Addition Of Dissolved Organic Carbon Research Articles

Reprint of “Chronic toxicity of silver nitrate to Ceriodaphnia dubia and Daphnia magna, and potential mitigating factors”

We investigated the chronic toxicity of Ag, as silver nitrate, using two freshwater aquatic cladoceran species, Ceriodaphnia dubia and Daphnia magna, to generate data for the development of a chronic ambient water quality criterion for Ag. Preliminary studies with C. dubia showed variable results which were related to the equilibration time between food and silver. Follow-up testing was conducted using a 3 h equilibration time, which stabilized dissolved Ag concentrations and the toxicity of Ag +. Results with C. dubia conducted individually (1 per cup, n = 10) and in mass (30 per chamber, n = 2) gave similar results once similar standardized equilibration times were used. The maximum acceptable toxicant concentration (MATC) of Ag to C. dubia and D. magna was 9.61 and 3.00 μg dissolved Ag/L, respectively. The chronic toxicity of Ag + to C. dubia was also evaluated in the presence of: (1) dissolved organic carbon (DOC) and (2) sulfide. The addition of DOC (0.4 mg/L) resulted in a ∼50% decrease in toxicity while the addition of sulfide (75.4 nM) deceased toxicity by 42%. Whole-body Ag concentration in D. magna was positively correlated with increased levels of Ag exposure, however; we observed a non-statistical decrease in whole-body Na levels, an estimator of sodium homeostasis.

Chronic toxicity of silver nitrate to Ceriodaphnia dubia and Daphnia magna, and potential mitigating factors

We investigated the chronic toxicity of Ag, as silver nitrate, using two freshwater aquatic cladoceran species, Ceriodaphnia dubia and Daphnia magna, to generate data for the development of a chronic ambient water quality criterion for Ag. Preliminary studies with C. dubia showed variable results which were related to the equilibration time between food and silver. Follow-up testing was conducted using a 3 h equilibration time, which stabilized dissolved Ag concentrations and the toxicity of Ag +. Results with C. dubia conducted individually (1 per cup, n = 10) and in mass (30 per chamber, n = 2) gave similar results once similar standardized equilibration times were used. The maximum acceptable toxicant concentration (MATC) of Ag to C. dubia and D. magna was 9.61 and 3.00 μg dissolved Ag/L, respectively. The chronic toxicity of Ag + to C. dubia was also evaluated in the presence of: (1) dissolved organic carbon (DOC) and (2) sulfide. The addition of DOC (0.4 mg/L) resulted in a ∼50% decrease in toxicity while the addition of sulfide (75.4 nM) deceased toxicity by 42%. Whole-body Ag concentration in D. magna was positively correlated with increased levels of Ag exposure, however; we observed a non-statistical decrease in whole-body Na levels, an estimator of sodium homeostasis.

The estuarine mixing behaviour of peatland derived dissolved organic carbon and its relationship to chromophoric dissolved organic matter in two North Sea estuaries (U.K.)

The mixing behaviour of dissolved organic carbon (DOC) was investigated in two U.K. estuaries with peatland derived river inputs (Tyne and Tweed), under varying discharge regimes and in different seasons. DOC removal (i.e. non-conservative) was common to both estuaries at low salinities, but DOC addition also occurred in the Tyne at higher salinities due to inputs from a large sewage treatment works and polluted urban rivers. DOC in freshwater endmembers was in the range 608.1–2326.5 μM (mean 1229.6 μM, n = 7) for the Tyne and was 226.1 and 668.2 μM on two occasions in the Tweed; the high DOC concentrations reflecting the peatland nature of the catchments and high river discharge. DOC removal in the Tyne estuary was in the range 0–58% ( n = 7) (one transect exhibits 0% removal as pollution sources of DOC dominate medium to high salinity samples so it was not possible to extrapolate a conservative mixing line back to low salinities). Highest percent removals were at highest river flows and correspondingly high turbidity and DOC concentrations in the Tyne estuary. In contrast, DOC removal in the Tweed estuary was consistently 6% ( n = 2), and appeared independent of river flow, turbidity and DOC concentration on the two transects conducted. Both estuaries exhibited a strong correlation between the chromophoric dissolved organic matter (CDOM) absorption coefficient at 350 nm ( a 350) and DOC which was seasonally robust; Tweed ( r 2 = 0.98, p < 0.001, n = 20), Tyne ( r 2 = 0.85, p < 0.001, n = 87) and Tyne without potentially anthropogenic or sedimentary derived DOM source samples ( r 2 = 0.93, p < 0.001, n = 74). The robust nature of these relationships implies that remotely sensed CDOM characteristics have the potential to improve the spatio-temporal resolution of estuarine DOC. This study highlights the variation in estuarine removal of peatland derived DOC between the two estuaries and in the Tyne estuary it shows how differences in river regime can result in significant changes in DOC removal at low salinities. Our findings demonstrate the need to conduct DOM sampling under varying seasons and hydrological regimes to accurately quantify the removal of terrigenous DOM in estuaries and its subsequent flux to the oceans.

Experimental inflow of groundwater induces a “biogeochemical regime shift” in iron‐rich and acidic sediments

In acid mine drainage (AMD) polluted lakes, sediments may show a stable iron‐ or sulphate‐reducing regime. We tested the hypothesis that increased inflow of groundwater can trigger a shift between these regimes. Schwertmannite‐bearing sediment, rich in iron (40–43%), sulphur (2.5%), and carbon (4.5%), was sampled from an acidic mine lake, and percolated (0, 5, and 20 L m−2 d−1) with two solutions (1/1 mmol L−1 versus 10/15 mmol L−1 sulphate/ferrous iron, pH 5) in sediment columns. A dissolved organic carbon (DOC) source was added after 46 days (∼2.5 mmol C L−1) to ease the carbon limitation of reductive processes. The sediment remained in an iron‐reducing and acidic regime during percolation without DOC. Percolation at a high rate and addition of DOC raised pH values in schwertmannite‐rich layers from 2.8 to 4.5–5. Schwertmannite transformation to goethite partly accelerated by a factor of &gt;2 but effectively buffered proton consumption only at the low percolation rate. Input of dissolved ferrous iron and sulphate reduced the mobilization of ferrous iron from the sediment. With pH increase, iron reduction slowed relative to CO2 production, sulphate was reduced, and iron sulphides were formed. Input of alkalinity and electron donors by percolation thus shifted the biogeochemical regime from iron to sulphate reduction. Similar regime shifts can be expected for other sediments containing immobile proton or redox buffers when exposed to the advective input of alkalinity and acidity, or electron acceptors and donors.

Effect of Dissolved Organic Matter on Copper - Zinc Competitive Adsorption by a Sandy Soil at Different pH Values

The effect of dissolved organic carbon from sewage sludge on copper and zinc adsorption and interaction on samples from the surface layer (0 - 10 cm) of a sandy soil (Gleyic Podzol) were studied at two pH levels (4 and 7). This soil presented acidification hazards and sewage sludge with a high content of Cu and Zn was used as a fertilizer. Soil samples were treated by solutions of Cu and Zn as nitrate salts with concentration levels up to 30 mg l−1 in a Ca(NO3)2 background at a constant level (180 mg l−1) of dissolved organic carbon (DOC). Copper and zinc sorption capacity decreased in the presence of DOC. As the pH increased, the decrease in Cu adsorption due to interaction with DOC was more obvious. Conversely, without addition of DOC, Cu and Zn adsorption increased with pH. More Cu than Zn was adsorbed. Soil sorption of these cations was described by equilibrium isotherms that fitted both Langmuir and Freundlich type equations, presenting however a better fit to the Freundlich equation (R2>98%). Adsorption dependence on DOC was more noticeable at pH 7.

DOC dynamics in a small temperate estuary: Simultaneous addition and removal processes and implications on observed nonconservative behavior

Dissolved organic carbon (DOC) dynamics in the Pawcatuck River estuary, a small temperate estuary in Rhode Island, United States, were examined through the use of field transect and in situ production studies. In late summer, when river discharge was minimal, phytoplankton blooms occurred in the upper reaches of the estuary and released large amounts of autochthonous DOC that accumulated in the middle reaches of the estuary. DOC production rates in August months, calculated both by mixing diagrams and in situ DOC incubations, ranged from 6.67 to 34.7 μmol C l−1 d−1 and were positively correlated with DCMU-enhanced fluorescence, an estimate of phytoplankton photosynthetic activity (r2=0.796, p<0.001). The percent extracellular release (PER) of DOC from phytoplankton, calculated from measured in situ DOC production and net phytoplankton production (NPP) rates, ranged from 5.8% to 40.6% and was negatively correlated with NPP (r2=0.80, p<0.01). Accumulated DOC was principally nonhumic in nature, and the humic DOC component behaved quite differently with either conservative mixing or significant removal at the head of the estuary. Humic removal at times amounted to approximately 50% of the humic material and 25% of the total incoming riverine DOC. These large humic losses were not observed in bulk DOC-salinity mixing diagrams but required distinct analyses of the humic and nonhumic components. DOC addition and removal processes co-occur in this system and observation of bulk DOC mixing diagrams may mask the true dynamic nature of the estuarine DOC pool. The net result of the DOC addition and removal processes is a seasonally variable transformation of a humic-rich incoming riverine DOC to a nonhumic enriched bulk DOC component that varies seasonally and with river discharge.

Production and food web interactions of Arctic freshwater plankton and responses to increased DOC

A gradient of dissolved organic carbon (DOC) was added to enclosures in a high Arctic lake (Svalbard, 79° N). The aim was to simulate the effect of increased concentrations of DOC that will be a predicted effect of increased temperature and precipitation. The study aimed to provide information on the overall effects of such increased levels of DOC on the pelagic food-web, as well as the increased attenuation of UV-radiation (UV-R) caused by increased DOC. The biomass development of phytoplankton, heterotrophic bacteria and ciliates from 15. July (shortly after ice-off) to 8. August revealed a consistent pattern across all enclosures. Initial phytoplankton biomass decreased from maxima around 600μg Cl -1 towards 50μg Cl -1 by the end of July. Similarly, ciliate biomass decreased from ∼100 to 5 μg Cl -1 , while heterotrophic bacteria decreased from initially 280μg Cl -1 to biomasses near 100μg Cl -1 . Over the same period, zooplankton biomass (almost a monoculture of Daphnia tenebrosa) increased from <40 to some 170μg Cl -1 . These patterns were reflected also in decreasing absolute production of bacteria and phytoplankton, while production:biomass ratio remained fairly constant. In general, the study demonstrated a very dynamic system over the brief ice-free season, where Daphnia grazing had a strong impact on the unicellular biota. Additions of DOC initially stimulated planktonic production, yet this effect was soon overruled by Daphnia grazing. This stimulating effect could be a result both of increased UV-R attenuation and some growth stimulating effect. Since no corresponding stimulating effect was observed in the bag shielded with Mylar filter to screen off UV-B, the latter cause seems most likely.

Effects of carbon substrate enrichment and DOC concentration on biodegradation of PAHs in soil.

Two common reasons to explain slow environmental biodegradation of polycyclic aromatic hydrocarbons (PAHs), namely lack of appropriate carbon sources for microbial growth and limited bioavailability of PAHs, were tested in a laboratory bioassay using a creosote-contaminated soil. The soil, containing a total of 8 mg g-1 of 16 PAHs, was sieved and incubated in bottles for 45 days. The first explanation was tested by enrichment with the analogue anthracene and the non-analogue myristic acid, and both failed to stimulate degradation of all PAHs except anthracene. The second explanation was tested by addition of different concentrations of dissolved organic carbon (DOC), with effects depending on the DOC concentration and the molecular size of the PAH. The degradation was enhanced from 10 to 35% for 12 PAHs when the soil was saturated. The degraded amounts of individual PAHs were proportional to their concentration in the soil. The slow in situ degradation of PAHs was enhanced by more than three times by adding water as a solvent. Addition of DOC facilitated the degradation of four- to six-ring PAHs. Bioremediation of PAH-contaminated sites may be facilitated by creating water-saturated conditions but retarded by addition of other carbon substrates, such as analogue compounds.

Responses in pigmentation and anti-oxidant expression in Arctic Daphnia along gradients of DOC and UV exposure

Responses in carapace melanization and expression of the major anti-oxidant catalase (CAT) and glutathione transferase (GST) in Arctic Daphnia were assessed in enclosures along a gradient of dissolved organic carbon (DOC). This gradient was created by adding freeze-dried humic matter to 2 m3 UV-transparent enclosures, yielding final nominal concentrations of 1, 2.5, 5 and 10 mg C l−1. The UV attenuation was strongly affected by additions of DOC, and attenuation coefficients at 320 nm increased from 3.0 in the control to approximately 3.5 and 11.0 m−1 in the 1 and 10 mg DOC treatments respectively. Most Daphnia showed pronounced carapace melanization, and the absorbance of short-wave radiation through the carapace was strongly related to the degree of melanization. Nevertheless, the different UV climate in the enclosures did not cause any short-term adaptation in Daphnia pigmentation over a 3 week period. The levels of CAT and GST were assessed over time in the control and at 10 mg DOC. These enzymes displayed opposite patterns, with somewhat lower activities of CAT at low DOC (control) relative to 10 mg DOC, while the opposite was found for GST. There was also a significant negative correlation between CAT and solar irradiation for GST in both bags, while no effects were found for GST.

Microbial respiration within a floodplain aquifer of a large gravel‐bed river

1. Aerobic respiration, productivity and the carbon turnover rate of microbial biofilms were determined at hyporheic and phreatic sites in the Kalispell Valley alluvial aquifer along a transect extending 3.9 km laterally from the main channel of the Flathead River, a sixth order river in Montana (U.S.A.). The effect of experimentally increasing bioavailable organic carbon (acetate) on the respiration rate of biofilms in this carbon‐poor [dissolved organic carbon (DOC) &lt; 2 mg L−1] aquifer was also measured.2. Chambers containing natural substratum were placed in‐situ and allowed to colonise for 20 weeks. After 4, 12 and 20 weeks, they were taken to the laboratory where oxygen flux was measured in a computer‐controlled, flow‐through respirometry system.3. Respiration ranged from 0.01 to 0.33 mg O2 dm−3 h−1 across sites, with means ranging from 0.10 to 0.17 mg O2 dm−3 h−1. Productivity estimates ranged from 0.18 to 0.32 mg C dm−3 day−1 (mean 0.25, SE 0.03). The total organic carbon (TOC) of the microbial biofilms ranged from 18.2 to 29.7 mg C dm−3. Turnover rate ranged from 3.2 to 5.6 year−1 with a mean of 4.2 year−1.4. At the hyporheic site very close to the river, respiration did not significantly increase when samples were supplemented with labile carbon. Respiration increased with increasing DOC addition at hyporheic sites more distant from the river, suggesting a carbon‐limitation gradient within the hyporheic zone. Microbes at the phreatic site did not respond to increasing DOC addition, suggesting that the phreatic biofilm is adapted to low carbon availability.5. Comparing the volume of the alluvial aquifer (about 0.7 km3) to that of the river benthic sediments (to 0.25 m depth, which amounts to about 1.6 × 10−4 km3) within the Flathead Valley, leads to the conclusion that interstitial microbial productivity is orders of magnitude greater than benthic productivity. Alluvial aquifers are often voluminous and microbial production is an enormous component of ecosystem production in rivers such as the Flathead.

Influence of Dissolved Organic Carbon and Initial Moisture on Zinc Sorption by Two Arid Soils

Sorption of added zinc to irrigated soils in arid regions is an important process that may control the availability of zinc to growing plants . Two soil surface samples varying in clay , organic matter , and calcium carbonate content were selected from central and southwestern regions of Saudi Arabia and prepared in order to give different initial moisture contents ranging from air dried to 100 % of field capacity . The sorption experiment was conducted using Zn concentrations ranging from 5 to 25 mg L 1, prepared from ZnSO4 either in distilled water or in solutions containing 75 mg L 1 dissolved organic carbon (DOC). Results indicate that the amount of Zn sorbed in the presence of DOC was relatively high compared with the absence of DOC and Zn retention was strongly affected by the initial soil moisture content . Also , equilibrium Zn concentrations were quite low , while Zn retentions were high in all treatments . Data of Zn sorption were described by the Freundlich isotherm , and two linear portions were found in most cases . In the absence of DOC , retentions of added Zn were controlled by the available exchange sites and / or the precipitation of Zn as sparingly soluble forms. Zn ions in the presence of DOC were able to form soluble - Zn organic complexes that adsorb on the soil surfaces . The extent of such behavior was related to the variations in clay , organic matter , and calcium carbonate contents as well as the initial moisture of the soil . Results indicate that addition of DOC reduces the amount of extractable Zn from either soil Zn or the sorbed Zn by ammonium bicarbonate diethylenetriamine pentaacetic acid ( AB DTPA ). More than 80 % of the sorbed Zn was extracted by AB-DTPA , and the percentage of extracted / sorbed Zn decreased with the increase in sorbed Zn . The obtained results give evidence that initial moisture content and addition of DOC reduce the extractability of applied inorganic Zn by AB-DTPA extract in arid soils.

Patchiness in Microbial Nitrogen Transformations in Groundwater in a Riparian Forest

AbstractWe measured microbial N transformations in 15 cm diam. by 40 cm intact horizontal sections of aquifer material (mesocosms), taken from a riparian forest in Rhode Island, USA, incubated under ambient conditions. The mesocosms allowed us to measure these transformations on the same scale as hydrologic tracer methods (Br−/NO−3 ratios) that measure net NO−3 removal. Our objective was to reconcile discrepancies between hydrologic tracer and microbial measurements in previous studies where laboratory‐based microbial NO−3 consumption measurements were much lower than in situ hydrologic measurements of net NO−3 removal. We hypothesized that small “patches” of organic matter in the aquifer matrix, which are easily missed when sampling for microbial measurements, are “hotspots” of NO−3 removal and are responsible for these discrepancies. Mesocosms were subjected to three treatments [Br− only, Br− + 15NO−3 Br− + 15NO−3 + dissolved organic carbon (DOC)]. Solution (NH+4, NO−3, dissolved organic N) and gaseous (N2O, 15N2O, and 15N2) inputs and outputs to the mesocosms were measured over a 132‐d incubation, followed by destructive sampling for the presence of patches and residual 15N in aquifer matrix and patch material. Total (gross) NO−3 consumption by denitrification and immobilization was greater than net removal of NO−3 measured by Br−/NO−3 ratios. Net NO−3 consumption was only observed in mesocosms that contained “patches” of organic matter and was not increased by addition of DOC, suggesting that these patches, which represent &lt;1% of aquifer weight, are critical to groundwater NO−3 removal in riparian forests.

Soil Dissolved Organic Matter Sorption as Influenced by Organic and Sesquioxide Coatings and Sorbed Sulfate

AbstractThe sorption of dissolved organic carbon (DOC) and its fractions as influenced by organic C (OC), sesquioxides [amorphous Al(OH)3, ferrihydrite, goethite], and extractable SO2−4 was investigated by modifying the properties of the Bs horizon of an Entic Haplorthod. Coating the Bs material with OC extracted from the Oa horizon of a mor forest floor layer decreased the sorption of total DOC. The effects on the sorption of the hydrophobic DOC fraction were little, whereas the sorption of the hydrophilic fraction was strongly reduced. At the highest OC coating levels, a net release of hydrophilic DOC occurred, indicating a displacement of indigenous hydrophilic substances from the soil by the sorbing hydrophobic dissolved organic matter (DOM). Coating of the soil material with sesquioxides clearly increased the sorption of DOC and its fractions. The strongest effects occurred when amorphous Al(OH)3 was added, followed by ferrihydrite and goethite. Increased contents of extractable SO2−4 did not affect the sorption of DOC on soil material. With increasing addition of DOC, however, the concentrations of SO2−4 in the equilibrium solution increased. Adsorbing DOM seems to displace SO2−4 bound to soil materials. Thus, DOM shows a higher affinity to sorb on soil material than SO2−4.