchment to site scales.

Ground-ice origin and age on Herschel Island (Qikiqtaruk), Yukon, Canada

Glacial legacies preserved in permafrost such as buried glacial ice of the last ice age are of increasing concern in the Western Canadian Arctic. Permafrost collapse due to melting ground ice largely follows the margins of the maximum Laurentide Ice Sheet extent and therefore predetermines the postglacial landscape evolution in this region. Another type of ground ice, (i.e., wedge ice) of late Pleistocene and Holocene age associated with permafrost aggradation, is also widespread here. Our study on Herschel Island (Qikiqtaruk, Beaufort Sea) re-examines previous data and applies stable-isotope and dissolved organic carbon (DOC) analyses as well as radiocarbon dating on DOC and particulate plant macrofossil remains preserved in massive ground ice, wedge ice and host deposits.Newly obtained DOC ages of the massive ice span from 32 220 to 25 830 cal BP and extend the only previously available direct age determination (CO2-derived radiocarbon age of 21 290 cal BP) properly into the Last Glacial Maximum as the formation time of the massive ice. Its newly obtained isotopic composition exhibits mean values of −33.1 ± 0.6‰ in δ18O, of −257 ± 4‰ in δD and 7.6 ± 0.9‰ in deuterium excess (d) fitting into previously reported respective data ranges. The very low (negative) stable isotope composition of the massive ice, the numerous enclosed spherical air bubbles as well as the very low mean DOC content of 0.7 ± 0.1 mg L−1 provide strong evidence for an origin as glacier ice that was buried and has survived since deglaciation.The newly studied wedge ice on Herschel Island formed during the Holocene between 9220 and 3470 cal BP, and shows a distinct isotopic composition with mean values of −18.4 ± 1.1‰ in δ18O, −138 ± 9‰ in δD and a mean deuterium excess (d) of 8.8 ± 1.4‰. Such isotopic wedge-ice record as well as its mean DOC concentration of 4.3 ± 2.1 mg L−1 fall within the range of previously studied Holocene ice wedges in the region. The directly-dated stable isotope record of ice-wedge growth on Herschel Island indicates a winter cooling trend towards the mid-Holocene, even though most available records attribute this cooling rather to the warm season while the winter temperatures over this period are not constrained yet.The extraordinarily rich ground-ice inventory of Herschel Island offers insights into the glacial and postglacial landscape evolution of the Western Canadian Arctic. Here, paleoenvironmental research highlights ice sheet-permafrost interactions over glacial, deglacial and postglacial timescales.

Groundwater Plays an Important Role in Controlling Riverine Dissolved Organic Matter in a Cold Alpine Catchment, the Qinghai–Tibet Plateau

AbstractIn cold alpine areas, the hydrological control on dissolved organic matter (DOM) export in relation to permafrost distribution remains uncertain on a catchment scale. To address this, the seasonal variations in DOM chemical features of groundwater and stream water were examined from 2013 to 2019 in a representative catchment on the northeastern Qinghai–Tibet Plateau, which is covered by both permafrost (at elevations higher than 3,500 m a.s.l.) and seasonally frozen areas. The results show that the highest dissolved organic carbon (DOC) concentration occurred during the thawing period (April–May), while the highest DOC flux occurred during the thawed period (June–early October). In the permafrost area, the freeze–thaw process significantly influenced the groundwater flow paths and DOM. During the thawing period, with shallow groundwater flow path through the organic soil layer, the stream water DOM recharged by suprapermafrost groundwater was characterized by high aromaticity and high DOC concentrations. During the thawed period, the deepening groundwater flow path through the mineral layer and dilution by overland flow, resulted in low DOC concentrations and DOM with lower aromaticity in stream water. When groundwater flowed from the permafrost area into the porous aquifers in the seasonally frozen area, the groundwater flow path also affected DOM dynamics. The stream water at the catchment outlet, which received groundwater discharge from the porous aquifer, showed a lower peak DOC concentration, a higher proportion of protein‐like DOM, and a higher contribution of cold season DOC flux to annual DOC flux when compared to the water flowing out from the permafrost area. Our work highlights the role of the thick, porous aquifer in the seasonally frozen area in regulating DOM export. The results suggest that the cold alpine area may exhibit a stronger positive feedback to climate warming than that in Arctic areas.

Study of the Interaction of Dissolved Organic Carbon, Available Nutrients, and Clay Content Driving Soil Carbon Storage in the Rice Rotation Cropping System in Northern Thailand

The appropriate management of crop residues in a rice rotation cropping system (RRCS) can promote carbon storage and contribute to soil health. The objective of this study was to determine and analyze the amount of organic carbon in the soil, the amount of labile carbon in a dissolved state in the soil, and the physicochemical properties of the soil and their relationship with soil organic carbon dynamics under the RRCS in northern Thailand. The RRCS can be divided into the following four categories by pattern: (1) Rice_F (rice (Oryza sativa) followed by a fallow period); (2) Rice_S (rice followed by shallots (Allium cepa L.); (3) Rice_Mixed crop (rice followed by tobacco (Nicotiana tabacum), vegetables, or maize (Zea mays)); and (4) Rice_P (rice followed by potatoes (Solanum tuberosum)). These patterns can be classified according to the dissolved organic carbon (DOC), the availability of nutrients from fertilization, and clay contents. In our study, the Rice-F and Rice-S patterns led to higher soil organic carbon (SOC) and dissolved organic carbon (DOC) in the soil, but when the Rice-P pattern was followed, the soil had a lower clay content, lower available phosphorus (Avail P), the lowest DOC, and high contents of calcium (Ca2+) and magnesium (Mg2+). This study also revealed that on the basis of relationships, clay content, Avail P, and DOC were the most important factors for the formation of SOC, while Ca2+ and Mg2+ were the subordinate factors for the decreased formation of SOC and carbon storage when the RRCS was followed. In addition, low SOC/clay when the Rice-P pattern was followed could reflect carbon saturation, while the percentages of DOC/SOC could indicate the decomposition and formation of SOC.

Chromophoric Dissolved Organic Matter and Dissolved Organic Carbon in Lakes Across an Elevational Gradient From the Mountains to the Sea

AbstractDissolved organic matter (DOM) in lakes across elevation gradients is a complex function of topography, climate, vegetation coverage, land use, and lake properties. To examine sources and processing of DOM from sea level to mountain lakes (3–1,574 m), we measured dissolved organic carbon (DOC) concentrations and chromophoric dissolved organic matter (CDOM) optical properties, lake characteristics, and water quality parameters in 62 freshwater lakes in the Pacific Northwest, USA. Higher elevation lakes had lower DOC concentrations and absorbance. These lakes had higher forest cover and minimal wetlands in their watershed, in addition to low nutrients, water temperatures, and chlorophyll a in the lake itself. Two humic‐like and one protein‐like fluorescent component were identified from excitation‐emission matrix spectroscopy. The index of recent autochthonous contribution (BIX), fluorescence index (FIX), and SR optical indices showed that most lakes were dominated by terrestrially derived material. The humification index (HIX) and specific ultra‐violet absorbance (SUVA254) were consistent with more aromatic humic CDOM at lower elevations. The lower fluorescence of humic‐like components at higher elevation was attributed to lower inputs from vegetation. The relative contribution of the protein‐like component increased at higher elevation. This may be due to reduced allochthonous terrestrial inputs relative to in situ production of autochthonous material or increased photochemical/biological degradation of allochthonous material. Differences in optical characteristics associated with the amount and source of CDOM were observed across the elevational gradient. These differences were driven by characteristics at both within‐lake and watershed scales.

Dissolved organic matter dynamics and spectral characteristics of twig litter from different Castanopsis carlesii forests in the middle subtropical region, China

To assess the dynamics and spectral characteristics of dissolved organic matter of twig litter in continuous increase stage, peak stage, and continuous decrease stage of twig litter production in different types of Castanopsis carlesii forest in middle subtropical China, a field experiment was conducted in C. carlesii natural forest, secondary forest and plantation. The results showed that litter production stage and forest type significantly affected the content and spectral characteristics of dissolved organic matter of twig litter were . Compared with the secondary forest and plantation, natural forest had higher dissolved organic carbon (DOC) content and lower special ultraviolet-visible absorption values at 254, 260 and 280 nm (SUVA254, SUVA260, SUVA280) at the continuous decrease stage of twig litter production, indicating high twig litter quality of natural forest and high cycling efficiency with dissolved organic matter in the natural forest at this stage. In contrast, the higher contents of total nitrogen (TN), total phosphorus (TP), total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), and lower DOC:TDP and TDN:TDP ratios of twig litter in the plantation were observed at the peak stage of twig litter production, while no differences were detected in dissolved organic matter contents and spectral values in the secondary forest among the stages. In addition, the DOC, TDN, TDP of twig litter were negatively correlated with temperature and precipitation in the natural forests and secondary forests, but TDN and TDP of twig litter were positively correlated with temperature and precipitation in the plantations. These results suggested that the higher nutrient content at the peak stage of twig litter production in the C. carlesii plantation might lead to more efficient material cycling and that there would be a higher efficiency of material cycling for twig litter dissolved organic matter in C. carlesii natural forest at reduction stage of twig litter production.

Biomass, community composition and N:P recycling ratios of zooplankton in northern high‐latitude lakes with contrasting levels of N deposition and dissolved organic carbon

Abstract Global changes are causing decreases in inorganic nitrogen (N) concentrations, increases in coloured dissolved organic carbon (DOC) concentrations, and decreases in dissolved inorganic N to total phosphorus ratios (DIN:TP) in northern lakes. The effects of these changes on phytoplankton and zooplankton biomass and the N:P recycling ratio of zooplankton remain unresolved. In 33 Swedish headwater lakes across subarctic‐to‐boreal gradients with different levels of N deposition (low N in the north [Västerbotten, boreal; Abisko, subarctic] vs. high N in the south [Värmland, boreal; Jämtland, subarctic]), we measured water chemistry, phytoplankton biomass (chlorophyll‐a [Chl‐a], Chl‐a:TP), seston mineral quality (C:P, N:P), as well as zooplankton biomass, community composition, and C:N:P stoichiometry. We estimated nutrient imbalances and the N:P recycling ratios of zooplankton using ecological stoichiometry models. There was a large‐scale gradient from low lake DIN and DIN:TP in the north to high DIN and DIN:TP in the south, with lower DIN:TP in lakes coinciding with higher DOC within each region. Lower lake DIN was associated with lower phytoplankton biomass (lower Chl‐a:TP). Lower lake DIN:TP was associated with richer seston mineral quality (lower seston C:P and N:P) and higher zooplankton biomass. Zooplankton community composition differed in the north vs. south, with a dominance of N‐requiring calanoid copepods with high N:P in the north and P‐requiring cladocerans with low N:P in the south. Also, greater differences in zooplankton community composition were found between subarctic regions (with lower DOC) than between boreal regions (with higher DOC), suggesting that increases in lake DOC and associated declines in lake DIN:TP reduce differences in zooplankton community composition. The combination of lower lake DIN, higher lake DOC, and lower lake DIN:TP led to reduced zooplankton N:P recycling ratios, possibly by reducing seston N:P and/or by enhancing calanoid copepod dominance in the zooplankton community. Our findings suggest that the combination of declining N deposition and increasing lake browning in northern high‐latitude lakes will reduce phytoplankton biomass, but will concurrently enhance seston mineral quality and probably also zooplankton biomass and their recycling efficiency of P relative to N.

Dissolved organic carbon affects the occurrence of deep chlorophyll peaks and zooplankton resource use and biomass

Abstract Concentrations of dissolved organic carbon (DOC) are increasing in many parts of the world and vary considerably among lakes. As a result, it is important to understand the effects of DOC on zooplankton, which are a key component of freshwater food webs. Deep chlorophyll maxima (DCMs) also are common in many lakes and numerous studies suggest that they may be an important resource for zooplankton. In a survey of eight boreal lakes spanning a gradient of DOC (3.5–9.2 mg/L), we assessed variations in the occurrence of DCMs, zooplankton biomass and their use of basal resources using stable isotopes of carbon (C), nitrogen (N) and hydrogen (H). DCMs occurred only in lakes with lower DOC concentrations and Bayesian stable isotope mixing models indicated that they contributed between 22% and 72% of the assimilated diet of zooplankton. Food quality as indicated by C:P and chlorophyll a: C ratios increased with depth and in DCMs. The proportion of C ultimately derived by zooplankton from terrestrial sources increased from 6% to 27% with DOC. Zooplankton biomass among lakes declined with increasing DOC concentrations, the absence of DCMs and with increasing allochthony. Our results suggest that increasing inputs of DOC may suppress zooplankton in many lakes by affecting both resource quality and the availability of metalimnetic phytoplankton resources. Because zooplankton occupy a central position in freshwater food webs, declines in their biomass may further affect phytoplankton dynamics and fish productivity.

Source apportionment and elevational gradient of dissolved organic matter over the Tibetan plateau

Our understanding of water-soluble organic constituents and their transformation in the unique aqueous continuum over cryosphere region is scarce. Here, dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and dissolved total nitrogen (DTN) and water-soluble inorganic ions in multiple water bodies from the eastern Tibetan Plateau (TP) cryosphere are systematically determined from a suite of field campaigns, laboratory experiments, linear regression analysis, and multiple comparisons, etc. We found that the water bodies located at high elevation have much lower DOC contents compared to the samples at lower elevation, there has significant altitude dependence of DOC abundance in water bodies over the study area (elevation range: 1971–4700 m asl). Comparison of optical properties, source apportionment, chemical analysis and model simulation of the water bodies provide evidence that the atmospheric deposition of organic species in high mountains is transported to plateau lakes in the northeast of TP via alpine runoff (45%) and snow/ice meltwater (20%). Further, dominance of anthropogenic activities in lower elevations can contribute (35%) to the observed altitudinal dependency. Thus, this preliminary study represents the first systematic investigation of the transport and cycling of organic carbonaceous matter and nitrogenous matter in eastern TP and warrants more robust in-situ observations and measurements in future in High Mountains of Asia.

Organic carbon and eukaryotic predation synergistically change resistance and resilience of aquatic microbial communities

With rapid global urbanization, anthropogenic activities alter aquatic biota in urban rivers through inputs of dissolved organic carbon (DOC) and nutrients. Microorganisms-mediated global element cycles provide functions in maintaining microbial ecology stability. The DOC (bottom-up control) and microbial predation (top-down control) may synergistically drive the competition and evolution of aquatic microbial communities, as well as their resistance and resilience, for which experimental evidences remain scarce. In this study, laboratory sediment-water column experiments were employed to mimic the organic carbon-driven water blackening and odorization process in urban rivers and to elucidate the impact of DOC on microbial ecology stability. Results showed that low (25–75 mg/L) and high DOC (100–150 mg/L) changed the aquatic microbial community assemblies in different patterns: (1) the low DOC enriched K-selection microorganisms (e.g., C39, Tolumonas and CR08G) with low biomass and low resilience, as well as high resistance to perturbations in changing microbial community assemblies; (2) the high DOC was associated with r-selection microorganisms (e.g., PSB-M-3 and Clostridium) with high biomass and improved resilience, together with low resistance detrimental to microbial ecology stability. Overall, this study provided new insight into the impact of DOC on aquatic microbial community stability, which may help guide sustainable urban river management.

Benthic–limnetic morphological variation in fishes: Dissolved organic carbon concentration produces unexpected patterns

AbstractVariation in traits related to foraging and locomotion in benthic and limnetic habitats has been observed in many fishes. Benthic and limnetic food chain productivity in lakes is strongly influenced by the concentration of dissolved organic carbon (DOC) in the water, suggesting that DOC might indirectly impose selection on these traits and lead to classic benthic forms at low DOC concentrations and limnetic forms at high DOC concentrations. We tested this hypothesis via geometric morphometric and meristic analyses of bluegill sunfish (Lepomis macrochirus, Centrarchidae) from 14 lakes with DOC concentrations ranging from 4 to 24 mg/L. These lakes, located in close proximity to each other, straddle the drainage divide between the Mississippi River and Laurentian Great Lakes basins in northern Wisconsin, USA. Bluegill morphology was consistently related to lake DOC concentration in both drainage basins, despite differences in morphology between basins. Fish from higher DOC lakes had deeper bodies and smaller heads, among other differences, though the proportion of shape variation described by DOC was low. Gill raker length and inter‐raker spacing were positively related to DOC concentration. Although some traits were thus related to DOC concentration, the directions of these relationships did not match the predicted benthic–limnetic patterns. Further, no relationships were evident between DOC and gill raker number, eye width, pectoral fin dimensions, or pectoral fin insertion angle in univariate analyses. These variable outcomes suggest that selection linked to DOC does not map neatly onto the classic benthic–limnetic axis, that high DOC favors a benthic–limnetic generalist rather than a limnetic specialist, or that the benthic–limnetic morphological dichotomy is less clear and universal than is often suggested.