Dissolved Organic Research Articles

Effects of permafrost thaw on nitrogen availability and plant–soil interactions in a boreal Alaskan lowland

Summary Increasing rates of permafrost thaw in boreal peatlands are converting conifer forests to waterlogged open wetlands. Permafrost thaw increases soil nitrogen (N) availability, but it is unclear whether such changes are due solely to changes in surface soil N mineralization or N mobilization from thawing permafrost soils at depth. We examined plant species composition and N availability along triplicate permafrost thaw gradients in Alaskan peatlands. Each gradient comprised four community types including: (i) a permafrost peatland with intact permafrost, (ii) a drunken forest experiencing active thaw, (iii) a moat representing initial complete thaw and (iv) a collapse scar bog representing several decades of post‐thaw succession. Concentrations of dissolved organic (DON) and inorganic N (DIN) in the upper 60 cm of soil increased along the permafrost thaw gradients. The drunken forest had the greatest mean concentrations of total dissolved N relative to the other community types, primarily due to greater concentrations of large molecular DON. The moat and collapse bog had significantly greater inorganic N concentrations than the permafrost or drunken forest, suggesting that changes in N availability are not a short‐term effect, but can be sustained for decades or centuries. Across all plant community types, DIN and DON concentrations increased with soil depth during maximum seasonal ice thaw (September), suggesting that deeper soil horizons are important reservoirs of N post‐thaw. Vegetation responses to permafrost thaw included changes in plant community composition shifting from upland forest species to hydrophilic vegetation with deeper rooting profiles in the collapse scar bogs and changes in foliar N and δ15N values. N concentrations in plant foliage and litterfall increased with concentrations of DIN during collapse bog succession, suggesting that plants are utilizing additional mineralized N. Synthesis. Our results suggest that the conversion of forest to wetlands associated with permafrost thaw in boreal lowlands increases N availability, at least in part by increasing turnover of deep soil organic matter. Plants appear to utilize these additional deeper N sources over timescales of years to centuries following permafrost thaw.

Importance of within-lake processes in affecting the dynamics of dissolved organic carbon and dissolved organic and inorganic nitrogen in an Adirondack forested lake/watershed

Abstract. Lakes nested in forested watersheds play an important role in mediating the concentrations and fluxes of dissolved organic matter. We compared long-term patterns of concentrations and fluxes of dissolved organic carbon (DOC) and dissolved organic (DON) and inorganic nitrogen (DIN) in aquatic ecosystems of the Arbutus Lake watershed to evaluate how a lake nested in a forested watershed affects the sources (e.g., production) and sinks (e.g., retention) of DOC and DON in the Adirondack Mountains of New York, USA. We observed no significant long-term changes of DOC and DON in the lake outlet since 1983 and 1994, respectively. However, the temporal patterns of DOC and DON concentrations in the lake inlet showed significant seasonality such as increases during the vegetation-growing season along with notable decreases in the dormant season. A comparison of mass balances between inlet and outlet for the period from 2000 to 2009 suggested that the lake was a sink of DOC (mean of influx minus outflux: +1140 mol C ha−1 yr−1). In contrast, the difference of discharge-weighted DON concentrations (mean of inlet minus outlet: −1.0 µmol N L−1) between inlet and outlet was much smaller than the discharge-weighted DOC concentrations (average of inlet minus outlet: + 87 µmol C L−1). DON fluxes showed considerable variation among years (mean of influx minus outflux: +8 mol N ha−1 yr−1; range of differences: −15 to 27 mol N ha−1 yr−1). DON exhibited low percent retention ((influx-outflux)/influx) (mean: 6.9 %, range: −34.8 to +31.2) compared to DOC (mean: 30.1 %, range: +9.2 to +44.1). The resultant increase of DON within the lake was closely linked with a net decrease of DIN through monthly Pearson correlation analysis, suggesting the importance of biotic factors in mediating lake DON dynamics. Our results show different relative retentions of DOC compared with DON, along with a larger retention of DIN than DON, suggesting that DOC and DON might display substantially different biogeochemical relationships in oligo-mesotrophic lakes nested forested watersheds and therefore different roles for a sink behavior for DOC compared to a producer of DON.

Isotopic evidence for determining the sources of dissolved organic sulfur in a forested catchment.

Understanding sulfur (S) biogeochemistry, especially in those watersheds subject to elevated levels of atmospheric S inputs, is needed for determining the factors that contribute to acidification, nutrient losses and the mobilization of toxic solutes (e.g., monomeric aluminum and methylmercury). S is found in a variety of both organic and inorganic forms undergoing a range of biotic and abiotic transformations. In watersheds with decreasing atmospheric S inputs, internal cycling is becoming dominant in affecting whether there is net loss or retention of S. Little attention has been given to the role of dissolved organic S (DOS) in affecting S biogeochemistry. DOS originates from assimilatory and bacterial dissimilatory S reduction (BDSR), the latter of which produces (34)S-depleted S. Within groundwater of the Archer Creek Catchment in the Adirondack Mountains (New York) there was reoxidation of reduced S, which was an important source of SO4(2-). DOS in surface waters had a higher variation of δ(34)S-DOS values (-6.0 to +8.4‰) than inorganic S with δ(34)S-SO4(2-) values ranging from +1.0 to +5.8‰. Inverse correlations between δ(34)S values of SO4(2-) and DOS suggested that BDSR played an important role in producing DOS.

Distinct bacterial-production–DOC–primary-production relationships and implications for biogenic C cycling in the South China Sea shelf

Abstract. Based on two summer spatio-temporal data sets obtained from the northern South China Sea shelf and basin, this study reveals contrasting relationships among bacterial production (BP), dissolved organic (DOC) and primary production (PP) in the transition zone from the neritic to the oceanic regions. Inside the mid-shelf (bottom depth <100 m), where inorganic nutrient supplies from river discharge and internal waves were potentially abundant, BP, DOC and PP were positively intercorrelated, whereas these three measurements became uncorrelated in the oligotrophic outer shelf and slope. We suggest that the availability of limiting minerals could affect the couplings/decouplings between the source (i.e. phytoplankton) and sink (i.e. bacteria) of organic carbon, and thus DOC dynamics. DOC turnover times were homogeneously low (37–60 days) inside the mid-shelf area and then increased significantly to values >100 days in the outer shelf, indicating that riverine (Pearl River) DOC might be more labile. The actual mechanism for this is unknown, but might relate to higher inorganic nutrient supply from river/terrestrial sources. The positive correlation of the BP / PP ratios vs. phosphate (and nitrate) concentrations in the inner shelf implies that if anthropogenic mineral loading keeps increasing in the foreseeable future, the near-shore zone may become more heterotrophic, rendering the system a stronger source of CO2.

Dynamic carbon budget of a large shallow lake assessed by a mass balance approach

To study the role of large and shallow hemiboreal lakes in carbon processing, we calculated a 3-year carbon mass balance for Lake Vortsjarv (Estonia) based on in situ measurements. This balance took into account hydrological and biogeochemical processes affecting dissolved inorganic (DIC), dissolved organic (DOC) and particulate organic (POC) carbon species. Accumulation varied greatly on a seasonal and yearly basis. The lake exported carbon during most of the year except during spring floods and in late autumn. In-lake processes were responsible for exporting POC and storing DOC while DIC switched between storage and export. The carbon cycle was alternatively dominated in 2009 by biogeochemical processes and in 2011 by riverine fluxes, whereas in 2010 the two process types were of the same magnitude. These results suggest that the role of large shallow lakes like Vortsjarv in the global C cycle is equally driven by hydrological factors, in particular seasonal water level changes, and by biogeochemical in-lake reactions.

Effect of the heterotrophic dinoflagellate Oxyrrhis marina and the copepod Acartia tonsa on vertical carbon flux in and around thin layers of the phytoflagellate Isochrysis galbana

Dynamics of material and energy flow through food webs differ when resources are allocated in patches in comparison to situations in which the same resources are distributed evenly throughout the water column. Thin layers of plankton are special cases of such resource patches. While previous studies have predominantly focused on the response of organisms to these layers, we investigated how 2 types of grazers in turn affect thin layers. In an experimental study with tightly controlled environmental conditions, we monitored the redistribution of particulate organic (POC), dissolved organic (DOC) and inorganic (DIC) carbon from thin layers of Isochrysis galbana. The 2 grazers (the protist Oxyrrhis marina and the copepod Acartia tonsa) had significant grazing impact on the thin layers despite the fact that their population maxima were observed outside the layers. Both grazers exported carbon from the thin layer as body burden (i.e. incorporated into cell tissue) and through release of DOC and DIC into the environment above and below the layers, albeit at different rates. The copepods released larger amounts of DIC and DOC within the thin layer, while the protist grazer exported more dissolved carbon (DOC and DIC) from the thin layers. In the copepod treat- ments, a net increase of DIC was observed inside the thin layer (as a result of increased respiration during feeding) and into the atmosphere above the water column due to their vertical migration between the thin layer and the water surface. Whether or not grazers made a positive contribution to DOC net release depended on the strength of grazing, with a negative effect when phytoplankton— itself releasing DOC—was depleted.

Extending membrane longevity by using MIEX as a pre-treatment

The use of membrane technology and in particular microfiltration (MF) is becoming more popular as a water treatment technology. The operational lifetime of MF membranes can be reduced by natural organic matter (NOM) which is a known foulant. The MIEX DOC (Magnetic Ion EXchange for Dissolved Organic Carbon removal) process was developed to remove NOM from raw water sources. It was anticipated that a reduction in NOM resulting from the MIEX DOC process would also improve the performance of MF systems. A pilot plant study was undertaken to directly measure the impact of MIEX pre-treatment on MF performance. A long term study with a regular cleaning strategy revealed little benefit in MIEX pre-treatment to improve the lifetime of the MF membrane. However, by applying less frequent cleaning it was immediately apparent that fouling was occurring at a faster rate without MIEX pre-treatment. Further studies showed that irreversible fouling also occurred at a faster rate on the membrane without pre-treatment.

Short-term CO 2 mineralization after additions of biochar and switchgrass to a Typic Kandiudult

Biochar additions to soil can increase soil organic carbon (SOC) concentrations; however, minimal information is available on relationships with soil nitrogen (N) cycle. We hypothesized that biochar additions to sandy soils should be resistant to microbial mineralization in short-term studies but may prime organic carbon (OC) mineralization of fresh residue that promotes N immobilization. A laboratory pot incubation study was conducted with a Norfolk loamy sand (Fine-loamy, kaolinitic, thermic, Typic Kandiudult) mixed with pecan-shell biochar at rates of 0, 5, 10 and 20 g kg − 1 and with 0 and 10 g kg − 1 dried, ground switchgrass ( Panicum virgatum L. ). On days 25 and 67 of the incubation, all pots were leached with 1.2 to 1.3 pore volumes of deionized H 2O and the leachate NO 3–N, NH 4–N, and dissolved organic (DOC) concentrations were measured. Also cumulative soil CO 2 fluxes after days 25 and 67 were determined. Biochar alone and mixed with soil and switchgrass after 67 days of incubation were characterized using Fourier transformed infrared spectroscopy (FT-IR). Mixing biochar with switchgrass after 67 days caused a significant increase in SOC content while soil total nitrogen (TN) and leachate DOC concentrations showed mixed results. Biochar mineralization by itself was found to be minimal, but by days 25 and 67, soil with biochar and switchgrass exhibited higher cumulative CO 2 fluxes implying stimulation of switchgrass mineralization. Significant NO 3–N immobilization occurred after 25 days in treatments with biochar + switchgrass; however, by day 67 the NO 3–N concentrations rebounded slightly. The FT-IR analysis revealed that switchgrass in the presence of biochar underwent some structural modifications. Biochar applications in the short-term can cause N immobilization resulting in temporary plant available NO 3–N concentration reductions.

EFEITO DE DIFERENTES CONCENTRAÇÕES DE CARBONO ORGÂNICO DISSOLVIDO E BACTÉRIAS NA DEGRADAÇÃO DE MICROCISTINAS (CIANOTOXINA)

EFFECTS OF DIFFERENT DISSOLVED ORGANIC CARBON CONCENTRATIONS AND BACTERIAL ACTIVITY ON BIODEGRADATION OF MICROCYSTIN (CYANOTOXIN). Microcystins degradation depends on several factors like initial concentration in water, presence of microorganisms, temperature, pH, presence of other available sources of organic carbon, and others. The aims of this study were to measure microcystin degradation under different concentrations of dissolved organic carbon (DOC), and determine the influence of bacterial community on this process. For this purpose, water from the Jacarepagua Lagoon (Rio de Janeiro, Brazil) was diluted in deionized water in order to obtain final concentrations of DOC of 60, 43 and 28mg L -1 (water with bacteria), and final DOC concentrations of 43 and 60mg L -1 (water without bacteria). A semi-purified microcystin extract was prepared and mixed

Ionic treatment for removal of sulfonamide and tetracycline classes of antibiotic

Self-decomposition and removal of antibiotics by ionic treatment was evaluated in this study. Seven sulfonamide classes (SA) and seven tetracycline classes (TA) of antibiotic were selected for this purpose. According to this study, self-decomposition of SAs and TAs was slow, and a considerable amount of antibiotics still remained after 15 days. Ionic treatment was effective for removal of SAs and TAs, but organic interference was observed. When dissolved organic (DOC) was present in raw water, the removal performance of antibiotics generally deteriorated due to competition with organics. SAs and TAs, which were present in ionic form at neutral pH, were removed through ion exchange. Their removal efficiencies were closely related to their chemical structure. Antibiotics with stronger electronegativity were easier to remove by ionic treatment. Equilibrium equations for removal of SAs and TAs by ionic treatment were also presented.

Dissolved Nitrogen, Phosphorus, and Sulfur forms in the Ecosystem Fluxes of a Montane Forest in Ecuador

The N, P, and S cycles in pristine forests are assumed to differ from those of anthropogenically impacted areas, but there are only a few studies to support this. Our objective was therefore to assess the controls of N, P, and S release, immobilization, and transport in a remote tropical montane forest. The study forest is located on steep slopes of the northern Andes in Ecuador. We determined the concentrations of NO3-N, NH4-N, dissolved organic N (DON), PO4-P, dissolved organic P (DOP), SO4-S, dissolved organic S (DOS), and dissolved organic C (DOC) in rainfall, throughfall, stemflow, lateral flow (in the organic layer), litter leachate, mineral soil solution, and stream water of three 8–13 ha catchments (1900–2200 m a.s.l.). The organic forms of N, P, and S contributed, on average, 55, 66, and 63% to the total N, P, and S concentrations in all ecosystem fluxes, respectively. The organic layer was the largest source of all N, P, and S species except for inorganic P and S. Most PO4 was released in the canopy by leaching and most SO4 in the mineral soil by weathering. The mineral soil was a sink for all studied compounds except for SO4. Consequently, concentrations of dissolved inorganic and organic N and P were as low in stream water (TDN: 0.34–0.39 mg N l−1, P not detectable) as in rainfall (TDN: 0.39–0.48 mg N l−1, P not detectable), whereas total S concentrations were elevated (stream water: 0.04–0.15, rainfall: 0.01–0.07 mg S l−1). Dissolved N, P, and S forms were positively correlated with pH at the scale of soil peda except inorganic S. Soil drying and rewetting promoted the release of dissolved inorganic N. High discharge levels following heavy rainstorms were associated with increased DOC, DON, NO3-N and partly also NH4-N concentrations in stream water. Nitrate-N concentrations in the stream water were positively correlated with stream discharge during the wetter period of the year. Our results demonstrate that the sources and sinks of N, P, and S were element-specific. More than half of the cycling N, P, and S was organic. Soil pH and moisture were important controls of N, P, and S solubility at the scale of individual soil peda whereas the flow regime influenced the export with stream water.

Seasonal fluctuation in microbial biomass and activity along a natural nitrogen gradient in a drained peatland

The effects of peat total N on the dissolved N and C concentrations and microbial biomass and activity and their range of seasonal fluctuation were studied in a drained peatland forest in Finland. Seasonal fluctuations in the concentrations of extractable dissolved organic (DON) and inorganic nitrogen (DIN) compounds and extractable dissolved organic carbon (DOC), microbial C and N, ergosterol, net and gross N mineralisation rates were investigated during two growing seasons along a natural peat N gradient in a drained peatland. Significant seasonal fluctuations in NH 4 + and DOC concentrations, microbial C and N, but not in ergosterol or microbial C-to-N ratios in the peat, were observed during the 1999 and 2000 growing seasons. The peat total N concentration affected extractable DON and DOC, but not DIN concentrations in the peat. A negative correlation was found between total N concentration in peat and microbial N and C, and a positive correlation between total N and ergosterol, in peat with N concentrations of up to 2%. Gross mineralisation rates did not show any correlation, whereas net mineralisation rates showed a significant positive correlation with the total N concentration of the peat in both 1999 and 2000.

Effects of water mass exchange on bacterial communities in an aquaculture area during summer

Bacterial abundance (BA) and bacterial production (BP) and their relations with chlorophyll a, dissolved organic carbon (DOC) and dissolved organic (DON) and dissolved inorganic nitrogen (DIN) concentrations, were measured in the eutrophic fish farming ground of the Hasama-ura inlet of Gokasho Bay in the summer of 1996. The BP in the aquaculture area was 172±154 μgC/l/day (mean±standard deviation; n=24) and was higher in the aquaculture area than in the mouth of the bay, especially at the lower chlorophyll a concentrations of the mid and bottom waters. It correlated well with the concentrations of DON, but not with chlorophyll a or DOC. The BP per unit chlorophyll a was higher in the aquaculture area than in the bay mouth, especially at the lower chlorophyll a concentrations of the middle and bottom waters. Our findings suggested that the allochthonous organic matter added by farmed fish supported the bacterial activity in the aquaculture area. An occasional water exchange by intrusion of oceanic water into the Hasama-ura inlet did not cause a significant reduction of BA or BP. However, the correlation coefficient between BP and the DON concentration increased significantly after the water exchange, indicating that the water exchange caused a change in the organic substrate supporting the bacterial community.

Soil organic sulfur dynamics in a coniferous forest

Sulfate microbial immobilization and the mineralization of organic S were measured in vitro in soil horizons (LFH, Ae, Bhf, Bf and C) of the Lake Laflamme watershed (47°17′ N, 71°14′ O) using 35SO4. LFH samples immobilized from 23 to 77% of the added 35SO4 within 2 to 11 days. The 35SO4 microbial immobilization increased with temperature and reached an asymptote after a few days. The mineral soil generally immobilized less than 20% of the added 35SO4, and an asymptote was reached after 2 days. An isotopic equilibrium was rapidly reached in mineral horizons. A two-compartment (SO4 and organic S) model adequately described 35SO4 microbial immobilization kinetics. The active organic reservoir in the whole soil profile represented less than 1% of the total organic S. The average concentrations of dissolved organic S (DOS) in the soil solutions leaving the LFH, Bhf and Bf horizons were respectively 334, 282 and 143 µg⋅L−1. Assuming that the DOS decrease with soil depth corresponded to the quantities adsorbed in the B horizons, we estimated that 12 800 kg⋅ha−1 of organic S could have been formed since the last glaciation, which is about 13 times the size of the actual B horizons reservoirs. Our results suggest that the organic S reservoirs present in mineral forest soils are mostly formed by the DOS adsorption resulting from incomplete litter decomposition in the humus layer. The capability of these horizons to immobilize SO4 from the soil solution would be restricted to a 1% active fraction composed of microorganisms. Despite their refractory nature, these reservoirs can, however, be slowly decomposed by microorganisms and contribute to the S-SO4 export from the watershed in the long term.

A contribution to the chemical characterization of rivers in the Rio Negro Basin, Brazil

Water samples were collected in Middle Amazonia from the Amazon River, Rio Negro and 17 tributaries of Rio Negro. The analyses consisted of pH, conductivity, and dissolved organic (DOC) measurements, as well as plasma source mass spectrometry (ICP-MS). Factor analysis revealed three factors, which explained 94% of the total variance. A plot of factor scores presented a cluster containing mostly samples from the Rio Negro Basin. Ultrafiltration tests confirmed that organics from the Rio Negro have higher molecular mass than in the Amazon, and that some metals are associated with these compounds. Heavy rare-earth elements (REE) are enriched relative to light REE in the dissolved fraction of most rivers of the Negro Basin; the opposite occurred in suspended matter.

Effects of organic matter on the conversion rates of nitrogenous compounds in a channel reactor under various flow conditions

This study investigated the effects of organic matter on the conservation rates of nitrogenous compounds in a deep mixed films channel reactor under high dissolved organic (DO) and various flow conditions. A series of batch tests was performed to examine the effects of organic matter, with and without the addition of glucose, on variations of the nitrogenous compounds. Experimental results indicated that increasing the organic matter concentration under all flow conditions decreased the concentrations of nitrate and total nitrogens; however, an enormous build-up of nitrite occurred in the water column. Also, the conversion pathways of nitrogenous compounds changed from ammonification and nitrification to assimilation, ammonium oxidation and denitrification. Moreover, changing the flow velocity significantly influenced the conversion rates of nitrification and denitrification. When adding organic matter, the rate of nitrite to gaseous products became greatly enhanced if the flow velocity increased within the turbulent regime; increasing velocity seemed only to inhibit the rate of conversion from nitrate to nitrite. In contrast, when organic matter was not added, the rates of ammonium oxidation exceeded those of nitrite oxidation within all flow regimes. Experimental results further demonstrated that nitrite oxidation was the rate-limiting process of nitrification under the low organic matter conditions. On the other hand, when sufficient amounts of organic matter were present, the processes of nitrite to gaseous products and nitrate to nitrite dominated the rate of denitrification under the laminar and turbulent regimes, respectively.

Bacteria, Dissolved Organics and Oxygen Consumption in Salinity Stratified Chesapeake Bay, an Anoxia Paradigm

Chesapeake Bay is a bacterially dominated ecosystem driven, at least under summer conditions, by high levels of labile dissolved organics. Bacterioplankton are exceptionally abundant (20 × 109 cells liter−1) and productive (7 × 109 cells liter−1 d−1), and their biomass can equal or exceed 60% of phytoplankton biomass. In the salinity stratified Chesapeake Bay bacterioplankton account for 60–100% of planktonic oxygen consumption, potentially driving the Bay to anoxia in days to weeks. Sulfide, released from sediments by sulfate reducing bacteria, chemically consumes oxygen at rates up to 9 mg O2 liter−1 d−1 maintaining the oxygen deficit. The organic matter driving this oxygen demand in the summer season is functionally dissolved. Dissolved organics, measured as biochemical oxygen demand, account for about 60% of microbially labile organics throughout the water column and 80% (sometimes 100%) in the subpycnoclinal water. Field studies suggest that reduced oyster stocks in Chesapeake Bay may be a major factor in the shift to this bacterially dominated trophic structure

Large-Scale Trends for Stream Benthic Respiration

Large-Scale Trends for Stream Benthic Respiration

Responsibilities of Authors

Previous articleNext article No AccessResponsibilities of AuthorsRosemary J. MackayRosemary J. Mackay Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by Volume 15, Number 1Mar., 1996 Article DOIhttps://doi.org/10.1086/jnortamerbentsoc.15.1.1467428 Views: 3Total views on this site Citations: 3Citations are reported from Crossref Journal History This article was published in the Journal of the North American Benthological Society (1986-2011), which is continued by Freshwater Science (2012-present). Copyright 1996 The North American Benthological SocietyPDF download Crossref reports the following articles citing this article:Steven M. Wondzell Effect of morphology and discharge on hyporheic exchange flows in two small streams in the Cascade Mountains of Oregon, USA, Hydrological Processes 20, no.22 (Jan 2006): 267–287.https://doi.org/10.1002/hyp.5902R. Moore, S.M. Wondzell PHYSICAL HYDROLOGY AND THE EFFECTS OF FOREST HARVESTING IN THE PACIFIC NORTHWEST: A REVIEW, Journal of the American Water Resources Association 41, no.44 (Aug 2005): 763–784.https://doi.org/10.1111/j.1752-1688.2005.tb04463.xSandra M. Clinton, Rick T. Edwards, Robert J. Naiman FOREST-RIVER INTERACTIONS: INFLUENCE ON HYPORHEIC DISSOLVED ORGANIC CARBON CONCENTRATIONS IN A FLOODPLAIN TERRACE, Journal of the American Water Resources Association 38, no.33 (Jun 2002): 619–631.https://doi.org/10.1111/j.1752-1688.2002.tb00984.x

Dissolved organic carbon and sulfur in southwestern Quebec lakes: Relationships with catchment and lake properties

In 59 southwestern Quebec lakes, dissolved organic S (DOS) concentration averages 185+92 bg liter I (9.2% of total S) and ranges from undetectable to 437 pg liter-’ (22% of total S). DOS can be predicted from dissolved organic C (DOC) concentrations: DOS (pg liter-‘) = 220 log,,(DOC) + 7, r* = 0.55. When applied to 1,238 lakes from five Quibec regions, this relationship indicates that DOS can account for 8.5% (Ottawa) to 22.2% (North Shore) of total S. These results stress the importance of evaluating DOS concentrations in studies of catchment S budgets. Multiple regression models using lake and catchment properties as independent variables respectively explain 75, 43, 49, and 69% of the variance in DOC, DOS, DOC : DOS, and color. The regressions found for DOC, color, and DOS include a flowpath index, accounting for dissolved organic matter (DOM) leaching from the catchment, and a term for in-lake losses [log(Z,,,) or l/Z,,,]. The DOC : DOS regression includes the flowpath index, lake perimeter, and altitude and suggests that DOM supply and composition also depend on catchment properties.