Different Dissolved Organic Carbon Research Articles

Effects of dissolved organic carbon and total ammonia nitrogen concentrations with the same DOC/TAN on biofloc performance

The regulation of the carbon-to‑nitrogen ratio (C/N) is at the core of biofloc technology. This study compared the performance of bioflocs under different dissolved organic carbon (DOC) and total ammonia nitrogen (TAN) concentrations but the same DOC/TAN ratio. Dissolved organic carbon and total ammonium nitrogen were set at 200 mg/L and 10 mg/L (Group 200/10), 100 mg/L and 5 mg/L (Group 100/5), and 50 mg/L and 2.5 mg/L (Group 50/2.5), respectively. The TAN removal rates of groups A, B, and C were 1.89 ± 0.02 mg N/g TSS·h, 1.63 ± 0.03 mg N/g TSS·h, and 1.32 ± 0.02 mg N/g TSS·h, respectively, and the differences were significant (P < 0.05). At the phylum level, the microbial community structure was dominated by Proteobacteria, Bacteroidetes, Actinobacteria, Verrucomicrobia, and Planctomycetes. At the genus level, Gemmobacter, Hydrogenophaga, and Microbacterium were the dominant genera. Most of bacteria community differed significantly among the three groups. As the concentration of carbon and nitrogen sources increased, the Shannon index showed a decrease in bacterial diversity, and the number of heterotrophic bacteria significantly increased. The crude protein content of flocs in group A ((29.22 ± 0.20)%) was significantly lower than that in groups B ((30.56 ± 0.32)%) and C ((30.83 ± 0.20)%). In groups A and B with higher carbon and nitrogen concentrations, the copy numbers of genes related to nitrification were relatively low, but the copy numbers of genes related to denitrification were relatively high. Therefore, it can be concluded that with a ratio of 20 of DOC/TAN, increasing DOC and TAN concentrations led to a significant increase in TAN removal capacity, heterotrophic bacteria count, denitrification function gene copies, and crude protein content of bioflocs, and a decrease in bacterial diversity, nitrification function gene copies, and C/N of bioflocs (P < 0.05). These results suggest that the influence of DOC and TAN concentrations on bioflocs performance may be more significant than that of DOC/TAN, and should be fully considered in practical bioflocs technology systems.

Invasive upside-down jellyfish tolerate organic eutrophication and warming

The upside-down jellyfish Cassiopea is a successful invasive organism due to high tolerance of different environmental conditions. So far, some single local or global factor experiments have been conducted to assess the effect on the ecophysiology of Cassiopea, but information about combined factor effects is scarce. Therefore, we performed a 42-d manipulation experiment exposing Cassiopea sp. to a local (organic eutrophication) and global factor (warming) simultaneously, i.e., four different dissolved organic carbon (DOC) concentrations [3 (Control), 10 (Low), 20 (Medium) and 40 (High) mg L–1], followed by a stepwise water temperature increase from 26 to 32 °C, in which we assessed four ecophysiological parameters: respiration, net photosynthesis, gross photosynthesis, and pumping rate. Our results show a steep increase in respiration and simultaneous decline in gross photosynthesis in response to Medium and High DOC enrichment causing medusae in the respective treatments to become net heterotrophic over time. This was accompanied by increases in pumping rates, likely to optimize gas exchange and potentially increase nutrient availability by advection of nutrients trapped in underlying sediments. No effect of warming was found for any of the assessed parameters. The observed transition from auto- to heterotrophy under high DOC concentrations together with the thermal tolerance of Cassiopea sp. aids to understand the current invasive success of members of the genus Cassiopea as nonnative organisms and may shed light on the future of Cassiopea sp. as potential native invaders.

A balancing act: Optimizing free chlorine contact time to minimize iodo-DBPs, NDMA, and regulated DBPs in chloraminated drinking water

Many drinking water treatment plants in the U.S. have switched from chlorination to chloramination to lower levels of regulated trihalomethane (THM) and haloacetic acid (HAA) disinfection byproducts (DBPs) in drinking water and meet the current regulations. However, chloramination can also produce other highly toxic/carcinogenic, unregulated DBPs: iodo-acids, iodo-THMs, and N-nitrosodimethylamine (NDMA). In practice, chloramines are generated by the addition of chlorine with ammonia, and plants use varying amounts of free chlorine contact time prior to ammonia addition to effectively kill pathogens and meet DBP regulations. However, iodo-DBPs and nitrosamines are generally not considered in this balancing of free chlorine contact time. The goal of our work was to determine whether an optimal free chlorine contact time could be established in which iodo-DBPs and NDMA could be minimized, while keeping regulated THMs and HAAs below their regulatory limits. The effect of free chlorine contact time was evaluated for the formation of six iodo-trihalomethanes (iodo-THMs), six iodo-acids, and NDMA during the chloramination of drinking water. Ten different free chlorine contact times were examined for two source waters with different dissolved organic carbon (DOC) and bromide/iodide. For the low DOC water at pH 7 and 8, an optimized free chlorine contact time of up to 1 h could control regulated THMs and HAAs, as well as iodo-DBPs and NDMA. For the high DOC water, a free chlorine contact time of 5 min could control iodo-DBPs and NDMA at both pHs, but the regulated DBPs could exceed the regulations at pH 7.

Controls on turnover of marine dissolved organic matter—testing the null hypothesis of purely concentration‐driven uptake: Comment on Shen and Benner, “Molecular properties are a primary control on the microbial utilization of dissolved organic matter in the ocean”

Controls on turnover of marine dissolved organic matter—testing the null hypothesis of purely concentration‐driven uptake: Comment on Shen and Benner, “Molecular properties are a primary control on the microbial utilization of dissolved organic matter in the ocean”

Under Ice and Early Summer Phytoplankton Dynamics in Two Arctic Lakes with Differing DOC

AbstractWe investigated Arctic lake phytoplankton response along vertical gradients in the water column during seasonal succession from ice‐covered to open‐water conditions. Two oligotrophic lakes in West Greenland with different dissolved organic carbon (DOC) concentrations were selected. We assessed which factors: (1) promote under‐ice growth of phytoplankton, and (2) trigger shifts in the community structure. Our results suggest that DOC is an important driver of the seasonal distribution of phytoplankton biomass—high DOC exacerbates light limitation under ice resulting in low phytoplankton biomass, but supports phytoplankton growth during the open‐water period when photolytic and biological degradation of organic matter contributes to the pool of available nutrients. Under‐ice phytoplankton biomass in a clear, low DOC lake was as high as during the open‐water period, suggesting the importance of under‐ice processes for lake metabolic balance, also evidenced by persistent oxic conditions in the hypolimnion. Species composition was dynamic, reflecting rapidly changing conditions over the course of the season. Our data suggest that phytoplankton under ice provides seed populations for early spring blooms and that the largest shifts in species composition occur later during the summer, likely as a result of nutrient depletion and intensified grazing pressure. Early spring temperatures in West Greenland have risen rapidly in recent decades, triggering shifts in the timing of lake ice‐out and the onset of stratification; therefore, it is important to understand phytoplankton seasonal dynamics to disentangle the effects of climate‐driven shifts on aquatic biota.

Dissolved Organic Carbon Source Influences Tropical Coastal Heterotrophic Bacterioplankton Response to Experimental Warming.

Global change impacts on marine biogeochemistry will be partly mediated by heterotrophic bacteria. Besides ocean warming, future environmental changes have been suggested to affect the quantity and quality of organic matter available for bacterial growth. However, it is yet to be determined in what way warming and changing substrate conditions will impact marine heterotrophic bacteria activity. Using short-term (4 days) experiments conducted at three temperatures (−3°C, in situ, +3°C) we assessed the temperature dependence of bacterial cycling of marine surface water used as a control and three different dissolved organic carbon (DOC) substrates (glucose, seagrass, and mangrove) in tropical coastal waters of the Great Barrier Reef, Australia. Our study shows that DOC source had the largest effect on the measured bacterial response, but this response was amplified by increasing temperature. We specifically demonstrate that (1) extracellular enzymatic activity and DOC consumption increased with warming, (2) this enhanced DOC consumption did not result in increased biomass production, since the increases in respiration were larger than for bacterial growth with warming, and (3) different DOC bioavailability affected the magnitude of the microbial community response to warming. We suggest that in coastal tropical waters, the magnitude of heterotrophic bacterial productivity and enzyme activity response to warming will depend partly on the DOC source bioavailability.

Adsorption-desorption of dimethenamid and fenarimol onto three agricultural soils as affected by treated wastewater and fresh sewage sludge-derived dissolved organic carbon

The use of treated wastewaters (TWW) in agriculture is widening in areas suffering drought, such as southern Europe, to preserve freshwater supply for human consumption. The composition of TWW, especially concerning their organic carbon (OC) content, has been demonstrated to influence the processes governing the behavior of non-ionic pesticides in soils. Three OC-poor agricultural soils (SV, RM1 and RM3) from the province of Granada (Spain) were chosen for the assessment of the adsorption and desorption of the herbicide dimethenamid (DIM) and the fungicide fenarimol (FEN). TWW and sewage sludge extracts at different dissolved OC (DOC) concentrations (30, 90 and 300 mg L−1) were considered to evaluate their effect on pesticide adsorption-desorption. As expected by their properties, DIM and FEN were weakly and moderately adsorbed to the soils, respectively. Soil OC seemed to be the major factor controlling FEN adsorption, whereas the mineral fraction played a key role in DIM adsorption, especially in RM1 with high clay:OC ratio. Although TWW did not significantly modify the adsorption of pesticides, it enhanced DIM desorption from the three soils. Adsorption of FEN to SV and RM3 was directly related to the concentration of DOC, possibly due to co-sorption phenomena. Hysteretic desorption was found in all cases, indicating partially reversible adsorption. While FEN desorption was not altered by the solutions, the use of sludge extracts at the highest DOC concentration (300 mg L−1) enhanced DIM desorption as occurred with TWW. Interactions with DOC in solution seemed to predominate for this less hydrophobic compound, thus increasing the risk of natural waters contamination if TWW will be used.

Prediction of Full-Scale GAC Adsorption of Organic Micropollutants

Abstract The principal goal of this research was to develop a new scale-up procedure with which the adsorption of organic micropollutants (MPs) by granular activated carbon (GAC) can be predicted from bench-scale tests. To develop scale-up approaches, the adsorption of 29 environmentally relevant MPs was studied in both pilot-scale GAC adsorbers and rapid small-scale column tests that were based on the proportional diffusivity design (PD-RSSCT). Four surface waters with different dissolved organic carbon (DOC) concentrations were spiked with MPs at ng/L levels and tested in PD-RSSCTs simulating full-scale empty bed contact times of 7 and 15 min. Corresponding pilot-scale tests showed MP breakthrough occurred earlier than in PD-RSSCTs, and MP-specific differences between the PD-RSSCT and pilot column breakthrough curves were described by fouling parameters. On average, the PD-RSSCT overpredicted pilot-scale MP breakthrough by a factor of 3.0 ± 1.2 (n = 101). Two approaches were developed for predicting bed...

Characterisation of organic foulants on full-scale UF membranes during filtration, backwash and chemical cleaning episodes

Understanding organic fouling on ultrafiltration (UF) membranes during water filtration and cleaning episodes has become one of the major factors driving UF technology forward. The aim of this study was to quantify and characterise the organic foulants on an UF membrane at a full-scale drinking water treatment plant when it is fed with surface water and groundwater with different dissolved organic carbon (DOC) contents. DOC characterisation was performed by high-performance size-exclusion chromatography and fluorescence excitation–emission matrix (FEEM). The masses of DOC (and its fractions) retained by the membrane over a whole filtration period (and detached during cleaning episodes) were calculated through mass balances. Under river water feeding conditions, DOC was retained by 22%, being biopolymers the most retained DOC fraction (59%), followed by humic substances (17%) and other minor organic fractions. Routine backwashing resulted in the detachment of only 8% of the total mass of DOC retained, with biopolymers as the most detached fraction (27%). Within biopolymers, proteins appeared to contribute more to hydraulically irreversible fouling than polysaccharides. Under groundwater feeding conditions, no apparent retention of DOC was observed. FEEM analyses showed neither significant removal of fluorescent components during filtration nor detachment from the UF membrane during routine backwashing.

Removal of Bisphenol A (BPA) in Surface Water by Ozone Oxidation Process

The removal of Bisphenol A in river water can be accomplished with the application of ozone. Ozone is widely used to disinfect drinking water due to its strong oxidizing properties. This study was conducted to investigate the removal of Bisphenol A in different areas of Skudai River. Batch experiments were conducted at initial Bisphenol A concentration of 0.5 mg L-1. The concentrations of Bisphenol A and dissolved ozone were measured using Ultra High Performance Liquid Chromatography (UHPLC) and UV-Visible spectrophotometer respectively. Based on the results obtained the stability of ozone in the water were very fast. Ozonewas quickly decomposed in water samples. The results indicated that the depletion of ozone varied from different dissolved organic carbon (DOC) value of water samples. Concentration of dissolved ozone was reducing rapidly after spiked into high DOC value (9.17 mg L-1) of river water. The results of the study have shown that ozonation could be used to effectively remove the Bisphenol A from water. The ozone depletions in river water at 120 seconds were about 87%-99%.

The Influence of the Removal of Specific NOM Compounds by Anion Exchange on Ozone Demand, Disinfection Capacity, and Bromate Formation

This research on a pilot scale focuses on the reaction of ozone with natural organic matter (NOM) for three water qualities with different dissolved organic carbon (DOC) concentrations and NOM compositions, obtained after several stages of an anion exchange process. It was shown that for the same ozone dosage per DOC, the ozone demand was higher, less bromate was formed and a lower disinfection capacity was reached for water containing mainly humic substances, than for water where the humic substances were partly removed. It can be concluded that NOM composition, specifically the humic substances, influences the ozone demand, disinfection capacity and bromate formation.

Using the critical body residue approach to determine the acute toxicity of cadmium at varying levels of water hardness and dissolved organic carbon concentrations

The linkage between acute adverse effects of cadmium and internal cadmium levels were investigated for the oligochaete worm Lumbriculus variegatus in water at varying degrees of hardness and two different dissolved organic carbon (DOC) concentrations. The LC 50s for the effect of cadmium on the survival of the worms greatly differed depending on water hardness and DOC. We found less variability in internal metal toxicity metrics (lethal residue; LR 50s) than in external toxicity metrics (lethal concentration; LC 50s): LC 50s varied from 2.4 to 66.1 μmol/L, while LR 50s varied only from 226 to 413 μmol/kg wet weight. The cadmium body burden appeared to be independent of exposure conditions. From our experimental data, a critical cadmium body residue (324±78 μmol/kg wet weight) associated with 50% lethality was derived. The protective role of DOC and water hardness against cadmium toxicity was evident.

Effects of natural organic matter (NOM) character and removal on ozonation for maximizing disinfection with minimum bromate and AOC formation

The objective of this study was to evaluate the effects of the character and the removal of natural organic matter (NOM) on the formation of assimilable organic carbon (AOC) and bromate during ozone disinfection. Natural waters from two locations with different dissolved organic carbon (DOC) concentrations were tested. In addition, the DOC concentration of one of the natural waters was reduced either by ion exchange (IEX) or by granular activated carbon (GAC) filtration. The resulting four water types were tested in conventional pilot-scale ozone bubble column reactors and in a bench-scale plug flow reactor with dissolved ozone dosing. For the tested waters NOM fractions were determined with size exclusion chromatography using DOC and UVA254 detection. As expected, it was observed that for the same ozone dosages the CT is higher when NOM is removed by IEX or GAC compared to the situation without NOM removal. It is concluded that, for the waters tested, removal of the humic compounds with a molecular weight of approximately 1,000 g/mol by using IEX led to less AOC formation and less bromate formation as compared to the removal of humic compounds with a molecular weight of 300–500 g/mol by using GAC.

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

A Multi-residue Method for the Analysis of Pesticides and Pesticide Degradates in Water Using HLB Solid-phase Extraction and Gas Chromatography–Ion Trap Mass Spectrometry

A method was developed for the analysis of over 60 pesticides and degradates in water by HLB solid-phase extraction and gas-chromatography/mass spectrometry. Method recoveries and detection limits were determined using two surface waters with different dissolved organic carbon (DOC) concentrations. In the lower DOC water, recoveries and detection limits were 80%-108% and 1-12 ng/L, respectively. In the higher DOC water, the detection limits were slightly higher (1-15 ng/L). Additionally, surface water samples from four sites were analyzed and 14 pesticides were detected with concentrations ranging from 4 to 1,200 ng/L.

Impact of coagulation and adsorption on DOC fractions of secondary effluent and resulting fouling behaviour in ultrafiltration

Membrane fouling by macromolecular dissolved organic compounds is still a fundamental drawback in low-pressure membrane filtration of secondary effluent. In this study, pre-treatment of secondary effluent by coagulation and/or adsorption was investigated in terms of removal of different dissolved organic carbon (DOC) fractions, especially macromolecular substances. DOC fractionation has been characterised by size exclusion chromatography. Adsorption tests using four commercially available activated carbons yielded a removal of small as well as larger organic compounds, revealing differences in the affinity towards macromolecules depending on the type of applied activated carbon. By contrast, coagulation removed predominantly larger molecules, i.e., biopolymers and humic substances. In terms of DOC reduction, the coagulant ferric chloride was superior to aluminium chloride. A combination of coagulation and adsorption resulted in the addition of individual removal efficiencies, suggesting that different fractions of organic compounds were involved in each of the processes. After removal of macromolecular organic compounds either by coagulation or by adsorption, a significant reduction of membrane fouling was observed in tests using two different types of ultrafiltration flat-sheet membranes in 20-h cross-flow filtration tests.

Contrasting uptake routes and tissue distributions of inorganic and methylmercury in mosquitofish (Gambusia affinis) and redear sunfish (Lepomis microlophus)

High Hg concentrations in freshwater fish are a concern for human health, yet we lack a clear understanding of the mechanisms that produce high Hg concentrations in fish. Controlled studies in natural surface waters that quantify the uptake and retention of Hg in fish tissues following exposures from the aqueous phase and from invertebrate prey diets are rare. Using 203Hg, we contrasted the accumulation of inorganic Hg (HgI) and methylmercury (MeHg) from the dissolved phase and from invertebrate food in mosquitofish (Gambusia affinis) feeding on Daphnia pulex (representing a pelagic food chain) and in redear sunfish (Lepomis microlophus) feeding on amphipods (Hyallela sp., representing a benthic/macrophyte-based chain). Experiments were conducted with environmentally realistic Hg concentrations in two freshwaters from the San Francisco Bay Delta (CA, USA) with significantly different dissolved organic carbon (DOC) concentrations. Mercury uptake rates following aqueous exposures were consistently higher for fish in the water with lower DOC, whereas efflux rates were similar for both water types. Approximately 50% of the ingested Hg, associated with invertebrate prey was lost from mosquitofish, and 90% or more from sunfish, within 48 h. Assimilation efficiencies for ingested MeHg for both fish were 86 to 94%, substantially higher than those for HgI regardless of water type. Biokinetic modeling using the parameters determined in these experiments accurately predicted Hg burdens for fish in the San Francisco Bay Delta system. Despite considerable accumulation of HgI from both aqueous and dietary exposure routes, the high assimilation efficiencies and slow loss of MeHg from dietary sources are the principal determinants of predicted Hg burdens in both fish species.

Environmental Constraints on Spawning Depth of Yellow Perch: The Roles of Low Temperature and High Solar Ultraviolet Radiation

AbstractThe roles of temperature and ultraviolet radiation (UVR) in determining the spawning success of yellow perch Perca flavescens were investigated in two Pennsylvania lakes with different dissolved organic carbon (DOC) concentrations. In situ incubation experiments were used to manipulate temperature and UVR and to examine hatching time and hatching success. Extensive scuba surveys were used to document actual spawning depths. Differences in the temperature and UVR profiles of the two lakes led to contrasting responses of incubated yellow perch eggs. Higher temperatures in the surface waters of the higher‐DOC lake led to hatching times that were 10–26 d shorter than those in the surface waters of the low‐DOC lake or in the deeper waters of the higher‐DOC lake. The high levels of UVR in the surface waters of the low‐DOC lake killed 100% of the eggs before hatching. Ultraviolet radiation had little effect on survival in the higher‐DOC lake or in deeper waters of the low‐DOC lake. Scuba surveys revealed that spawning in the low‐DOC lake occurred at greater depths than previously recognized. Ninety‐two percent of the eggs spawned in the low‐DOC lake were located at depths greater than 3 m, while 76% of eggs in the higher‐DOC lake were spawned in water less than 1 m deep. Temperature and UVR are both important in determining among‐lake differences in spawning depths of yellow perch. Yellow perch are able to spawn at shallow depths in higher‐DOC lakes, where warmer temperatures accelerate developmental rates and DOC blocks potentially damaging UVR. In low‐DOC lakes, yellow perch must spawn at greater depths to avoid UVR damage. Spawning at greater depths may be costly due to the substantially slower developmental rates at lower temperatures. Our data suggest that the conflicting selective pressures of UVR and temperature create an optimal spawning‐depth range for yellow perch that differs among lakes as a function of DOC concentration.

Atrazine uptake, elimination, and bioconcentration by periphyton communities and Daphnia magna: Effects of dissolved organic carbon

AbstractThe bioconcentration and toxicokinetics of atrazine in three different periphyton communities and in laboratory reared Daphnia magna were studied in natural and artificial waters with different dissolved organic carbon (DOC) concentrations and qualities. The exposure concentrations were similar to those short‐lived peak concentrations found in contaminated waters. Atrazine uptake and elimination were very fast, and the bioconcentration was low both in periphyton and D. magna. The bioconcentration factors in D. magna were approx. 16% of those in periphyton. The uptake and elimination rates were also higher in periphyton than in Daphnia. The periphyton properties affected the bioconcentration of atrazine more than the DOC concentration and water quality. A steady state was reached within a few hours of uptake by the periphyton and within 24 h by D. magna. A residue of atrazine was left in the periphyton and D. magna after an 8‐h biphasic depuration with clearly distinct fast and slow phases. In D. magna, atrazine may be eliminated during the normal life span of the organism. These results suggest that DOC has a minor influence on the toxicokinetics of atrazine.

Effect of dissolved organic matter on the uptake of trace metals by American oysters.

To examine the effects of dissolved organic matter on metal bioavailability, uptake of trace metals (Cd, Co, Hg, Cr, Ag, Zn) by American oysters (Crassostrea virginica) was compared between treatments with different dissolved organic carbon (DOC) concentrations and contrasting low molecular weight (LMW, 1 kDa) and high molecular weight (HMW, 1 kDa-0.2 micron) DOC fractions, using radiotracer techniques and short-term exposure experiments. Uptake rate constants (mL g-1 h-1) of metals, in general, increased with increasing DOC concentrations, with an initial decrease at lower DOC concentrations. Oyster dry weight concentration factors (DCF, mL g-1), determined at the end of exposure experiments (8 h), also increased for Cd, Co, Cr, Ag, and Zn, but decreased for Hg, with increasing DOC concentrations. Changes of metal uptake rate constants and DCF values with DOC concentration suggest that metal uptake pathways by American oysters vary from predominantly uptake (by diffusion of neutral) of free ionic, inorganically complexed, and LMW organic ligand complexed metals at very low DOC concentration to direct ingestion and digestion of HMW or colloidally complexed metals at higher DOC concentrations. Measured partition coefficients (Kc) between dissolved and colloidal phases were comparable between metals, ranging from 10(5.12) to 10(5.75) mL g-1. However, DCF values and uptake rate constants differed considerably between metals, with the highest DCF values and uptake rate constants found for B-type metals, e.g., Ag, Hg, Zn, and Cd, and the lowest ones for several intermediate-type metals (e.g., Co, Cr). Metal types and thus the interaction of metals with organic ligands, such as strong complexation of B-type metals with S-containing organic ligands, may play an important role in the bioavailability and toxicity of metals to aquatic organisms. Differences in metal uptake in contrasting LMW and HMW DOC treatments suggest a generally depressed bioavailability of colloidally complexed metals at low DOC concentration (0.5 ppm) but a generally enhanced uptake at higher DOC concentrations.