Bioavailable Dissolved Organic Carbon Research Articles

Bacterial carbon cycling in a subarctic fjord: A seasonal study on microbial activity, growth efficiency, and virus‐induced mortality in Kobbefjord, Greenland

In this seasonal study, we examined the environmental controls and quantitative importance of bacterial carbon consumption in the water column and the sediment in the subarctic Kobbefjord, Greenland. Depth‐integrated bacterial production in the photic zone varied from 5.0 ± 2.7 mg C m−2 d−1 in February to 42 ± 28 mg C m−2 d−1 in May and 34 ± 7 mg C m−2 d−1 in September, corresponding to a bacterial production to primary production ratio of 0.34 ± 0.14, 0.07 ± 0.04, and 0.08 ± 0.06, respectively. Based on measured bacterial growth efficiencies (BGEs) of 0.09–0.10, pelagic bacterial carbon consumption was 54 ± 59 mg C m−2 d−1, 1194 ± 329 mg C m−2 d−1, and 689 ± 115 mg C m−2 d−1 in February, May, and September, respectively, which corresponded to 367%, 71%, and 87% of pelagic primary production. The average annual sediment respiration corresponded to 121 mg C m−2 d−1 and accounted for 17% of total microbial respiration in the system. Concentration of bioavailable dissolved organic carbon (BDOC) ranged from 144 ± 37 µg C L−1 in the riverine input to the fjord to a maximum of 341 ± 37 µg C L−1 in the surface waters in September. Parallel enrichment experiments showed that bacterial production and BGE were positively correlated with BDOC concentration, suggesting that organic carbon availability was limiting bacterial activity and carbon conversion efficiency. Viral production was low (0.8–1.8 × 104 viruses mL−1 h−1) as compared to low‐latitude environments, suggesting a relatively small effect of viruses on bacterial mortality (4–36% of cell production) and carbon cycling. Heterotrophic bacterial consumption was closely coupled with autochthonous BDOC production, and the majority of the primary production was consumed by pelagic bacteria at all seasons. The relatively low measured BGE emphasized the importance of solid on‐site BGE estimates for assessing carbon budgets in marine environments.

Assessing the Relative Bioavailability of DOC in Regional Groundwater Systems

It has been hypothesized that the degree to which a hyperbolic relationship exists between concentrations of dissolved organic carbon (DOC) and dissolved oxygen (DO) in groundwater may indicate the relative bioavailability of DOC. This hypothesis was examined for 73 different regional aquifers of the United States using 7745 analyses of groundwater compiled by the National Water Assessment (NAWQA) program of the U.S. Geological Survey. The relative reaction quotient (RRQ), a measure of the curvature of DOC concentrations plotted versus DO concentrations and regressed to a decaying hyperbolic equation, was used to assess the relative bioavailability of DOC. For the basalt aquifer of Oahu, Hawaii, RRQ values were low (0.0013 mM(-2)), reflecting a nearly random relationship between DOC and DO concentrations. In contrast, on the island of Maui, treated sewage effluent injected into a portion of the basalt aquifer resulted in pronounced hyperbolic DOC-DO behavior and a higher RRQ (142 mM(-2)). RRQ values for the 73 aquifers correlated positively with mean concentrations of ammonia, dissolved iron, and manganese, and correlated negatively with mean pH. This indicates that greater RRQ values are associated with greater concentrations of the final products of microbial reduction reactions. RRQ values and DOC concentrations were negatively correlated with the thickness of the unsaturated zone (UNST) and depth to the top of the screened interval. Finally, RRQ values were positively correlated with mean annual precipitation (MAP), and the highest observed RRQ values were associated with aquifers receiving MAP rates ranging between 900 and 1300 mm/year. These results are uniformly consistent with the hypothesis that the hyperbolic behavior of DOC-DO plots, as quantified by the RRQ metric, can be an indicator of relative DOC bioavailability in groundwater systems.

Recycling versus export of bioavailable dissolved organic matter in the coastal ocean and efficiency of the continental shelf pump

At least 15% of the 2 Pg y−1 of dissolved organic carbon (DOC) that accumulates in the surface layer of the open ocean has been exported from the ocean margins. The C: N: P stoichiometry of the production and microbial degradation of dissolved organic matter (DOM) in the coastal ocean conditions the quality of the exported substrates. In this work, DOC, dissolved organic nitrogen (DON) and phosphorus (DOP) bioavailability measurements from published bottle incubation experiments have been compiled and reanalyzed to examine the role of bioavailable DOM (BDOM) in the coastal ocean. DOM bioavailability decreased significantly (p< 0.001) in the sequence DOP > DON > DOC, with bioavailable DOC (BDOC) representing 22 ± 12% (mean ± SD) of the total DOC, bioavailable DON (BDON) 35 ± 13% of the total DON and bioavailable DOP (BDOP) 70 ± 18% of the total DOP. This suggests that the role of DOM on the recycled and export production of the coastal ocean is more relevant for the P than for the N and for the N than for the C biogeochemical cycles. First‐order microbial degradation rate constants (κ) of BDOM (normalized to 15°C) increased significantly (p < 0.05) in the same sequence, with κC being 0.066 ± 0.065 day−1, κN 0.111 ± 0.096 and κP 0.154 ± 0.137 day−1. Significant (p < 0.001) power relationships were found among κC, κN and κP (R2 = 0.84–0.87). The C: N: P molar ratio of the DOM that resists microbial degradation was extremely depleted in N and P, 2835 (±3383): 159 (±187): 1, compared with the BDOM fraction, 197 (±111): 25 (±16): 1. The flushing time (τ) of the coastal ocean in relation to the turnover time of BDOM (1/κ), i.e., τ · κ, dictates the fate ―degradation versus accumulation― of the large scale export of BDOM, which could fuel parts of the oceanic new production and influence the N/P limitation of the open ocean.

Cycling of two carbon substrates of contrasting lability by heterotrophic biofilms across a nutrient gradient of headwater streams

Anthropogenic impacts can significantly alter stream nutrient and dissolved organic carbon (DOC) delivery and composition. Nutrient and DOC cycling in headwater streams, however, are linked via a variety of complex feedbacks that are, in part, influenced by DOC composition emphasizing the need to investigate coupled nutrient–DOC interactions. This study assessed differential incorporation and mineralization of 13C labeled glucose and vanillin by heterotrophic microbes within epilithic biofilm communities in four temperate headwater streams spanning a 100-fold range in total dissolved nitrogen and soluble reactive phosphorous concentrations. The substrates were traced via 13C analyses of DOC, dissolved inorganic carbon, bulk biofilm, and individual biofilm phospholipid fatty acids (PLFA) to assess total incorporation of the substrates and the distribution of substrate use within the heterotrophic community. Results indicate greater nutrient uptake by high nutrient streams with glucose additions relative to vanillin additions and support the hypothesis that nutrient retention in high nutrient streams is hampered by a lack of labile C sources. Vanillin-derived C uptake was only detectable in PLFA from the highest nutrient stream and was dominated by eukaryotic organisms, likely including fungi. This suggests biofilms in high nutrient streams are better adapted to access relatively slow turnover substrates perhaps due to their composition and overall structure. PLFA-based glucose use efficiencies were greatest in the lowest nutrient stream supporting the hypothesis that labile DOC sources are used more efficiently by heterotrophs in less impacted streams, while biofilms of high nutrient streams are better adapted to utilizing a wider array of DOC sources. This adaption is likely a result of exposure to the lower quality DOC pools in high-nutrient streams resulting from high DOC uptake supported, in part, by fast turnover autochthonous sources of DOC. Nutrient retention in nutrient-rich streams, however, is still likely limited by readily bioavailable DOC leading to lower nutrient retention and downstream nutrient enrichment.

Submersible UV-Vis Spectroscopy for Quantifying Streamwater Organic Carbon Dynamics: Implementation and Challenges before and after Forest Harvest in a Headwater Stream

Organic material, including total and dissolved organic carbon (DOC), is ubiquitous within aquatic ecosystems, playing a variety of important and diverse biogeochemical and ecological roles. Determining how land-use changes affect DOC concentrations and bioavailability within aquatic ecosystems is an important means of evaluating the effects on ecological productivity and biogeochemical cycling. This paper presents a methodology case study looking at the deployment of a submersible UV-Vis absorbance spectrophotometer (UV-Vis spectro∷lyzer model, s∷can, Vienna, Austria) to determine stream organic carbon dynamics within a headwater catchment located near Campbell River (British Columbia, Canada). Field-based absorbance measurements of DOC were made before and after forest harvest, highlighting the advantages of high temporal resolution compared to traditional grab sampling and laboratory measurements. Details of remote deployment are described. High-frequency DOC data is explored by resampling the 30 min time series with a range of resampling time intervals (from daily to weekly time steps). DOC export was calculated for three months from the post-harvest data and resampled time series, showing that sampling frequency has a profound effect on total DOC export. DOC exports derived from weekly measurements were found to underestimate export by as much as 30% compared to DOC export calculated from high-frequency data. Additionally, the importance of the ability to remotely monitor the system through a recently deployed wireless connection is emphasized by examining causes of prior data losses, and how such losses may be prevented through the ability to react when environmental or power disturbances cause system interruption and data loss.

Dissolved Oxygen as an Indicator of Bioavailable Dissolved Organic Carbon in Groundwater

Concentrations of dissolved oxygen (DO) plotted vs. dissolved organic carbon (DOC) in groundwater samples taken from a coastal plain aquifer of South Carolina (SC) showed a statistically significant hyperbolic relationship. In contrast, DO-DOC plots of groundwater samples taken from the eastern San Joaquin Valley of California (CA) showed a random scatter. It was hypothesized that differences in the bioavailability of naturally occurring DOC might contribute to these observations. This hypothesis was examined by comparing nine different biochemical indicators of DOC bioavailability in groundwater sampled from these two systems. Concentrations of DOC, total hydrolysable neutral sugars (THNS), total hydrolysable amino acids (THAA), mole% glycine of THAA, initial bacterial cell counts, bacterial growth rates, and carbon dioxide production/consumption were greater in SC samples relative to CA samples. In contrast, the mole% glucose of THNS and the aromaticity (SUVA(254)) of DOC was greater in CA samples. Each of these indicator parameters were observed to change with depth in the SC system in a manner consistent with active biodegradation. These results are uniformly consistent with the hypothesis that the bioavailability of DOC is greater in SC relative to CA groundwater samples. This, in turn, suggests that the presence/absence of a hyperbolic DO-DOC relationship may be a qualitative indicator of relative DOC bioavailability in groundwater systems.

The origin and function of dissolved organic matter in agro-urban coastal streams

[1] Streams draining urban and agriculture catchments are often a source of inorganic nutrients to downstream aquatic ecosystems, but little is known about how changes in land use influence the quality and biodegradability of dissolved organic matter (DOM). We used parallel factor analysis of excitation‐emission fluorescence spectroscopy and biodegradation incubations to examine how DOM composition influences bioavailable dissolved organic carbon (DOC) in surface waters of urban and agricultural catchments during summer (low flow), winter (high flow) and spring (flow recession). Percent bioavailable DOC was variable for all catchments (2–57%) and negatively related to percent humic‐like fluorescence, but positively related to percent protein‐like fluorescence and simple fluorescence metrics of DOM precursor material (fluorescence index and b:a values). Conversely, highly variable DOC concentrations (2–140 mg L −1 ) were negatively related to protein‐like fluorescence and positively related to humic‐like fluorescence. Elevated concentrations of DOC (>30 mg L −1 ) in agro‐urban streams revealed fluorescence indices (<1.3) typical of wetland and forest‐dominated ecosystems, suggesting that enriched stream DOM is either derived from the destabilization of legacy soil carbon or currently produced from remnant wetlands and patches of native vegetation. Overall, we demonstrate that fluorescence characteristics can be used to predict bioavailable DOC in human‐dominated catchments to better understand the flow of carbon and nutrients in aquatic food webs for improved monitoring and management of coastal ecosystems.

Reconstructing the various facets of dissolved organic carbon bioavailability in freshwater ecosystems

We explored various aspects of freshwater dissolved organic carbon (DOC) lability by comparing short‐term (&lt; 2 d) bacterial C consumption (STCC; derived from bacterial respiration measurements) with long‐term (28 d) C consumption (LTCC) in DOC bioassays in lakes, rivers, and marshes located within the same complex drainage basin in southern Québec. We also combined STCC and LTCC measurements to estimate the proportion of DOC removed, and to derive a first‐order decay constant (k). STCC rates were, on average, 25% higher than LTCC, and both parameters showed distinct patterns, reaching their lowest and highest values in lakes and marshes, respectively. STCC and LTCC were correlated to DOC concentration across these freshwater ecosystems, whereas in lakes, STCC was positively correlated to chlorophyll and LTCC to terrestrial C inputs. k showed no ecosystem‐specific patterns but was negatively correlated to chlorophyll across systems. The size of the DOC pools supporting STCC and LTCC, as well as k, were related to distinct components of the DOC pool, as revealed by a parallel factor analysis of fluorescent dissolved organic matter excitation‐emission spectra. Short‐ and long‐term lability and C consumption, and the resulting k, are shown to be complementary facets of DOC bioavailability, which may play very different roles on aquatic ecosystem functioning.

Comparison of Dissolved Organic Carbon Bioavailability from Native and Invasive Vegetation along a Hawaiian River

Riparian litter fall is an important source of organic matter to rivers and accounts for a large fraction of their dissolved organic carbon (DOC) load. DOC is metabolically important in rivers, and therefore changes in riparian vegetation species composition should affect riverine DOC bioavailability. Worldwide, invasive vegetation composes a large percentage of riparian vegetation. In Hawai'i, riparian vegetation changes from native to invasive species with decreasing elevation. To assess how changes in riparian vegetation affect riverine DOC dynamics, we compared DOC bioavailability from native (Acacia koa and Metrosideros polymorpha) and invasive (Falcataria moluccana and Psidium cattleianum) riparian trees to freshwater and estuarine bacteria from the Wailuku River on Hawai'i Island through dark bioassays. DOC bioavailabilities in riverine and estuarine waters were similar among all riparian vegetation types. In contrast, vegetation-derived DOC was more bioavailable (52% ± 4%) than the riverine and estuarine DOC (14% ± 3%). Combining DOC bioavailability and leaf litter input data from our native and invaded riparian sites suggests that a shift in leaf litter inputs from native to invasive species may increase the amount of bioavailable DOC entering Hawaiian rivers and streams. This DOC input has the potential to impact the metabolism and food webs of downstream ecosystems.

Bioavailability of terrestrial organic carbon to lake bacteria: The case of a degrading subarctic permafrost mire complex

Permafrost degradation can result in the loss of significant amounts of carbon, through release to the atmosphere in the form of carbon dioxide and/or methane and through export downstream to lakes and rivers. The fate of this carbon in lake ecosystems is poorly understood. We investigated the capacity of lake bacteria to utilize carbon from different soils from an adjacent mire. Dark bioassays were undertaken to measure the dynamics of the bioavailability and chemical character of dissolved organic carbon (DOC). The soils ranged from already degraded minerotrophic fens to ombrotrophic active layer and soils from the permafrost layer. Our study shows that soil DOC was rapidly consumed by bacteria collected from lake water, particularly within the first 48 h (about 85% of the total consumed DOC). The mean DOC consumption by lake bacteria was 0.087 mg L−1 d−1 when supplied with lake water DOC and varied between 0.382 mg L−1 d−1 (permafrost soil) and 0.491 mg L−1 d−1 (degraded fen soil) when supplied with terrestrial DOC. Thus, the data suggest that export of DOC from degrading permafrost mires at any stage of degradation can potentially increase rates of respiration by fourfold to sevenfold and can have pronounced effects both on receiving lake ecosystems and on the land‐atmosphere carbon balance. In this study we also propose simple predictive models, incorporating weight‐averaged molecular weight and specific UV absorption in combination with other simple qualitative parameters for the estimation of potential bioavailability of soil DOC in aquatic ecosystems.

Bioavailability of DOC in leachates, soil matrix solutions and soil water extracts from beech forest floors

The biodegradability of dissolved organic carbon (DOC) in different fractions from the forest floor was studied. Soil leachate (SL, the soil solution in macropores which is freely drained from forest floor after rainfall), the soil matrix solution (SMS, the soil solution in meso-/micropores of the soil matrix), and soil water extracts (SWE) from two different beech forest floors were compared. Zero-tension and tension lysimeters were used to collect SL and SMS, respectively. Loss of DOC (during 21 days) and respiration of CO 2–C (during 7 days) were used as conventional measures of the availability of DOC. Bacterial production, measured using the leucine incorporation technique, and bacterial growth efficiency were also estimated. All methods were used to study differences in biodegradability between plots with and without ground flora ( Deschampsia flexuosa or Anemone nemorosa) and different type of forest floor (with an organic (O) horizon or a mull (A) horizon). There were no differences in bioavailability of DOC from soil solutions extracted from plots with and without ground flora. The bioavailability of DOC in the different collected soil solutions varied, however. DOC in SWE was the most available, with a mean of 39% of DOC-loss in 21 days, and 18% of DOC being respired in 7 days. DOC in soil matrix solution was the least available of the soil solutions (7% respired), significantly less than DOC in soil leachate (11% respired). The methods measuring biodegradation of DOC, DOC-loss and CO 2–C respiration gave similar results and were comparable to bacterial production and bacterial growth efficiency, with the exception of SWE from the O-horizon at the D. flexuosa site, which had low bacterial production and bacterial growth efficiency, indicating a limitation of the bacterial growth. This study is one of the first to use bacterial production and bacterial growth efficiency for measuring bioavailability in terrestrial environments, giving an extra dimension for the process of biodegradation of DOC.

Bioavailability and export of dissolved organic matter from a tropical river during base‐ and stormflow conditions

Concentrations, bioavailability, and export of dissolved organic matter (DOM), particulate organic matter (POM), and nutrients from the Wailuku River, Hawai'i, U.S.A., were examined under base‐ and stormflow conditions. During storms, DOM and POM concentrations increased approximately by factors of 2 and 11, respectively, whereas NO3− concentrations decreased by a third. The organic carbon (OC) pool was dominated by dissolved OC (DOC) during baseflow conditions, whereas DOC and particulate OC contributed equally during storms. The nitrogen (N) pool was dominated by NO3− during baseflow conditions and by particulate organic N (PON) during storms. Dissolved organic N (DON) comprised a similar percentage of the N pool under base‐ and stormflow conditions. DOC and DON bioavailability both decreased by half during storms in the Wailuku River. Differences in DOM concentrations and bioavailability between base‐ and stormflow conditions suggest that DOM flowpaths to the river changed from deep to shallow, and that DOM sources switched from autochthonous to allochthonous. Although DOM bioavailability decreased during storms, calculations suggest that storms contributed &gt;90% to the annual OC and N yields and &gt;80% to the bioavailable DOM yield from the Wailuku River to coastal waters. Overall, our results suggest that storms are important for delivering bioavailable DOM to coastal waters, where they may stimulate primary and secondary production.

Stream dissolved organic matter bioavailability and composition in watersheds underlain with discontinuous permafrost

We examined the impact of permafrost on dissolved organic matter (DOM) composition in Caribou-Poker Creeks Research Watershed (CPCRW), a watershed underlain with discontinuous permafrost, in interior Alaska. We analyzed long term data from watersheds underlain with varying degrees of permafrost, sampled springs and thermokarsts, used fluorescence spectroscopy, and measured the bioavailabity of dissolved organic carbon (DOC). Permafrost driven patterns in hydrology and vegetation influenced DOM patterns in streams, with the stream draining the high permafrost watershed having higher DOC and dissolved organic nitrogen (DON) concentrations, higher DOC:DON and greater specific ultraviolet absorbance (SUVA) than the streams draining the low and medium permafrost watersheds. Streams, springs and thermokarsts exhibited a wide range of DOC and DON concentrations (1.5–37.5 mgC/L and 0.14–1.26 mgN/L, respectively), DOC:DON (7.1–42.8) and SUVA (1.5–4.7 L mgC−1 m−1). All sites had a high proportion of humic components, a low proportion of protein components, and a low fluorescence index value (1.3–1.4), generally consistent with terrestrially derived DOM. Principal component analysis revealed distinct groups in our fluorescence data determined by diagenetic processing and DOM source. The proportion of bioavailable DOC ranged from 2 to 35%, with the proportion of tyrosine- and tryptophan-like fluorophores in the DOM being a major predictor of DOC loss (p < 0.05, R 2 = 0.99). Our results indicate that the degradation of permafrost in CPCRW will result in a decrease in DOC and DON concentrations, a decline in DOC:DON, and a reduction in SUVA, possibly accompanied by a change in the proportion of bioavailable DOC.

Bioavailability and bacterial degradation rates of dissolved organic matter in a temperate coastal area during an annual cycle

The bioavailability and bacterial degradation rates of dissolved organic matter (DOM) were determined over a seasonal cycle in Loch Creran (Scotland) by measuring the decrease in dissolved organic carbon (DOC), nitrogen (DON) and phosphorous (DOP) concentrations during long-term laboratory incubations (150 days) at a constant temperature of 14 °C. The experiments showed that bioavailable DOC (BDOC) accounted for 29 ± 11% of DOC (average ± SD), bioavailable DON (BDON) for 52 ± 11% of DON and bioavailable DOP (BDOP) for 88 ± 8% of DOP. The seasonal variations in DOM concentrations were mainly due to the bioavailable fraction. BDOP was degraded at a rate of 12 ± 4% d – 1 (average ± SD) while the degradation rates of BDOC and BDON were 7 ± 2% d – 1 and 9 ± 2% d – 1 respectively, indicating a preferential mineralization of DOP relative to DON and of DON relative to DOC. Positive correlations between concentration and degradation rate of DOM suggested that the higher the concentration the faster DOM would be degraded. On average, 77 ± 9% of BDOP, 62 ± 14% of BDON and 49 ± 19% of BDOC were mineralized during the residence time of water in Loch Creran, showing that this coastal area exported C-rich DOM to the adjacent Firth of Lorne. Four additional degradation experiments testing the effect of varying temperature on bioavailability and degradation rates of DOM were also conducted throughout the seasonal cycle (summer, autumn, winter and spring). Apart from the standard incubations at 14 °C, additional studies at 8 °C and 18 °C were also conducted. Bioavailability did not change with temperature, but degradation rates were stimulated by increased temperature, with a Q 10 of 2.6 ± 1.1 for DOC and 2.5 ± 0.7 for DON (average ± SD).

Microbial availability and degradation of dissolved organic carbon and nitrogen in two coastal areas

Microbial availability and degradation rates of dissolved organic carbon (DOC) and nitrogen (DON) were estimated at two coastal stations (Horsens Fjord and Darss Sill) in Denmark, by measuring the decrease in DOC and DON concentrations during long-term laboratory incubations (150 days). The experiments used two different treatments: one only receiving a microbial inoculum and another additionally to the inoculum, carbon and phosphate to ensure nitrogen limitation. The additions of carbon and phosphate led to increased DON bioavailability in all experiments. The incubations showed that bioavailable DOC (BDOC) accounted for 22 ± 13% of total DOC in Horsens Fjord and 14 ± 5% at Darss Sill. Bioavailable DON (BDON) accounted for 43 ± 10% (Horsens Fjord) and 28 ± 12% of DON (Darss Sill). The linear relations between BDOM and DOM suggested that the DOC variations in Horsens Fjord were controlled by the available fraction, while this was only partly the case for DOC at Darss Sill and DON (both stations), showing that the refractory pool to some degree controlled the seasonal variations in DOM at these coastal stations. Additionally we found that DOC and DON were cycled at approximately the same speed, probably due to a high carbon demand of the microbial community. Calculating the amounts of DON degraded within the two areas using the obtained decay rates showed that compared with the ambient inorganic nitrogen levels BDON contained a large proportion (52 ± 37%, Horsens Fjord and 74 ± 19%, Darss Sill) of the bioavailable nitrogen (BDON + DIN). These calculations further suggested that bioavailable DOM was washed out of the respective areas and could contribute to heterotrophic growth in adjacent waters.

Bioavailability and composition of dissolved organic carbon and nitrogen in a near coastal catchment of south-western Australia

The bioavailability and composition of dissolved organic carbon (DOC) and nitrogen (DON) were examined in 10 major sub-catchments of the Swan-Canning estuary, which bisects the city of Perth, in south-western Australia. Catchments contain a mix of forest, agriculture, and urban-dominated land-use, with the degree of development increasing near the city center. We incubated water samples from the 10 sub-catchments for 14 days at 25°C, and measured changes in DOC and DON and dissolved inorganic nitrogen (DIN). A greater proportion of DON (4–44%) was decomposed compared to DOC (1–17%). Both agricultural and urban catchments had high proportions of bioavailable DOC and DON, but overall DOC and DON losses were greatest in urban catchments. Using resin isolation techniques, we found that DOC was concentrated in the hydrophobic (humic) fraction, whereas DON had both hydrophobic and hydrophilic (non-humic) fractions. Hydrophobic DOC content was positively related to DOC decomposition. In contrast, DON decomposition was highly correlated with hydrophilic DON content and inversely related to the hydrophilic DOC/DON ratio, indicating a labile fraction of DON from non-humic sources. Taken together, these relationships suggest that bioavailable DOC may be supplied in part from terrestrial plant material, but bioavailable DON is likely to be from highly labile sources, possibly autochthonous or anthropogenic. Overall, labile DON was greater than initial DIN concentration at seven of ten sites and was even dominant in highly developed catchments. This study highlights the importance of organic N in urbanizing coastal catchments that, in addition to DIN, may serve as a readily available source of N for in-stream and estuarine production.

Twenty years apart: Comparisons of DOM uptake during leaf leachate releases to Hubbard Brook Valley streams in 1979 versus 2000

In seven experiments conducted in late summer 1979 and 2000 extracts of autumn‐shed sugar maple leaves and spruce needles were added to two stream reaches of the Hubbard Brook Experimental Forest at concentrations similar to those found during peak leaf fall. Uptake rates for dissolved organic carbon (DOC) ranged from 7 to 408 mg m−2 h−1. In Bear Brook uptake rates for spruce leachate were lower than for sugar maple leachate in both 1979 and 2000. Uptake velocities tended to be higher for more bioavailable DOC fractions: in 1979, monomeric carbohydrates in DOM were removed from the water column more rapidly than the bulk DOC pool in 4 of 4 cases; in 2000, uptake velocities for N‐containing DOC (DON) were higher than those measured for bulk DOC in 2 of 3 cases. Both results suggest that biological assimilation is an important component of DOM uptake, in contrast to earlier reports that DOM uptake in HBEF streams was primarily through abiotic sorption. In both years, uptake velocities for the same forms of DOM were lower in the high DOC stream Cascade Brook than for the low DOC stream Bear Brook. Experiments in 1979 found no change in inorganic nitrogen concentrations during leachate additions, while all leachate releases in 2000 led to reductions in stream NO3− concentrations.

Investigation of the photocatalytic degradation of brown water natural organic matter by size exclusion chromatography

Herein we report on the photocatalytic degradation of natural organic matter from a bog lake (Lake Hohloh, Black Forest, Germany) as followed by size exclusion chromatography with dissolved organic carbon (DOC) and ultraviolet (λ=254nm) detection (SEC-DOC and SEC-UV). When irradiating this humic-rich water in the presence of 0.5gL−1 titanium dioxide (P25), we found a preferential degradation of the higher molecular weight DOC fraction. This was coupled to growth of the middle and small DOC fractions. Mineralization proceeded in two distinct steps: a first stage without noticeable mineralization, followed by a phase of steady decrease in dissolved organic carbon. Formic, oxalic, succinic and glutaric acids were found in the irradiated samples and contribute significantly to the bioavailable DOC after irradiation, which favors bacterial regrowth.

Land Management Impacts on Dairy‐Derived Dissolved Organic Carbon in Ground Water

Dairy operations have the potential to elevate dissolved organic carbon (DOC) levels in ground water, where it may interact with organic and inorganic contaminants, fuel denitrification, and may present problems for drinking water treatment. Total and percent bioavailable DOC and total and carbon-specific trihalomethane (THM) formation potential (TTHMFP and STHMFP, respectively) were determined for shallow ground water samples from beneath a dairy farm in the San Joaquin Valley, California. Sixteen wells influenced by specific land management areas were sampled over 3 yr. Measured DOC concentrations were significantly elevated over the background as measured at an upgradient monitoring well, ranging from 13 to 55 mg L(-1) in wells downgradient from wastewater ponds, 8 to 30 mg L(-1) in corral wells, 5 to 12 mg L(-1) in tile drains, and 4 to 15 mg L(-1) in wells associated with manured fields. These DOC concentrations were at the upper range or greatly exceeded concentrations in most surface water bodies used as drinking water sources in California. DOC concentrations in individual wells varied by up to a factor of two over the duration of this study, indicating a dynamic system of sources and degradation. DOC bioavailability over 21 d ranged from 3 to 10%, comparable to surface water systems and demonstrating the potential for dairy-derived DOC to influence dissolved oxygen concentrations (nearly all wells were hypoxic to anoxic) and denitrification. TTHMFP measurements across all management units ranged from 141 to 1731 microg L(-1), well in excess of the maximum contaminant level of 80 microg L(-1) established by the Environmental Protection Agency. STHMFP measurements demonstrated over twofold variation ( approximately 4 to approximately 8 mmol total THM/mol DOC) across the management areas, indicating the dependence of reactivity on DOC composition. The results indicate that land management strongly controls the quantity and quality of DOC to reach shallow ground water and hence should be considered when managing ground water resources and in any efforts to mitigate contamination of ground water with carbon-based contaminants, such as pesticides and pharmaceuticals.

Untangling the complex issue of dissolved organic carbon uptake: a stable isotope approach

Summary1. We estimated uptake of stream water dissolved organic carbon (DOC) through a whole‐stream addition of a 13C‐DOC tracer coupled with laboratory measurements of bioavailability of the tracer and stream water DOC.2. The tracer, a leachate of 13C‐labelled tree tissues, was added to the head waters of White Clay Creek, Pennsylvania, U.S.A., over a 2‐h period and followed 1.27 km downstream to generate mass transfer coefficients for DOC lability classes within the tracer.3. From the longitudinal 13C uptake curve, we resolved labile and semi‐labile DOC classes within the 13C‐DOC tracer comprising 82% and 18% of the tracer respectively.4. Plug‐flow laboratory bioreactors colonized and maintained with stream water were used to determine the concentration of stream water DOC fractions that had a similar lability to the labile and semi‐labile classes within the tracer and we assumed that stream water DOC and tracer DOC with comparable lability fractions in the bioreactors behaved similarly in the stream, i.e. they had the same mass transfer coefficients.5. A small fraction (8.6%) of the stream water DOC was labile, travelling 238 m downstream before being taken up. The remaining bioavailable stream water DOC was semi‐labile and transported 4.5 km downstream before being taken up. These uptake lengths suggest that the labile DOC is an energy source within a stream reach, while the semi‐labile DOC is exported out of the reach to larger rivers and the downstream estuary, where it may provide energy for marine microbial communities or simply be exported to the oceans.