Organic Carbon In Surface Waters Research Articles

Controlling factors of latitudinal distribution of dissolved organic matter in the upper layers of the Indian Ocean

We studied chromophoric (CDOM) and fluorescent (FDOM) dissolved organic matter (DOM) and dissolved organic carbon in surface waters to determine the factors controlling the geographical distribution of DOM along two meridional transects in the Indian Ocean. For CDOM, we calculated the absorption coefficients, spectral slope, and absorption coefficient ratio from the observed absorption spectra. For FDOM, we calculated the biological (BIX) and humification (HIX) indices from the excitation emission matrices (EEMs); parallel factor analysis of the EEMs identified three fluorescent components, i.e., two humic-like and one protein-like. Using these DOM parameters, a factor analysis extracted fewer latent variables than the observed variables to account for the geographical distributions. We obtained three factors (F1, F2, and F3), which explained ~ 84% of the variance in the observed data. From the factor loadings, F1, F2, and F3 were interpreted as the effects of net primary production-derived DOM and its horizontal transport, photodegradation, and vertical transport by physical processes. We characterized seven marine biogeochemical provinces by factor scores. F1 scores gradually decreased from the northernmost to the Antarctic province, with a small maximum around the subtropical front. F2 scores were highest in the subtropical province and decreased in both the northward and southward directions. F3 scores were high in the Antarctic and northernmost provinces, and lowest in the subtropical province. Only BIX was insufficiently explained by these factors. BIX was highest in the northern part of the subtropical province, where photodegradation of DOM was the most intense. This suggests that the possible interaction between photodegradation, autochthonous production, and reworking by heterotrophic bacteria of DOM occurs in the subtropical province.

Comprehensive distribution characteristics and risks of heavy metals in typical intertidal zones and their relationship with urban economic indicators

Human activities have increased heavy metal pollution in intertidal environments. However, the comprehensive ecological risks of heavy metals in the active zones and their relationship with urban economic indicators are still poorly understood. This study investigated the multiphase distribution characteristics of heavy metals (Cr, Ni, Cu, Zn, Pb, and Cd) in 14 typical intertidal zones in the Guangdong–HongKong–Macao Greater Bay Area. The results showed that the concentrations of heavy metals varied greatly among different cities, and the highest average concentrations in Dongguan city were 695.27, 243.79, 1482.36, 980.24, 66.36 and 4.95 mg.kg−1, respectively. The proportion of bioavailable state Cd was the highest as 54.20%, followed by Zn > Cu > Pb > Ni > Cr. The comprehensive ecological risks (RCER) of the two intertidal zones in Dongguan were 1.45 ∼ 7.70 times higher than those of other sampling areas, indicating a very high ecological risk. Correlation analysis showed that total organic carbon in surface water, ammonia nitrogen in pore water, and solid organic carbon and the proportion of clay in sediments were significantly correlated with RCER values, and the linear regression coefficients were 0.784, 0.919, 0.962 and 0.914, respectively. The modified environmental Kuznets curve (EKC) model was used to describe the relationship between RCER values and urban economic indicators, and an inverted U shaped curve relationship was found, indicating that regional heavy metal pollution seemed to have reached a peak and entered a period of emission reduction. Nevertheless, given the increasing proximity of the intertidal zone to human daily life, long-term follow-up assessments are recommended.

Influence of reservoir management on the source and transport of particulate organic carbon in surface waters of the lower Yellow River

Influence of reservoir management on the source and transport of particulate organic carbon in surface waters of the lower Yellow River

Distribution of particulate and dissolved organic carbon in surface waters of northern scottish fjords

Coastal waters can act concurrently as a source and sink of carbon (C), with coastal sediments trapping and storing significant quantities of C, while C is simultaneously released to the atmosphere through biogeochemical processes in the water column. The mid-to high-latitude fjords are recognised as hotspots for the burial and storage of OC in their sediments and for their potential to provide a long-term climate regulation service. Yet the distribution of particulate (POC) and dissolved organic carbon (DOC) in fjord water columns remains a significant knowledge gap, that is particularly acute in temperate vegetated systems such as those in the North Atlantic. Here we present POC and DOC data from surface waters across four Scottish fjords with differing meteorological, catchment and submarine geomorphological characteristics to both quantify and understand the factors governing the spatial distribution of POC and DOC in fjords. The measured POC and DOC concentrations in the surface waters of the four fjords are broadly analogous to other temperate vegetated fjord systems around the world. Within these systems, local factors such as submarine geomorphology is the primary factor driving POC and DOC dispersal in fjord surface waters. The data highlights that fjords are important pathways for the transport and storage of POC and DOC in the coastal ocean and that a greater focus is required on the water column OC to allow the role fjords play in near- (water column) and long-term (sediment) climate regulation to be quantified.

The Sources and Burial of Marine Organic Carbon in the Eastern China Marginal Seas

The ecological functions and biogeochemical processes of continental marginal seas are important for the global carbon cycle. In the eastern China marginal seas (ECMS), phytoplankton productivity has increased significantly in recent decades, but the sources and burial processes of marine organic carbon (OC) remain under-studied. We analyzed the contents of marine lipid biomarkers (brassicasterol, dinosterol, and C37 alkenones) in surface suspended particles obtained from seven cruises between 2010 and 2015, and in surface sediments from eight cruises between 2006 and 2012 in the ECMS, to estimate marine OC sources and burial. The correlations between lipid biomarkers and environmental factors were quantified to reveal controlling factors. The study area was divided into four regions according to cluster analysis conducted based on sediment parameters. Our results showed that the concentrations of marine lipid biomarkers in surface water were high near large estuaries such as the Changjiang River Estuary and the Yellow River Estuary, but those in surface sediments were high in mud areas. Nutrient concentration was a key factor controlling phytoplankton biomass in surface water, with high nutrients enhancing the growth of diatoms and dinoflagellates, while haptophytes were more abundant in low-nutrient, high-salinity and cold environments. High marine OC contents calculated from total OC δ13C were mainly associated with fine sediments transported by coastal currents, and finally deposited in mud areas. The proto-burial efficiency of marine OC in the ECMS (7–19%) was markedly higher than the mean value in global marginal seas, with high values being located in the western coast of the ECMS. The proto-burial efficiency of total marine lipid biomarkers (6–24%) was quantified for the first time in our study, with overall values and spatial patterns similar to that of marine OC. A key parameter for marine OC and marine biomarker proto-burial efficiency was sediment grain size. The smaller the sediment grain size was, the better the marine OC and marine biomarker were retained. Our study provides an important basis to elucidate spatial distribution patterns and forcing mechanisms of marine OC in surface water (production process) and surface sediments (burial process), and to estimate carbon budgets in large marginal seas.

Ocean color algorithms to estimate the concentration of particulate organic carbon in surface waters of the global ocean in support of a long-term data record from multiple satellite missions

As the concentration of particulate organic carbon (POC) in the surface ocean plays a key role in marine biogeochemical cycles and ecosystems, its assessment from satellite observations of the global ocean is of significant interest. To achieve a global multi-decadal data record of POC by merging observations from multiple satellite ocean color missions, we formulated a new suite of empirical POC algorithms for several satellite sensors. For the algorithm development we assembled a field dataset of concurrent POC and remote-sensing reflectance, Rrs(λ), measurements collected in all major ocean basins encompassing tropical, subtropical, and temperate latitudes as well as the northern and southern polar latitudes. This dataset is characterized by a globally-representative probability distribution of POC with a broad range of values between about 10 and 1000 mg m−3. This development dataset was created with the use of additional inclusion and exclusion criteria based on well-assured and documented consistency of measurement protocols as well as specific bio-optical and particle characteristics of seawater which are consistent with vast areas of open-ocean pelagic environments.To formulate the algorithms the development dataset was subject to parametric regression analysis. Overall we evaluated over seventy formulas for estimating POC from Rrs(λ) using seven distinctly different algorithmic categories, each with a fundamentally different definition of independent variable involving Rrs(λ). Through the goodness-of-fit analysis, we selected the best candidate POC algorithms, referred to as the hybrid algorithms, which are tuned specifically for the spectral bands of SeaWiFS, MODIS, VIIRS, MERIS, and OLCI satellite sensors. These hybrid algorithms consist of two components, the MBR (Maximum Band Ratio)-OC4 cubic polynomial function and BRDI (Band Ratio Difference Index) quintic polynomial function. The MBR-OC4 uses four spectral bands and the BRDI three spectral bands from the blue-green spectral region. The MBR-OC4 algorithm is used for POC > 25 mg m−3 and the BRDI for POC < 15 mg m−3. In the transition region the weighting approach is applied to POC derived from the two algorithmic formulas. While the main role of the BRDI is to improve POC estimates in ultraoligotrophic waters where POC is very low, the MBR-OC4 provides improvements, compared with the predecessor algorithms, over a broader range of POC but especially for relatively high POC values. A preliminary analysis of field-satellite matchup datasets based on SeaWiFS and MODIS-Aqua observations shows improved performance of hybrid algorithms compared with current standard algorithms for both SeaWiFS and MODIS. In addition, a reasonable consistency is demonstrated between POC derived from hybrid algorithms applied to example satellite observations with SeaWiFS, MODIS-Aqua, and VIIRS-SNPP. The suite of newly developed algorithms provides the potential next generation version of global algorithms that better represents the spatial and temporal variability within a broader range of POC than the predecessor global algorithms, while also offering a capability to generate a long-term sensor-to-sensor consistent data record of POC that begins with the launch of SeaWiFS mission in 1997.

Seasonal Changes of Organic Carbon Mixing, Degradation and Deposition in Yangtze River Dominated Margin Related to Intrinsic Molecular and External Environmental Factors

AbstractThe East Asian monsoon is characterized by warm, humid summer and cool, dry winter that results in seasonal changes in fluvial sediment and organic carbon (OC) discharge. Here, we report measurements of elemental, isotopic, and optical compositions of OC in surface waters and sediments that were collected along the lower Yangtze River, Estuary, and East China Sea continuum in March and July 2019. The principal component analysis separates samples into three groups that correspond to fresh water area, mixing area, and seawater area. The terrigenous signal in the mixing area, indicated by water salinity and δ13C, is stronger in July. The applications of binary mixing models based on water salinity, dissolved OC (DOC) concentration and specific terrigenous fluorescence component suggest: (a) net removal of DOC from waters in July, but net addition of DOC into waters in March, attributed to more extensive degradation and larger deposition of OC in summer; and (b) more conservative behaviors of terrigenous OC compared to marine or mixed OC, suggesting an important role of intrinsic molecular characteristics on the persistence of DOC. The estimated monthly OC burial flux in the estuary and continent shelf is 0.48 × 1012 g in July and 0.081 × 1012 g of OC in March, equal to 15.5% and 2.6% of the annual OC discharge by the Yangtze River, respectively. Our study suggests that the burial of OC mainly occurs in the summer monsoon due to the larger sediment load of the Yangtze River, prevailing southeastern wind, and intensified Kuroshio Current.

Spatial distribution of epifaunal communities in the Hudson Bay system

The seasonal sea ice cover and the massive influx of river runoff into the Hudson Bay System (HBS) of the Canadian Arctic are critical factors influencing biological production and, ultimately, the dynamics and structure of benthic communities in the region. This study provides the most recent survey of epibenthic communities in Hudson Bay and Hudson Strait and explores their relationships with environmental variables, including mean annual primary production and particulate organic carbon in surface water, bottom oceanographic variables, and substrate type. Epibenthic trawl samples were collected at 46 stations, with a total of 380 epibenthic taxa identified, representing 71% of the estimated taxa within the system. Three communities were defined based on biomass and taxonomic composition. Ordination analyses showed them to be associated primarily with substrate type, salinity, and annual primary production. A first community, associated with coarse substrate, was distributed along the coastlines and near the river mouths. This community was characterized by the lowest density and taxonomic richness and the highest biomass of filter and suspension feeders. A second community, composed mostly of deposit feeders and small abundant epibenthic organisms, was associated with soft substrate and distributed in the deepest waters. A third community, associated with mixed substrate and mostly located near polynyas, was characterized by high diversity and biomass, with no clearly dominant taxon. The overall analysis indicated that bottom salinity and surface-water particulate organic carbon content were the main environmental drivers of these epibenthic community patterns. In the face of climate change, projections of increased river inflow and a longer open water season for the HBS could have major impacts on these epibenthic communities, emphasizing a need to continually improve our ability to evaluate and predict shifts in epibenthic richness and distribution.

Ecological aesthetic assessment of a rebuilt wetland restored from farmland and management implications for National Wetland Parks.

While wetlands are usually used as a natural approach to remove biodegradable pollutants in surface water, their purification efficiencies coupled with their aesthetic features are of less concern. The water quality, plant landscape, acoustic environment and odour indicators were investigated in the surface water inlet and outlet of the Fujin National Wetland Park (FNWP), restored from farmlands in Northeast China. Major concentrations of pollutants in the inlet and the outlet subjected to surface flow wetland treatment were monitored, and the removal efficiencies were calculated based on 54 water samples (6 sites×3 seasons×3 replicates). The results showed that the total nitrogen (TN) and organic carbon in surface water decreased significantly after the wetland treatment, while the total phosphorus (TP) did not decrease significantly. The removal efficiencies for TN and BOD5 changed seasonally and reached 69.08% and 60.44%, respectively. An ecological aesthetic index (EAI) was developed based on the trophic state index coupled with plant landscape, acoustic and odour indicators, and the calculated EAI showed that the outlet delivered a more aesthetically harmonious appearance than the inlet in spring and autumn, but not in summer. Based on the current aquatic macrophyte species and documented purification efficiencies in FNWP, we recommend an improved ecological aesthetic management approach that utilizes and arranges diverse native plants from the surrounding wetlands (e.g. Scirpus validus) in addition to local Nelumbo nucifera, Nymphaea tetragona and Myriophyllum spicatum, and conserves the indicative and endangered species (Aldrovanda vesiculosa), from the visual appeal of the waterscape.

Towards an assessment of riverine dissolved organic carbon in surface waters of the western Arctic Ocean based on remote sensing and biogeochemical modeling

Abstract. Future climate warming of the Arctic could potentially enhance the load of terrigenous dissolved organic carbon (tDOC) of Arctic rivers due to increased carbon mobilization within watersheds. A greater flux of tDOC might impact the biogeochemical processes of the coastal Arctic Ocean (AO) and ultimately its capacity to absorb atmospheric CO2. In this study, we show that sea-surface tDOC concentrations simulated by a physical–biogeochemical coupled model in the Canadian Beaufort Sea for 2003–2011 compare favorably with estimates retrieved by satellite imagery. Our results suggest that, over spring–summer, tDOC of riverine origin contributes to 35 % of primary production and that an equivalent of ∼ 10 % of tDOC is exported westwards with the potential of fueling the biological production of the eastern Alaskan nearshore waters. The combination of model and satellite data provides promising results to extend this work to the entire AO so as to quantify, in conjunction with in situ data, the expected changes in tDOC fluxes and their potential impact on the AO biogeochemistry at basin scale.

High total organic carbon in surface waters of the northern Arabian Gulf: Implications for the oxygen minimum zone of the Arabian Sea

Measurements of total organic carbon (TOC) for two years in Kuwaiti waters showed high TOC levels (101.0–318.4, mean 161.2 μM) with maximal concentrations occurring within the polluted Kuwait Bay and decreasing offshore, indicating substantial anthropogenic component. Analysis of winter-time data revealed a large increase in density over the past four decades due to decrease in Shatt Al-Arab runoff, implying that the dissolved/suspended organic matter in surface waters of the northern Gulf could be quickly injected into the Gulf Deep Water (GDW). Our measurements together with an analysis of previously collected/published data suggest that the recent summer-time declining trend in oxygen in the GDW might be related to eutrophication. Higher preformed TOC and lower preformed dissolved oxygen contents of the high-salinity water mass that flows out of the Gulf and ventilates the mesopelagic oxygen minimum zone (OMZ) of the Northwestern Indian Ocean may cause expansion/intensification of the regional OMZ.

Trends of dissolved organic carbon in surface water treated by innate coagulants

Trends of dissolved organic carbon in surface water treated by innate coagulants

The conservative behavior of dissolved organic carbon in surface waters of the southern Chukchi Sea, Arctic Ocean, during early summer.

The spatial distribution of dissolved organic carbon (DOC) concentrations and the optical properties of dissolved organic matter (DOM) determined by ultraviolet-visible absorbance and fluorescence spectroscopy were measured in surface waters of the southern Chukchi Sea, western Arctic Ocean, during the early summer of 2013. Neither the DOC concentration nor the optical parameters of the DOM correlated with salinity. Principal component analysis using the DOM optical parameters clearly separated the DOM sources. A significant linear relationship was evident between the DOC and the principal component score for specific water masses, indicating that a high DOC level was related to a terrigenous source, whereas a low DOC level was related to a marine source. Relationships between the DOC and the principal component scores of the surface waters of the southern Chukchi Sea implied that the major factor controlling the distribution of DOC concentrations was the mixing of plural water masses rather than local production and degradation.

Spatial patterns and environmental controls of particulate organic carbon in surface waters in the conterminous United States

Carbon cycling in inland waters has been identified as an important, but poorly constrained component of the global carbon cycle. In this study, we compile and analyze particulate organic carbon (POC) concentration data from 1145U.S. Geological Survey (USGS) gauge stations to investigate the spatial variability and environmental controls of POC concentration. We observe substantial spatial variability in POC concentration (1.43±2.56mgC/L, mean±one standard deviation), with the Upper Mississippi River basin and the Piedmont region in the eastern U.S. having the highest POC concentration. Further, we employ generalized linear models (GLMs) to analyze the impacts of sediment transport and algae growth as well as twenty-one other environmental factors on the POC variability. Suspended sediment and chlorophyll-a explain 26% and 17% of the variability in POC concentration, respectively. At the national level, the twenty-one environmental factors combined can explain ca. 40% of the spatial variance in POC concentration. At the national scale, urban area and soil clay content show significant negative correlations with POC concentration, whereas soil water content and soil bulk density correlate positively with POC. In addition, total phosphorus concentration and dam density correlate positively with POC concentration. Furthermore, regional scale analyses reveal substantial variation in environmental controls of POC concentration across eighteen major water resource regions in the U.S. The POC concentration and associated environmental controls also vary non-monotonically from headwaters to large rivers. These findings indicate complex interactions among multiple factors in regulating POC concentration over different spatial scales and across various sections of the river networks. This complexity, together with the large unexplained uncertainty, highlights the need for considering non-linear interplays of multiple environmental factors and developing appropriate methodologies to track the transformation and transport of POC along the terrestrial-aquatic interfaces.

Phytoplankton Dynamics and Its Further Implication for Particulate Organic Carbon in Surface Waters of a Tropical/Subtropical Estuary

Phytoplankton succession along with salinity in estuaries not only influences the riverine ecosystem but also interferes our understanding of riverine materials (e.g., organic carbon) transport to the sea. Four cruises were conducted in a mid-size river and its estuary (i.e., the Wanquan River) to elucidate the riverine phytoplankton decease along increasing salinity and to quantify the amount of algal particulate organic carbon (POC) present in the system at different seasons. CHEMTAX calculations suggested that chlorophytes were the main chlorophyll a (CHLa) contributor in the dissolved inorganic phosphorus-limited river, which contributed over 60 % of the total CHLa. Microscopy further revealed that the dominant species was Scenedesmus sp. In the estuary, phytoplankton succession along with salinity in the estuary was observed. Chlorophyte contribution to total CHLa dramatically decreased from over 60 % in the river (S = 0) segment to 20, suggesting the removal of riverine algae in the estuary. The mean algal POC concentration ranged from 80 μg/L (summer) to 140 μg/L (winter), and the riverine algal POC accounted for 6–56 % of the bulk riverine POC. The annual flux of riverine algal POC was estimated to be 660 tons.

Tracing groundwater geochemistry using δ13C on San Salvador Island (southeastern Bahamas): implications for carbonate island hydrogeology and dissolution

Mixing dissolution is a widely accepted process of karstification on carbonate platforms, but regional differences in climate and geology indicate that a universal application of this model is insufficient to assess water–limestone interactions in more specific island settings. A two-phase study investigating δ13C, carbon concentration, and other geochemical parameters took place on San Salvador Island, The Bahamas, to better understand its hydrologic characteristics and identify local controls on dissolution. In the initial phase, Crescent Pond and adjacent Crescent Top Cave, both with conduit connections to one another and to open marine water, were monitored over 1.5 normal tidal cycles and found to have little geochemical variation. Contrasting geochemical compositions between these two sites and the ocean illustrates the complexity of subsurface hydrology, while lower pH and δ13CDIC values in the cave suggest the potential for bacterially mediated dissolution. The second phase included a more comprehensive geochemical survey of 12 of the island’s surface/subsurface water bodies, and found that water geochemistry was governed primarily by connectivity to the ocean and secondarily by topographic and vegetative settings. Geochemical relationships illustrated by regression analyses showed that biologic activity exerted additional controls over water geochemistry, with photosynthesis removing biotically respired CO2 and elevating organic carbon in surface waters, while biotically respired CO2 accumulates and supports dissolution in the subsurface. These data underscore the importance of including the role of biotic processes with climate and geologic settings when identifying dissolution mechanisms and using them to estimate modern and historical dissolution processes.

Dissolved organic carbon and bacterial populations in the gelatinous surface microlayer of a Norwegian fjord mesocosm

The sea surface microlayer is the interfacial boundary layer between the marine environment and the troposphere. Surface microlayer samples were collected during a fjord mesocosm experiment to study microbial assemblage dynamics within the surface microlayer during a phytoplankton bloom. Transparent exopolymer particles were significantly enriched in the microlayer samples, supporting the concept of a gelatinous surface film. Dissolved organic carbon and bacterial cell numbers (determined by flow cytometry) were weakly enriched in the microlayer samples. However, the numbers of Bacteria 16S rRNA genes (determined by quantitative real-time PCR) were more variable, probably due to variable numbers of bacterial cells attached to particles. The enrichment of transparent exopolymer particles in the microlayer and the subsequent production of a gelatinous biofilm have implications on air-sea gas transfer and the partitioning of organic carbon in surface waters.

Biogeochemical Records at Shangsi Section, Northeast Sichuan in China: The Permian Paleoproductivity Proxies

The marine primary producers assimilate the atmospheric CO2 to form the organic carbon in surface water. The organic carbon then settles down through the water column and is removed from the oceans by final preservation in sediments in the form of petroleum or nature gases. The reconstruction of paleoproductivity will thus improve our understanding of the biological processes in the formation of fossil energy resource and help to locate new sites for future exploration. In this study, biorelated elements P, Cd, Al, Ba, as well as redox sensitive element Mo, were analyzed in the 448 rock samples collected from Permian strata at the Shangsi Section, Guangyuan, Northeast Sichuan in China. On the basis of the Ti content, the nondetrital contents of P, Ca, and Al, denoted as Pxs, Cdxs, and Alxs, were calculated and found to coincide with the TOC content throughout the whole section, with some enrichment intervals being found in the middle part of Chihsia Formation, topmost Maokou Formation, and Dalong Formation. This suggests that the biorelated elements could be used as proxies for the paleoproductivity here in this section. Baxs, a paleoproductivity indicator widely used in the paleoceanography, shows insignificant correlation with TOC, Pxs, Cdxs, and Alxs, probably arising from the loss of biological barium in anoxic conditions. Compiled with the data of TOC content and Pxs, Cdxs, and Alxs, three episodes of enhanced paleoproductivity were identified in Permian strata including the middle part of Chihsia Formation, topmost Maokou Formation, and Dalong Formation.

Accumulation and Export of Dissolved Organic Carbon in Surface Waters of Subtropical and Tropical Pacific Ocean

Dissolved organic carbon (DOC) concentrations in surface waters of the Pacific Ocean during October–November, 1995, were determined using a high-temperature combustion method. The DOC in the surface mixed-layer was approximately homogeneous with a concentration between 55 and 89 µmol C l−1. This homogeneity indicates that there is a strong control of the vertical distribution of DOC by mixing processes. The DOC concentrations in the mixed-layer in the subtropical region were up to 27 µmol C l−1 higher than in the tropical region. This difference reflects the subtropical accumulation and the tropical export of DOC. There is a significant positive correlation between DOC and chlorophyll a concentrations in the mixed-layer of the North Pacific subtropical region, suggesting that phytoplankton is the primary source of DOC accumulated in this region. Calculations using simple box models suggest that DOC export in the tropical region (0–50 m depth, 10°N-10°S, along 160°W) occurs primarily by poleward advection at a rate of 0.5–3 mmol C m−2day−1. A comparison with estimates of the export rate of particulate organic carbon published in previous studies leads us to conclude that DOC export may contribute less to the carbon budget in the tropical region than has recently been supposed.

Concentrations and characteristics of organic carbon in surface water in Arizona: influence of urbanization

Dissolved (DOC) and total (TOC) organic carbon concentrations and compositions were studied for several river systems in Arizona, USA. DOC composition was characterized by ultraviolet and visible absorption and fluorescence emission (excitation wavelength of 370 nm) spectra characteristics. Ephemeral sites had the highest DOC concentrations, and unregulated perennial sites had lower concentrations than unregulated intermittent sites, regulated sites, and sites downstream from wastewater-treatment plants ( p<0.05). Reservoir outflows and wastewater-treatment plant effluent were higher in DOC concentration ( p<0.05) and exhibited less variability in concentration than inflows to the reservoirs. Specific ultraviolet absorbance values at 254 nm were typically less than 2 m −1(milligram DOC per liter) −1 and lower than values found in most temperate-region rivers, but specific ultraviolet absorbance values increased during runoff events. Fluorescence measurements indicated that DOC in desert streams typically exhibit characteristics of autochthonous sources; however, DOC in unregulated upland rivers and desert streams experienced sudden shifts from autochthonous to allochthonous sources during runoff events. The urban water system (reservoir systems and wastewater-treatment plants) was found to affect temporal variability in DOC concentration and composition.