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Dissolved organic matter (DOM) influences several physical and chemical drivers of phytoplankton habitat. Increasing variability in lakewater dissolved organic carbon (DOC) concentrations in recent decades has raised uncertainty over temporal responses of phytoplankton communities to changing DOM. We conducted limnological surveys in four lakes with low (~ 2 mg l−1) to moderate (~ 4 mg l−1) DOC in Maine, USA, from February to November 2020 to assess variability in phytoplankton metrics and habitat gradients. Low-DOC lakes had similar thermal structure patterns compared to moderate-DOC lakes, but more stable euphotic depths. While moderate-DOC lakes had greater seasonal heterogeneity of biomass and vertical distribution of chlorophyll-a, the low-DOC lake had greater phytoplankton community turnover (Morisita-Horn Dissimilarity Index), underscored by a shift in dominance from autotrophs in the winter under ice to mixotrophs throughout the open water season. Long-term trends showed DOC concentrations generally increased for one decade beginning in 1995, followed by decreases in all four lakes for 12–15 years until 2020 by rates of 0.02–0.15 mg l−1 year−1, during which time water clarity increased in one lake. Small differences in or changes to DOC elicit greater variability of phytoplankton in lakes with low to moderate DOC concentrations. [ABSTRACT FROM AUTHOR]

Paddy fields are the main agricultural source of greenhouse gas methane (CH4) emissions. To enhance rice yield, various fertilization practices have been employed in rice paddies. However, the key microbial and abiotic factors driving CH4 emissions under different fertilization practices in paddy fields remain largely uncharted. This study conducted field experiments in a traditional double-cropping rice area in South China, utilizing five different fertilization practices to investigate the key factors influencing CH4 emissions. High-throughput sequencing and PICRUSt2 functional prediction were employed to investigate the contributions of soil physicochemical properties, CH4-metabolizing microorganisms (methanogens and methanotrophs), and key genes (mcrA and pmoA) on CH4 emissions. The results showed that CH4 emission fluxes exhibited seasonal variations, with consistent patterns of change observed across all treatments for both early- and late-season rice. Compared to the no-fertilization (NF) treatment, cumulative CH4 emissions were lower in early-season rice with green manure (GM) and straw returning (SR) treatments, as well as in late-season rice with GM treatment, while rice yields were maintained at higher levels. High-throughput sequencing analysis revealed that potential methanogens were primarily distributed among four orders: Methanobacteriales, Methanocellales, Methanomicrobiales, and Methanosarcinales. Furthermore, there was a significant positive correlation between the relative abundance of the CH4-related key gene mcrA and these microorganisms. Functional analysis indicated that these potential methanogens primarily produce methane through the acetoclastic and hydrogenotrophic pathways. Aerobic CH4-oxidizing bacteria, predominantly from the genus Methylocystis, were detected in all the treatments, while the CH4 anaerobic-oxidizing archaea ANME-1b was only detected in chemical fertilization (CF) and cow manure (CM) treatments. Our random forest analysis revealed that the relative abundance of two methanogens (Methanocellales and Methanosarcinales) and two environmental factors (pH and DOC) had significant impacts on the cumulative CH4 emissions. The variance decomposition analysis highlighted the CH4-metabolizing microorganisms explained 50% of the variance in the cumulative CH4 emissions, suggesting that they are the key microbial factors driving CH4 emissions. These findings provide guidance for the development of rational measures to reduce CH4 emissions in paddy fields. [ABSTRACT FROM AUTHOR]

Seaweed farming is emerging as a scalable and effective carbon dioxide removal (CDR) strategy, offering direct sequestration benefits through carbon uptake pathways and indirect climate advantages by substituting carbon-intensive products. This paper evaluates the potential of seaweed farming within the context of South Korea, leveraging its advanced aquaculture infrastructure, extensive coastal resources, and supportive policy frameworks. By synthesizing current literature, this study examines key sequestration mechanisms, including biomass storage, dissolved organic carbon (DOC) release, and particulate organic carbon (POC) burial, while addressing uncertainties such as the stability of recalcitrant DOC and the efficiency of POC burial under site-specific conditions. The analysis highlights South Korea's unique strengths, such as its established seaweed farming industry and innovative technological developments, alongside challenges like ecological trade-offs, nutrient competition, and the absence of robust monitoring, reporting, and verification (MRV) systems. The paper identifies opportunities to scale offshore farming, adopt integrated multi-trophic aquaculture, and enhance lifecycle climate benefits through product innovation and renewable energy integration. To guide future research and policy, the paper outlines critical gaps, including the need for precise quantification of long-term carbon sequestration pathways, development of MRV frameworks, and exploration of socio-economic impacts. By addressing these gaps, seaweed farming can become a central pillar of South Korea's climate mitigation strategy, providing valuable insights for other regions seeking to integrate marine-based solutions into global CDR efforts. [ABSTRACT FROM AUTHOR]

The interaction between redox-active species and dissolved organic carbon (DOC) is crucial in driving lacustrine benthic microbial processes. In lacustrine porewaters, many redox-active species exist in their reduced form, while DOC acts as a substrate and an electron acceptor. Understanding the types and abundance of redox-active species in porewaters along with their complementary DOC substrate is pivotal for gaining insights into benthic processes, particularly in regions susceptible to climate change. We report the in-situ measurement of redox-active species in sediment porewaters, alongside the ex-situ measurement of DOC extracted from cores collected from two Arctic lakes (Toolik and Fog 1). Fe2+ was abundantly detected below 4 cm of the sediment-water interface in all cores and was inversely related to dissolved O2. Additionally, two distinct Fe(III)-complexes were identified. DOC ranged in the order of 10s of mg/L and either remained stable or increased with depth. A comparison between Toolik and Fog 1 lakes revealed a higher accumulation of Fe2+ and DOC in the latter. This study marks the first of its kind to assess spatial distributions of redox-active species and DOC as a function of depth from multiple sites in Arctic lacustrine porewaters. [ABSTRACT FROM AUTHOR]

The interaction between redox-active species and dissolved organic carbon (DOC) is crucial in driving lacustrine benthic microbial processes. In lacustrine porewaters, many redox-active species exist in their reduced form, while DOC acts as a substrate and an electron acceptor. Understanding the types and abundance of redox-active species in porewaters along with their complementary DOC substrate is pivotal for gaining insights into benthic processes, particularly in regions susceptible to climate change. We report the in-situ measurement of redox-active species in sediment porewaters, alongside the ex-situ measurement of DOC extracted from cores collected from two Arctic lakes (Toolik and Fog 1). Fe2+ was abundantly detected below 4 cm of the sediment-water interface in all cores and was inversely related to dissolved O2. Additionally, two distinct Fe(III)-complexes were identified. DOC ranged in the order of 10s of mg/L and either remained stable or increased with depth. A comparison between Toolik and Fog 1 lakes revealed a higher accumulation of Fe2+ and DOC in the latter. This study marks the first of its kind to assess spatial distributions of redox-active species and DOC as a function of depth from multiple sites in Arctic lacustrine porewaters.

Fulvic acids (FA) are environmentally prevalent components of dissolved organic carbon. Little research has evaluated their potential influence on the bioavailability of herbicides to non-target aquatic plants. This study evaluated the potential impacts of FA on the bioavailability of atrazine (ATZ) to the aquatic plant Lemna minor. Plants were exposed to 0, 15, 30, 60, 125, and 750 µg/L ATZ in media containing three FA concentrations (0, 5, and 15 mg/L) in a factorial study under static conditions. Fronds were counted after 7- and 14-days exposure and intrinsic growth rates (IGR) and total frond yields were calculated for analysis. Atrazine NOAECs and LOAECs within each FA treatment series (0, 5, or 15 mg/L) were identified and EC50s were estimated. NOAEC/LOAECs for yield and IGR were 60/125 µg/L except for yield in the 0 mg/L-FA series (30/60) and IGR in the 5 mg/L-FA series (30/60). NOAEC/LOAECs were 30/60 µg/L for all treatments and both endpoints after 14 days exposure. EC50s ranged from 88.2 to 106.1 µg/L (frond production 7 DAT), 158.0–186.0 µg/L (IGR, 7 DAT), 74.7–86.3 µg/L (frond production, 14 DAT), and 144.1–151.3 µg/L (IGR, 14 DAT). FA concentrations did not influence the toxicity of ATZ. [ABSTRACT FROM AUTHOR]

The study investigated the adsorption of dissolved organic carbon (DOC) on Chinese soils from four long-term experiments involving five treatments: (1) no fertilization (control), (2–4) inorganic nitrogen (N), phosphorus (P), and potassium (K) fertilization (N, NP, and NPK), (5) NPK plus manure (NPKM). The results showed that DOC adsorption followed a modified Langmuir isotherm model effectively (R2 = 0.912 ~ 0.991). The maximum adsorption capacity (Qmax) (i.e. saturation deficits) generally increased with increasing soil organic carbon (SOC) due to fertilization in Luvic Phaeozem, Haplic Gypsisols, and Eutric Cambisols. However, it decreased significantly in acidic soils (Haplic Acrisols) from 15.59 g/kg for control, to 13.96, 13.03, 9.30 and 8.02 g/kg for N, NP, NPK and NPKM, respectively. Moreover, long-term fertilization, particularly with organic fertilization (e.g. NPKM), resulted in an increase of carbon (C) saturation by 1.42 g/kg (5.52%) in Luvic Phaeozem, 9.27 g/kg (44.29%) in Haplic Gypsisols, and 3.75 g/kg (27.66%) in Eutric Cambisols; however, there was a slight decrease by 1.70 g/kg (−7.49%) in Haplic Acrisols. Canonical correlation analysis (CCA) revealed that clay content and soil available iron (AFe) might be the crucial factors controlling C deficits, and subsequently, C saturation, along with current SOC. In conclusion, long-term fertilization contributed to C saturation in neutral-to-alkaline soils, but not in acidic soils. Thus, further attention should be paid to the long-term effects of fertilization on C saturation. [ABSTRACT FROM AUTHOR]

AbstractDissolved organic carbon (DOC) in soils is released into streams, working as a main component of the carbon cycle. DOC in terrestrial and aquatic ecosystems participates in many biogeochemical reactions and processes such as heterotrophic respiration, sorption of metals, and transport of pollutants. In order to understand the connectivity of organic carbon among soil, soil water, and forest streams, we investigated the concentrations and dual carbon isotope ratios (δ13C and Δ14C) of soil organic carbon (SOC) and water-extractable organic carbon (WEOC) obtained from soils beneath two tree species stands, and compared these with stream Δ14C-DOC of the forest watershed. Soil samples were collected at different depths (0–10, 10–30, and 30–50 cm) beneath Japanese cedar (Cryptomeria japonica) and sawtooth oak (Quercus acutissima). Although the SOC concentration was not significantly different between the two tree species, the WEOC concentration ([WEOC]) of soil at 0–10 cm depth under Cryptomeria japonica was higher than that of Quercus acutissima, in general. The δ13C-SOC and δ13C-WEOC increased, while the Δ14C-SOC and Δ14C-WEOC decreased with increasing soil depth. The Δ14C-WEOC was higher than the Δ14C-SOC, indicating that WEOC could be primarily derived from the young, hydrophilic, and exchangeable fraction of SOC, rather than from SOC strongly bonded to mineral soils. However, the stream Δ14C-DOC was lower than Δ14C-WEOC in general, except during summer storms. The 14C-depleted DOC released from deep soils or groundwater might lower Δ14C-DOC of a stream, suggesting that relatively old DOC could be released into streams during baseflow. This is contrary to the results of previous studies that have reported positive stream Δ14C-DOC from temperate forests. The discrepancy warrants future research on forest stream Δ14C-DOC across entire seasons, particularly under Asian monsoon climates.

Atmospheric acid deposition disrupted terrestrial-aquatic carbon cycling by drastically lowering dissolved organic carbon (DOC) loads in many lakes across NE North America and northern Europe during the 20th century. However, little is known about how acid deposition has altered the role of lakes as long-term carbon sinks. We present contemporary (n = 80) organic carbon accumulation rates (OCAR) and OCAR trends over the past ~ 150 years (n = 8), and other supporting infrared spectroscopic, isotopic, and elemental geochemical proxies, for lakes in and near Sudbury, Ontario, Canada – an area heavily affected by acid deposition from smelting activities in the late-19th and 20th centuries. Contemporary OCAR varied between 4.9 and 35.3 g m–2 yr–1 among study lakes (mean: 13.5±6.4 g m–2 yr–1). Sediment-inferred trends in lake-water DOC showed a strong response in DOC loadings to the effects of acid deposition during the past century, which is corroborated by increasing observed lake-water DOC concentrations (i.e., lake browning) since the 1980s. Despite these changes in DOC, as well as changes in water acidity, only lakes with direct physical watershed disturbances showed short-lived increases in OCAR, whereas OCAR changed little in remote Sudbury-region lakes with minimal direct human disturbances (mean OCAR: 14.3 ± 8.7 g m–2 yr–1). This is in stark contrast to many other northern forest lakes with minimal direct catchment disturbances that experienced significant increases in OCAR during the 20th century. Our results caution that lake browning may not be a dominant driver behind the widespread increase in organic C burial in northern lakes during recovery from acid deposition in recent decades. [ABSTRACT FROM AUTHOR]

Peatlands are globally important stores of soil carbon (C) formed over millennial timescales but are at risk of destabilization by human and climate disturbance. Pools are ubiquitous features of many peatlands and can contain very high concentrations of C mobilized in dissolved and particulate organic form and as the greenhouses gases carbon dioxide (CO2) and methane (CH4). The radiocarbon content (14C) of these aquatic C forms tells us whether pool C is generated by contemporary primary production or from destabilized C released from deep peat layers where it was previously stored for millennia. We present novel 14C and stable C (δ13C) isotope data from 97 aquatic samples across six peatland pool locations in the United Kingdom with a focus on dissolved and particulate organic C and dissolved CO2. Our observations cover two distinct pool types: natural peatland pools and those formed by ditch blocking efforts to rewet peatlands (restoration pools). The pools were dominated by contemporary C, with the majority of C (~50%–75%) in all forms being younger than 300 years old. Both pool types readily transform and decompose organic C in the water column and emit CO2 to the atmosphere, though mixing with the atmosphere and subsequent CO2 emissions was more evident in natural pools. Our results show little evidence of destabilization of deep, old C in natural or restoration pools, despite the presence of substantial millennial‐aged C in the surrounding peat. One possible exception is CH4 ebullition (bubbling), with our observations showing that millennial‐aged C can be emitted from peatland pools via this pathway. Our results suggest that restoration pools formed by ditch blocking are effective at preventing the release of deep, old C from rewetted peatlands via aquatic export. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) plays an important role in biogeochemistry of lake ecosystems. Studies measuring DOM at short intervals in lakes are rare, and thus its short-term dynamics are largely unknown. We investigated DOM temporal variation in large, shallow, eutrophic Lake Võrtsjärv (Estonia) during one growing season (May–Sep 2016) using a field-deployable in situ spectrometer to measure absorbance spectra (wavelength range 200–708 nm) at a 2 h interval coupled with monthly discrete water sampling. Collected spectra were analyzed together with some in-lake variables, lake metabolic rates, and meteorological and hydrological data using boosted regression tree (BRT) and random forest (RF) models. Different spectral parameters were used to assess total and allochthonous DOM quantity and relative share of autochthonous DOM. All parameters (i.e., DOM quantity and quality) varied on a large scale. For example, dissolved organic carbon (DOC) concentrations ranged from 12.0 to 17.3 mg L−1. High levels of DOM were mainly of allochthonous origin, and a strong relationship with inflow indicated the same. The relative share of autochthonous DOM increased with rising air temperature as primary production rose in warm water; however, we found no direct relationships with gross primary production. RF and BRT models explained up to 38% and 63% of DOM temporal variability, respectively. Our results showed that monthly water samples did not capture large variation in DOM. Therefore, high-frequency measurements using in situ spectrometry improve temporal representativeness of DOM monitoring in lakes compared to traditional sampling methods. [ABSTRACT FROM AUTHOR]

The "priming effect" (PE) is well documented in soils, but contradictory results have been reported in aquatic ecosystems. To investigate whether the presence of easily degradable labile dissolved organic matter (LDOM) changes the biodegradation rate of nonlabile DOM (NLDOM), incubations were performed with various sources of LDOM and NLDOM in microcosms amended with nutrients and a microbial inoculum. Stable carbon isotope analysis of DOM was used for the first time to estimate the PE. A significant positive PE (15%–34% of initial NLDOM) was measured, but only with mixed (glucose + amino acids) or complex (disaccharide) LDOM. Regardless of the LDOM, no PE was measured with more recalcitrant NLDOM (half‐life of 128 days here), when the LDOM/NLDOM ratio was low (0.3 versus 1), and when the NLDOM and microbial inoculum were from the same water (no mixing). The presence of sediments likely enhances microbial diversity and NLDOM degradation rate, but it did not increase the PE. Microbial use of LDOM produced new microbial NLDOM that should be discriminated (here by isotopes) from initial NLDOM for accurate PE measurements. The PE was temporary and lasted about 2 weeks after one LDOM addition. In areas with frequent additions of LDOM, a long‐lasting PE is expected and may significantly increase CO2 production. Exposing refractory DOM to new conditions and microbial community, such as during natural water mixing, seems to exert a strong control on its dynamics. Here, the addition of a new microbial inoculum had a stronger effect than adding LDOM or the PE. Plain Language Summary: Fresh‐ and sea‐waters produce large quantities of dissolved organic matter (DOM) every year. The majority of this DOM is labile and rapidly consumed by microorganisms and converted to CO2. Yet, a small fraction of it is less reactive and persists for months and years, contributing to a huge reservoir of nonlabile dissolved organic carbon, similar in size to atmospheric CO2. The biodegradation of labile organic matter can both produce nonlabile DOM and change (increase or decrease) its degradation rate. This interactive effect, between different pools of substrates, on microbial degradation impacts CO2 production and regroups several mechanisms known as the "priming effect (PE)." The PE is well known in soils, but contradictory results have been reported in aquatic ecosystems. Our experiments showed a temporary PE, but only when mixed (i.e., glucose + amino acids) or complex (i.e., disaccharide) labile substrates were added and when the background DOM was not too recalcitrant. Microbial DOM was always produced from the added labile substrate. This contribution is often overlooked, but must be discriminated for accurate estimates of the PE. Our results can explain why an absence of PE was often reported in aquatic studies. Key Points: Experiments were performed to test the priming effect (PE) of labile organic substrates on nonlabile dissolved organic matter degradationA PE was measured when mixed/complex labile substrates were added and when background organic matter was not too recalcitrantMicrobial dissolved organic matter produced from the added labile substrate should be considered in PE tests [ABSTRACT FROM AUTHOR]

Widespread 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. Key Points: Dissolved organic carbon concentration, maximum lake depth, and their interaction explained 87% of the variability in hypolimnion volume ratioThe relative change in hypolimnion volume ratio over the course of summer stratification had a strong, negative correlation with mean Schmidt stability [ABSTRACT FROM AUTHOR]

Our current knowledge of the chemical composition of ocean dissolved organic matter (DOM) is limited, mainly because of its extreme molecular diversity, low concentration of individual compounds and the elevated ionic strength of ocean waters. As a result, many analytical methods require a previous extraction step. The efficiency and selectivity of the extraction method defines the representativeness of the extracted DOM fraction. Nowadays, the most widespread procedure for concentrating DOM is solid phase extraction (SPE) using styrene divinyl benzene polymer cartridges (PPL). Here, we investigate the effect of SPE-PPL on DOM elemental and optical properties to assess the efficiency and selectivity of this extraction method on water samples from the main intermediate and deep water masses of Arctic, Mediterranean and Antarctic origin present in the Cape Vert Frontal Zone (CVFZ, NW Africa). Furthermore, North and South Atlantic Central waters converge in this area and coastal DOM is injected by the giant upwelling filament of Cape Blanc. On one side, the colored fraction of DOM (CDOM) presented extraction efficiencies comparable to that of the bulk dissolved organic carbon (DOC), but decreased significantly with increasing wavelength, suggesting an affinity of PPL cartridges for low molecular weight organic compounds. While the protein-like fluorescent fraction of DOM (FDOM) was also extracted with the same efficiency than DOC, the extraction efficiency of the humic-like fraction was comparatively much higher. On the other side, dissolved organic nitrogen (DON) extraction efficiencies were about half that of DOC. These contrasting extraction efficiencies of the different DOM pools indicated that the extracts were enriched in N-poor, low molecular weight and recalcitrant DOM, therefore showing less variability than the corresponding bulk DOM. Furthermore, DOC, DON, CDOM and FDOM extracted were not homogeneous through the water column but displayed certain significant differences among water masses in both efficiency and selectivity.

Meromictic Crawford Lake, located in SW Ontario, Canada is characterized by varved sediments, making it suitable for high-resolution paleoecological studies. Freeze cores, the only coring method available that reliably preserves the fragile laminations representative of seasonal deposition in the lake, were used to document siliceous diatom and chrysophyte community structure at an annual resolution from 1930– 1990CE. Stratigraphically constrained cluster analysis identified major assemblage changes that are believed to have been caused by local, regional and possibly global anthropogenic impacts. The assemblage changes within the siliceous algae are attributed to regional weather and increased industrial emissions and related effects of acid deposition on the lake’s catchment associated with the Great Acceleration –the massive economic, industrial, and demographic expansion beginning in the mid-20th century. Observed increases in spheroidal carbonaceous particles (SCPs) in varved lake sediment dating to the early 1950s record rapidly expanding steel production without emission controls around 30 km upwind of the lake. The findings reported here reflect major changes in earth systems that the Anthropocene Working Group recommends for a proposed epoch to be termed the Anthropocene, providing support for the laminated sediments from Crawford Lake as a potential Global boundary Stratotype Section and Point (GSSP). [ABSTRACT FROM AUTHOR]

As the largest organic carbon pool in water, dissolved organic carbon (DOC) plays a key role in the carbon cycle. In inland rivers and lakes, DOC is closely related to chromophoric dissolved organic matter (CDOM) with optical attenuation. In this study, the spatial distribution and seasonal variations of DOC and CDOM in Poyang Lake in 2014–2016 were investigated. The results demonstrated that the DOC concentration in Poyang Lake had a range of 1.34–5.56 mg/L with an average of 2.12 ± 0.54 mg/L. The absorption coefficient of CDOM at 355 nm had a range of 1.24–5.70 m−1 with an average of 2.71 ± 0.83 m−1. In terms of the spatial distribution, the concentrations of DOC and CDOM in the south of Poyang Lake were higher than those in the north of the lake. In terms of seasonal variations, the concentrations of DOC and CDOM were higher in spring and summer than in autumn and winter. The absorption coefficients of CDOM and DOC concentrations in Poyang Lake exhibited a significant linear correlation. The correlation between DOC and CDOM in some sections of Poyang Lake varied spatially and seasonally. The highest correlation was observed in wetland waters of the southern Poyang Lake in spring, while there was no significant correlation in northern section of the lake in most of the periods. The results revealed that water level, precipitation and the vegetation cover pattern had determining effects on the spatial heterogeneity of DOC and CDOM. The spectral characteristic parameters demonstrated that the main source of the CDOM in Poyang Lake was from terrestrial input. [ABSTRACT FROM AUTHOR]

This study shows the results of long-term inland water monitoring using Sentinel-2 data for Głuszyńskie Lake in the years 2015–2022. Four water quality parameters: biological oxygen demand (BOD), dissolved organic carbon (DOC), chlorophyll concentration (CHL) and electrical conductivity (EC) were calculated according to formulas found in the literature. The results were validated based on measurements conducted in 2021 and 2022, where for BOD, DOC and CHL high determination coefficients (0.77 and 0.79) were observed, and the EC determination coefficient was equal to 0.45. The results show that empirical formulas can be used for qualitative analyses of inland water quality, while for quantitative analyses more extensive field work needs to be performed.

Dissolved organic matter (DOM) is a key soil quality property, indicative of the organic matter stored in the soil, which may also be a function of temporal variation. This study examines whether DOM is a robust property of the soil, controlling fertility, or if it may change with time. Altogether eight sets of soil samples were collected in 2018 and 2019 from the cultivated topsoil (0–10 cm) of cropland and from a nearby grassland near Martonvásár, Hungary. The study sites were characterized by Chernozem soil and were part of a long-term experimental project comparing the effects of manure application and fertilization to the control under maize and wheat monocultures. DOM was extracted from the samples with distilled water. The dissolved organic carbon (DOC), total dissolved nitrogen (DN), biological index (BIX), fluorescence index (FI), humification index (HIX), carbon nitrogen (C/N) ratio and specific ultraviolet absorbance at 254 nm (SUVA254) index were studied in the arable soils, and the results showed that all the DOM samples were humified, suggesting relevant microbiological contributions to the decomposition of OM and its conversion into more complex molecules (FI = 1.2–1.5, BIX = ~0.5, and HIX = ~0.9). Temporal variations were detected only for the permanent grassland where higher DOM concentration was found in spring. This increased DOM content mainly originated from humified, solid phase associated, recalcitrant OM. In contrast, there were no differences among fertilization treatments and sampling dates under cropfield conditions. Moreover, climatic conditions were not proven as a general ruler of DOM properties. Therefore, momentary DOM alone is not necessarily the direct property of soil organic matter under cropfield conditions. The application of this measure needs further details of sampling conditions to achieve adequate comparability. [ABSTRACT FROM AUTHOR]

This study shows the results of long-term inland water monitoring using Sentinel-2 data for Głuszyńskie Lake in the years 2015-2022. Four water quality parameters: biological oxygen demand (BOD), dissolved organic carbon (DOC), chlorophyll concentration (CHL) and electrical conductivity (EC) were calculated according to formulas found in the literature. The results were validated based on measurements conducted in 2021 and 2022, where for BOD, DOC and CHL high determination coefficients (0.77 and 0.79) were observed, and the EC determination coefficient was equal to 0.45. The results show that empirical formulas can be used for qualitative analyses of inland water quality, while for quantitative analyses more extensive field work needs to be performed. [ABSTRACT FROM AUTHOR]

This document describes best practices for analysis of dissolved organic matter (dissolved organic carbon and total dissolved nitrogen) in seawater samples. Included are SOPs for sample collection and storage, details for laboratory analysis using high temperature combustion analysis on Shimadzu TOC analyzers, and suggestions for best practices in quality control and quality assurance. Although written specifically for GO-SHIP oceanographic community practices, many aspects of sample collection and processing are relevant to DOM determination across oceanic regimes and this document aims to provide updated methodology to the wider marine community.

Vibrio cholerae is a ubiquitously distributed human pathogen that naturally inhabits marine and estuarine ecosystems. Two serogroups are responsible for causing cholera epidemics, O1 and O139, but several non-O1 and non-O139 V. cholerae (NOVC) strains can induce cholera-like infections. Outbreaks of V. cholerae have previously been correlated with phytoplankton blooms; however, links to specific phytoplankton species have not been resolved. Here, the growth of a NOVC strain (S24) was measured in the presence of different phytoplankton species, alongside phytoplankton abundance and concentrations of dissolved organic carbon (DOC). During 14-day experiments, V. cholerae S24 was cocultured with strains of the axenic phytoplankton species Actinocyclus curvatulus, Cylindrotheca closterium, a Pseudoscourfieldia sp., and a Picochlorum sp. V. cholerae abundances significantly increased in the presence of A. curvatulus, C. closterium, and the Pseudoscourfieldia sp., whereas abundances significantly decreased in the Picochlorum sp. coculture. V. cholerae growth was significantly enhanced throughout the cogrowth experiment with A. curvatulus, whereas when grown with C. closterium and the Pseudoscourfieldia sp., growth only occurred during the late stationary phase of the phytoplankton growth cycle, potentially coinciding with a release of DOC from senescent phytoplankton cells. In each of these cases, significant correlations between phytoplankton-derived DOC and V. cholerae cell abundances occurred. Notably, the presence of V. cholerae also promoted the growth of A. curvatulus and Picochlorum spp., highlighting potential ecological interactions. Variations in abundances of NOVC identified here highlight the potential diversity in V. cholerae-phytoplankton ecological interactions, which may inform efforts to predict outbreaks of NOVC in coastal environments. [ABSTRACT FROM AUTHOR]

In this study, the spatial and temporal distribution characteristics of dissolved organic carbon (DOC) concentration in Gehu Lake, Taihu Lake Basin, China, and its influencing factors were studied through annual surveys conducted in 2018 and 2019 to aid its management and restoration. The DOC concentration ranged from 1.92 to 5.60 mg·L-1, and the average DOC concentration was 3.30±0.78 mg·L-1. The DOC concentration was highest in winter, followed by spring, summer and autumn; it was significantly higher in winter and spring than in summer and autumn. The DOC concentration was lower in the northern area and higher in coastal areas. Linear fitting showed that DOC concentration was strongly positively correlated with the absorption coefficient of chromophoric dissolved organic matter, the permanganate index, particulate phosphorus and particulate nitrogen (p<0.01) and significantly positively correlated with organic suspended solids (0.01

Through the fixation of large quantities of dissolved inorganic carbon (DIC), macroalgae facilitate the energetic foundation of highly productive coastal ecosystems. While the processes controlling photosynthesis and carbon fixation by macroalgae are well known, the fate of organic matter fixed by macroalgae is less well understood. This study quantified release rates of DOC by three ecologically significant Baltic macroalgae species: the perennial habitat forming Fucus vesiculosus and Furcellaria lumbricalis, and the seasonal fast-growing Ulva intestinalis, under both light and dark conditions. The released products were assessed using bacterial incubations whereby radiolabeled leucine was used to evaluate the uptake and lability of these products by marine heterotrophic bacteria. DOC was found to be released by both F. vesiculosus and U. intestinalis at rates of 0.27 mg C·h−1 under light and 0.13 mg C·h−1 per unit of dry mass under dark treatments, respectively, whereas F. lumbricalis DOC release was observed to be negligible under both light and dark. Our findings further validate previous hypotheses that factors such as photosynthetic activity are a primary driver behind DOC release and that DOC release is not an entirely passive process. Additionally, we reaffirm the need to relate a given species life characteristics and habitat in order to understand why DOC products are released. The consumption of macroalgae-derived DOC by heterotrophic bacteria reveals that released DOC is variable in its lability. After a period of 12 h and under maximum photosynthetic conditions, the release of DOC by F. vesiculosus and U. intestinalis achieved a peak rate of 219 µg C·L−1·day−1 and 214 µg C·L−1·day−1 for each gram of dry weight material, respectively, directly into the microbial loop via heterotrophic bacterial consumption. In contrast, F. lumbricalis’ low rate of DOC release and the subsequent low bacterial consumption indicate that habitats dominated by this species have a reduced importance in the transfer energy via the microbial loop. These findings have implications for how we view carbon transfer within coastal food webs and highlight how changes in species composition and coverage may dramatically affect coastal ecosystem productivity through the microbial loop.

Future climate scenarios indicate increasing drought intensity that threatens ecosystem functioning. However, the behavior of ecosystems during intense drought, such as the 2018 drought in Northern Europe, and their respective response following rewetting is not fully understood. We investigated the effect of drought on four different vegetation types in a temperate climate by analyzing dissolved organic matter (DOM) concentration and composition present in soil leachate, and compared it to two accompanying years. DOM is known to play an important role in ecosystem recovery and holds information on matter flows between plants, soil microorganisms and soil organic matter. Knowledge about DOM opens the possibility to better disentangle the role of plants and microorganisms in ecosystem recovery. We found that the average annual DOM concentration significantly decreased during the 2018 drought year compared to the normal year. This suggests a stimulation of DOM release under normal conditions, which include a summer drought followed by a rewetting period. The rewetting period, which holds high DOM concentrations, was suppressed under more intense drought. Our detailed molecular analysis of DOM using ultrahigh resolution mass spectrometry showed that DOM present at the beginning of the rewetting period resembles plant matter, whereas in later phases the DOM molecular composition was modified by microorganisms. We observed this pattern in all four vegetation types analyzed, although vegetation types differed in DOM concentration and composition. Our results suggest that plant matter drives ecosystem recovery and that increasing drought intensity may lower the potential for ecosystem recovery. [ABSTRACT FROM AUTHOR]

Background: Soil moisture, pH, dissolved organic carbon and nitrogen (DOC, DON) are important soil biogeochemical properties in switchgrass (SG) and gamagrass (GG) croplands. Yet their spatiotemporal patterns under nitrogen (N) fertilization have not been studied. Aims: The objective of this study is to investigate the main and interactive effects of N fertilization and bioenergy crop type on central tendencies and spatial heterogeneity of soil moisture, pH, DOC and DON. Methods: Based on a 3‐year long fertilization experiment in Middle Tennessee, USA, 288 samples of top horizon soils (0–15 cm) under three fertilization treatments in SG and GG croplands were collected. The fertilization treatments were no N input (NN), low N input (LN: 84 kg N ha−1 in urea) and high N input (HN: 168 kg N ha−1 in urea). Soil moisture, pH, DOC and DON were quantified. And their within‐plot variations and spatial distributions were achieved via descriptive and geostatistical methods. Results: Relative to NN, LN significantly increased DOC content in SG cropland. LN also elevated within‐plot spatial heterogeneity of soil moisture, pH, DOC and DON in both croplands though GG showed more evident spatial heterogeneity than SG. Despite the pronounced patterns described above, great plot to plot variations were also revealed in each treatment. Conclusion: This study informs the generally low sensitivity of spatiotemporal responses in soil biogeochemical features to fertilizer amendments in bioenergy croplands. However, the significantly positive responses of DOC under low fertilizer input informed the best practice of optimizing agricultural nutrient amendment. [ABSTRACT FROM AUTHOR]

Boreal peatlands are vast carbon (C) stores but also major sources of dissolved organic C (DOC) and nutrients to surface waters. Drainage and forest harvesting accelerates DOC leaching. Continuous cover forestry (CCF) is considered to cause fewer adverse environmental effects. Yet, the effects of CCF on DOC processes are unrecognised. We study DOC production and quality in unharvested, CCF, and clear-cut drained peatland forests and in a non-forested alluvial sedge fen. Parallel replicate peat columns with ground vegetation are collected from the uppermost 50 cm at each site, and the water table (WT) is set to −20 or −40 cm depths on the columns. During the eight-month ex situ incubation experiment, the soil water samples are extracted monthly or bi-monthly. The samples are incubated at 15 °C for multiple 72 h incubation cycles to study pore water quality and biodegradation of DOC. The CO2 production occurs during the first three days. The DOC concentrations and the CO2 release per volume of water are significantly lower in the sedge fen than in the drained peatland forests. The WT has a negligible effect on DOC concentrations and no effect on DOC quality, but the higher WT has generally higher CO2 production per DOC than the lower WT. The results suggest that peat in the drained peatlands is not vulnerable to changes per se but that forest management alters biotic and abiotic factors that control the production, transport, and biodegradation of DOC. [ABSTRACT FROM AUTHOR]

We investigated changes in the quantity and quality of dissolved organic matter (DOM) from rain to stream water in a forested watershed at Yamashiro Experimental Forest (YMS) in southern Kyoto prefecture. The dissolved organic carbon (DOC) concentration and specific UV absorbance at 254 nm (SUVA254) in rainwater increased in the order of bulk rain, throughfall, stemflow, and O layer leachate because of the DOM supply from tree tissue and O layer. Decreases in DOC concentration and SUVA254 with soil depth were not observed in the soil-percolating water. This finding may have been caused by the low free oxide content of the soil and the collection of soil water with a tension-free lysimeter. The DOC concentration was very low in both seepage and stream waters; seasonal variation with a high concentration in summer was observed in the stream water. An increase in K+ concentration in summer was also observed in the stream water; thus, we presumed that DOC seasonal variation was caused by the DOM supply with the accumulated decomposition of litter in the streambed. The significant correlation between DOC concentration and absorbance at 254 nm (UV254) was observed for all sample types of observation target in the watershed; the ratio of DOC concentration to UV254 was different, while the correlation coefficient between DOC concentration and UV254 value differed among sample types in the watershed. We concluded that UV254 which can be measured by simply and easily is a good indicator for estimating DOC concentration in liquid samples in forested watersheds. [ABSTRACT FROM AUTHOR]

Future climate scenarios indicate increasing drought intensity that threatens ecosystem functioning. However, the behavior of ecosystems during intense drought, such as the 2018 drought in Northern Europe, and their respective response following rewetting is not fully understood. We investigated the effect of drought on four different vegetation types in a temperate climate by analyzing dissolved organic matter (DOM) concentration and composition present in soil leachate, and compared it to two accompanying years. DOM is known to play an important role in ecosystem recovery and holds information on matter flows between plants, soil microorganisms and soil organic matter. Knowledge about DOM opens the possibility to better disentangle the role of plants and microorganisms in ecosystem recovery. We found that the average annual DOM concentration significantly decreased during the 2018 drought year compared to the normal year. This suggests a stimulation of DOM release under normal conditions, which include a summer drought followed by a rewetting period. The rewetting period, which holds high DOM concentrations, was suppressed under more intense drought. Our detailed molecular analysis of DOM using ultrahigh resolution mass spectrometry showed that DOM present at the beginning of the rewetting period resembles plant matter, whereas in later phases the DOM molecular composition was modified by microorganisms. We observed this pattern in all four vegetation types analyzed, although vegetation types differed in DOM concentration and composition. Our results suggest that plant matter drives ecosystem recovery and that increasing drought intensity may lower the potential for ecosystem recovery.

The effects of benthic dissolved organic carbon (DOC) flux on the dynamics of DOC in the deep continental margins (200 – 2000 m depth) is poorly understood. We investigated heterotrophic prokaryotes (hereafter bacteria) production (BP) and the bio-reactive properties of sediment-derived dissolved organic matter (SDOM) to elucidate microbially mediated cause-effect relationships regarding the rapid consumption of dissolved oxygen (DO) and accumulation of humic-like fluorescent DOM (FDOMH) in the deep-water column (750 – 2000 m depth range) of the Ulleung Basin (UB) in the East Sea. BP in the deep water (2.2 μmol C m-3 d-1) of the UB was among the highest reported for various deep-sea sites. The high DOC concentration (55 μM) likely supported the high BP seen in the deep-water column of the UB. Concentrations of DOC and C1 component of the FDOMH, which is indicative of microbial metabolic by-products, were 13-fold and 20-fold greater, respectively, in pore water than in the overlying bottom water, indicating that the sediment in the continental margins is a significant source of DOM in the overlying water column. Fine-scale water sampling revealed that BP near the sediment (0 – 30 m above the seafloor; 2.78 μmol C m-3 d-1) was 1.67 times higher than that measured in the water column above (30 – 100 m above the seafloor; 1.67 μmol C m-3 d-1). In addition, BP increased in the bottom water incubation amended with SDOM-containing pore water (PW). The results demonstrated that SDOM contains bio-reactive forms of DOM that stimulate heterotrophic microbial metabolism at the expense of oxygen in the bottom water layer. The accumulation of C1 component in both PW-amended and unamended bottom water incubation (i.e., without an extra DOM supply from sediment) further indicated that refractory DOM is produced autochthonously in the water column via heterotrophic metabolic activity. This explains in part the microbially mediated accumulation of excess FDOMH in the deep-water column of the UB. Overall results suggest that the benthic release of bio-reactive DOM may be of widespread significance in controlling microbial processes in the deep-water layer of marginal seas.

We reconstructed historical diatom and algal pigment records from a clear-water versus a brown-water lake in the northeastern U.S. to compare ecological responses to reduced acid deposition in recent decades in the context of a 140-year record, during which time multiple external drivers influenced both lakes. In the clear-water lake, diatom community structure changed continually from the beginning of the record, while it was more static in the brown-water lake over time, with the period of greatest change occurring after 1990. Concentrations of algal pigments were low in the clear-water lake until 1940, then increased during the 1940 to 1990 period, after which they slightly declined. The opposite pattern occurred in the brown-water lake—algal pigments were high until 1940, decreased in the period from 1940 to 1990, and increased after 1990. The clear-water lake was more responsive to long-term climate warming beginning at the end of the Little Ice Age. During the period of higher acid deposition, light availability was more important for controlling algal responses in the clear-water lake, while nutrient subsidies from allochthonous DOC were likely the primary control in the brown-water lake. With reductions in atmospheric sulfate deposition, both lakes showed signs of recovery toward pre-acidification conditions, but these changes were dampened in the clear-water lake, suggesting greater sensitivity to acid deposition and other external drivers such as effects of climate change. Our study highlights strong and divergent algal responses in a clear-water vs. a brown-water lake during a period of higher acid deposition, and considers these changes in the context of multiple drivers of environmental change over the past 140 years. [ABSTRACT FROM AUTHOR]

Oomycete pathogens in freshwaters, such as Saprolegnia parasitica and Aphanomyces astaci, are responsible for fish/crayfish population declines in the wild and disease outbreaks in aquaculture. Although the formation of infectious zoospores in the laboratory can be triggered by washing their mycelium with natural water samples, the physico-chemical properties of the water that might promote sporulation are still unexplored. We washed the mycelia of A. astaci and S. parasitica with a range of natural water samples and observed differences in sporulation efficiency. The results of Partial Least Squares Regression (PLS-R) multivariate analysis showed that SAC (spectral absorption coefficient measured at 254 nm), DOC (dissolved organic carbon), ammonium-N and fluoride had the strongest positive effect on sporulation of S. parasitica, while sporulation of A. astaci was not significantly correlated with any of the analyzed parameters. In agreement with this, the addition of environmentally relevant concentrations of humic acid, an important contributor to SAC and DOC, to the water induced sporulation of S. parasitica but not of A. astaci. Overall, our results point to the differences in ecological requirements of these pathogens, but also present a starting point for optimizing laboratory protocols for the induction of sporulation. [ABSTRACT FROM AUTHOR]

Summary: The evolutionary and ecological story of coccolithophores poses questions about their heterotrophy, surviving darkness after the end‐Cretaceous asteroid impact as well as survival in the deep ocean twilight zone. Uptake of dissolved organic carbon might be an alternative nutritional strategy for supply of energy and carbon molecules.Using long‐term batch culture experiments, we examined coccolithophore growth and maintenance on organic compounds in darkness. Radiolabelled experiments were performed to study the uptake kinetics. Pulse–chase experiments were used to examine the uptake into unassimilated, exchangeable pools vs assimilated, nonexchangeable pools.We found that coccolithophores were able to survive and maintain their metabolism for up to 30 d in darkness, accomplishing about one cell division. The concentration dependence for uptake was similar to the concentration dependence for growth in Cruciplacolithus neohelis, suggesting that it was taking up carbon compounds and immediately incorporating them into biomass. We recorded net incorporation of radioactivity into the particulate inorganic fraction.We conclude that osmotrophy provides nutritional flexibility and supports long‐term survival in light intensities well below threshold for photosynthesis. The incorporation of dissolved organic matter into particulate inorganic carbon, raises fundamental questions about the role of the alkalinity pump and the alkalinity balance in the sea. [ABSTRACT FROM AUTHOR]

The carbon cycle in lakes has been receiving great concerns and has become a hot issue. This study aimed at investigating the characteristics of Dissolved Organic Carbon (DOC) and its influence on methane emission based on continuous field sampling in a year cycle from algal-dominated zones of Lake Taihu. Results showed that the DOC with mean value of 4. 15 mg/L varied spatially due to external input and internal algal blooms. Overall, peak DOC occurred in Northwest Zone and Meiliang Bay, and temporal variations of DOC in the two zones were associated with basin precipitation, especially for Northwest Zone (R²=0. 67,P<0. 01) . It should be noted that the temporal variation of DOC in Central Zone were highly positively correlated with algal biomass mostly due to the less external input. The CH4 diffusion emission flux with mean value of 0. 083 mmol·m-2·d-1 varied greatly across zones. Algal blooms significantly stimulated the CH4 emissions via increasing DOC concentration. Overall, the algal-dominated lake was a hot spot of CH4 emission due to DOC enrichment. Considering the effects of DOC on lake CH4 emission was regulated by multiple factors related to internal metabolic activities and external loading, further studies are needed to reveal the potential control mechanism. [ABSTRACT FROM AUTHOR]

Purpose: In the natural environment, microplastics and biochar are normally coexistent. However, interactions between them on bioavailability and degradation of organic pollutants in soils are not clear. Materials and methods: Here, wheat straw biochar produced at 400 °C (BC) and polyethylene (PE) with particle size ranges of 840–2000 and 104–178 μm were selected and named as LBC, SBC, LPE, and SPE, respectively. Their effects on bioavailability and degradation of phenanthrene in soils with or without earthworms were investigated. Results and discussion: Sorption ability (lgKf) of PE-BC (1:1) mixtures for phenanthrene was higher than that of their corresponding individual PE or BC in general, which is mainly related to the change in DOC content in the equilibrium solution. Both initial desorbing fraction of phenanthrene extracted by n-butanol and degradation ratio of phenanthrene were more obviously decreased by the addition of 2% PE-BC mixtures than by the addition of 2% PE or BC in general. Correlation analysis results indicated that particle sorption played a key role in controlling phenanthrene bioavailability and then phenanthrene degradation no matter whether earthworm Pheretima guillelmi was present or not. Earthworm presence enhanced phenanthrene degradation in soils with SPE, LBC, or LPE but had no obvious effect on the degradation in soils with PE-BC mixtures due to the change in ingestion of SPE by earthworms or DOC solubilization. Conclusion: The enhanced sorption ability and the reduced SPE ingestion by earthworms by coexistence of PE and BC inhibited phenanthrene degradation, leading to higher persistence of phenanthrene. [ABSTRACT FROM AUTHOR]

The East Asian monsoon is characterized by warm, humid summer and cool, dry winter that results in seasonal changes in fluvial sediment and organic carbon (OC) discharge. Here, we report measurements of elemental, isotopic, and optical compositions of OC in surface waters and sediments that were collected along the lower Yangtze River, Estuary, and East China Sea continuum in March and July 2019. The principal component analysis separates samples into three groups that correspond to fresh water area, mixing area, and seawater area. The terrigenous signal in the mixing area, indicated by water salinity and δ13C, is stronger in July. The applications of binary mixing models based on water salinity, dissolved OC (DOC) concentration and specific terrigenous fluorescence component suggest: (a) net removal of DOC from waters in July, but net addition of DOC into waters in March, attributed to more extensive degradation and larger deposition of OC in summer; and (b) more conservative behaviors of terrigenous OC compared to marine or mixed OC, suggesting an important role of intrinsic molecular characteristics on the persistence of DOC. The estimated monthly OC burial flux in the estuary and continent shelf is 0.48 × 1012 g in July and 0.081 × 1012 g of OC in March, equal to 15.5% and 2.6% of the annual OC discharge by the Yangtze River, respectively. Our study suggests that the burial of OC mainly occurs in the summer monsoon due to the larger sediment load of the Yangtze River, prevailing southeastern wind, and intensified Kuroshio Current. Plain Language Summary: Large rivers export an enormous amount of terrigenous sediment and carbon into the ocean. Under the combined influence of rapid climate change and intense human disturbance, the carbon cycling in estuaries and adjacent seas is highly dynamic. Here, we examine elemental, isotopic, and optical characteristics of organic carbon in waters and sediments along the Yangtze River, Estuary, and East China Sea continuum, and compare organic carbon concentration, composition, and deposition between March and July of 2019, corresponding to normal and high‐water discharge, respectively. We have obtained three key findings of the transport, mixing, and deposition processes of organic carbon in this large river‐dominated margin. (a) There is a net removal of organic carbon from waters in July, and net addition of organic carbon into waters in March. (b) Terrigenous dissolved organic carbon is more conservative than aquatic/marine dissolved organic carbon. (c) The organic carbon burial flux in the estuary and shelf sea is 0.48 × 1012 g in July, five‐fold greater than that in March (0.081 × 1012 g). Thus, the accumulation of organic carbon mainly occurs in the summer monsoon mainly attributed to the larger sediment load of the Yangtze River, prevailing southeastern wind, and intensified Kuroshio Current. Key Points: Terrigenous dissolved organic carbon (DOC) is more conservative than aquatic/marine DOC during seaward transport and mixing processesThere is net removal of DOC in July and net addition of DOC in March in the Yangtze River estuary and shelfThe OC burial in the Yangtze River estuary and shelf in July is five‐fold greater than that in March [ABSTRACT FROM AUTHOR]

Peninsula Lake, Ontario, Canada, is a Precambrian Shield lake that has experienced many environmental stressors since European settlement of the watershed in the mid-1800s, including forest clearance, water-level management, sewage inputs, and land-use changes. The deterioration of water quality by the 1970s prompted mitigation efforts intended to reduce inputs of nutrients and untreated sewage to the lake. Paleolimnological analysis of a sediment core from Peninsula Lake, collected in 1995, revealed that efforts to address eutrophication had been somewhat successful, in that the lake had returned to oligotrophic status by the early 1990s. Nevertheless, there had been little to no improvement in inferred deepwater oxygen conditions. We used similar paleolimnological techniques on a core collected in 2014 to examine ecological responses to environmental stressors and remediation efforts in the intervening years, with a particular focus on changes in the amount of oxygenated habitat available to lake trout. Specifically, we inferred trends in volume-weighted hypolimnetic oxygen (VWHO), through analysis of sedimentary chironomid assemblages, to assess long-term changes in coldwater fisheries habitat. Potential drivers of VWHO were also investigated, and included diatom-inferred total phosphorus, spectrally derived trends in sedimentary chlorophyll a and its main diagenetic products, and lake-water dissolved organic carbon. The sedimentary chironomid assemblages indicate a long period of VWHO depletion, concurrent with known anthropogenic activities from the early twentieth century to the 1990s, followed by recent recovery to near pre-impact levels. The sedimentary diatom assemblages indicate nutrient enrichment throughout most of the twentieth century, which was also followed by a return to pre-impact conditions, albeit with recent indications of renewed nutrient enrichment. In contrast, strikingly different trajectories were observed in two other regional waterbodies that support lake trout, but experienced relatively little nutrient enrichment over the past ~ 150 years. Peninsula Lake appears to be recovering from nutrient enrichment and hypolimnetic oxygen depletion, but water quality improvements have nevertheless been accompanied by increased primary production, perhaps because climate warming has extended the ice-free season and stratification period. Therefore, instead of returning to pre-impact conditions, Peninsula Lake appears to be developing a novel ecological state. Any future improvements in lake trout habitat that result from nutrient abatement may be offset by rising temperatures. [ABSTRACT FROM AUTHOR]

The advective supply of allochthonous dissolved organic carbon (DOC) from open ocean to marginal seas through western boundary current intrusion influences the regional carbon inventory and microbial activities. However, there is limited observation about this process and its biogeochemical impacts on marginal seas. In this study, we investigated the biodegradation of allochthonous DOC carried by the intrusion of the Kuroshio Current into South China Sea (SCS). Using an isopycnal mixing model, the exchange and biodegradation processes of Kuroshio‐intruded DOC were quantified. We estimated that approximately 10% of the surface DOC was remineralized due to the enhanced biodegradation in the SCS. This result was supported by the on‐deck bioassay experiments that were conducted under different environmental contexts. The results of modeling and on‐deck incubations indicate that DOC biodegradation was enhanced by the sharp gradient of environment factors, including nutrients supply, microbial species, and bio‐lability of DOC in the frontal zone during the surface water mass mixing. The amount of carbon released from the enhanced DOC degradation by Kuroshio intrusion was estimated to be approximately equal to 8.6 Tg C yr−1. Concomitantly, the amount of nitrogen released could contribute 0.19–0.70 mmol N m−2 d−1 to the surface of SCS which is comparable to the total supply from deeper water and nitrogen fixation in surface waters. This study suggests that the enhanced biodegradation of DOC during the western boundary currents intrusion could serve as an important sink of oceanic DOC, and thus provide an additional nutrient source to marginal seas. Plain Language Summary: The movement and breakdown of dissolved organic carbon (DOC) in the ocean are two vital processes that regulate atmospheric carbon dioxide concentrations. DOC that accumulates in surface ocean can be transported into deep ocean where the majority of it is degraded by microbes to inorganic components. The horizontal transport of DOC through wind‐driven currents in surface is also an important process that regulates carbon and nutrient cycling. In this study, the transport of DOC by the Kuroshio Current (a western boundary current in the Pacific Ocean) and its subsequent breakdown via microbes were investigated in the South China Sea (SCS). The degradable amount of DOC was determined using two independent approaches: a physical mixing model and on‐deck incubation experiments that simulated the bacterial breakdown of DOC during the mixing of Kuroshio and SCS waters. Our results demonstrate that the Kuroshio intrusion enhances DOC biodegradation due to the changing environmental conditions such as nutrients and microbial communities, releasing a significant amount of nutrients as a result. This influx of nutrients may help support primary production in the SCS. Thus, this study highlights that the intrusion of western boundary currents into marginal seas could be an important process in carbon and nutrient cycles. Key Points: Biodegradation of oceanic dissolved organic carbon (DOC) was enhanced during the Kuroshio Current intrusion to South China Sea (SCS)Changes of environmental contexts, such as nutrients supply and microbial species, stimulated the biodegradationThe remineralization of Kuroshio‐intruded DOC may partially support the export production in SCS [ABSTRACT FROM AUTHOR]

Streams and rivers are known to be significant processors and emitters of carbon (C) to the atmosphere. There have been many large-scale estimates of fluvial C emissions, yet many lack quantification of temporal variability. Here, we compared the relative importance of spatial versus seasonal variability of CO 2 and CH 4 concentrations and fluxes, and DOC concentration in 23 streams and rivers spanning seven Strahler stream orders in two regions with contrasting watershed features, geomorphology and climate in boreal Québec. Much of the overall variance in pCO 2 , pCH 4 and DOC concentration could be explained by region and site-specific properties rather than season. Region was the main driver of pCO 2 , pCH 4 , and DOC concentration followed by network-specific variability. pCO 2 had the strongest seasonal patterns, yet season accounted only for 16% of the total variation, whereas intrinsic network and landscape properties accounted for 60%. Seasonality explained even less of the overall variability for pCH 4 (5%) and DOC (6%) once the region and network features were accounted for. Despite significant spatial and temporal variation in pCO 2 and pCH 4 point fluxes were not significantly different between regions and seasons, because of a modulating interplay between concentration and gas exchange at the whole network scale. The declining trend of pCO 2 and pCH 4 with flow distance across all watersheds and seasons (even under ice and snow) suggests a predominance of network degassing independent of season, and together with the relative constancy of fluxes across regions and time further suggests that seasonality may be dampened when considering fluvial emissions at large spatial scales and especially when integrating these emissions at the whole network scale. [ABSTRACT FROM AUTHOR]