Christopher Osburn | North Carolina State University (original) (raw)

Papers by Christopher Osburn

Research paper thumbnail of Seasonal variation in the quality of dissolved and particulate organic matter exchanged between a salt marsh and its adjacent estuary

Fluorescence was used to examine the quality of dissolved and particulate organic matter (DOM and... more Fluorescence was used to examine the quality of dissolved and particulate organic matter (DOM and POM) exchanging between a tidal creek in a created salt marsh and its adjacent estuary in eastern North Carolina, USA. Samples from the creek were collected hourly over four tidal cycles in May, July, August, and October 2011. Absorbance and fluorescence of chromophoric DOM (CDOM) and of base-extracted POM (BEPOM) served as the tracers for organic matter quality while dissolved organic carbon (DOC) and base-extracted particulate organic carbon (BEPOC) were used to compute fluxes. Fluorescence was modeled using parallel factor analysis (PARAFAC) and principle components analysis (PCA) of the PARAFAC results. Of nine PARAFAC components (C) modeled, C3 represented recalcitrant DOM and C4 represented fresher soil-derived source DOM. Component 1 represented detrital POM, and C6 represented planktonic POM. Based on mass balance, recalcitrant DOC export was 86 g C m À2 yr À1 and labile DOC export was 49 g C m À2 yr À1 ; no planktonic DOC was exported. The marsh also exported 41 g C m À2 yr À1 of detrital terrestrial POC, which likely originated from lands adjacent to the North River estuary. Planktonic POC export from the marsh was 6 g C m À2 yr À1. Assuming the exported organic matter was oxidized to CO 2 and scaled up to global salt marsh area, respiration of salt marsh DOC and POC transported to estuaries could amount to a global CO 2 flux of 11 Tg C yr À1 , roughly 4% of the recently estimated CO 2 release for marshes and estuaries globally.

Research paper thumbnail of Optical Proxies for Terrestrial Dissolved Organic Matter in Estuaries and Coastal Waters

Dissolved organic matter (DOM) absorbance and fluorescence were used as optical proxies to track ... more Dissolved organic matter (DOM) absorbance and fluorescence were used as optical proxies to track terrestrial DOM fluxes through estuaries and coastal waters by comparing models developed for several coastal ecosystems. Key to using these optical properties is validating and calibrating them with chemical measurements, such as lignin-derived phenols—a proxy to quantify terrestrial DOM. Utilizing parallel factor analysis (PARAFAC), and comparing models statistically using the OpenFluor database (http://www.openfluor.org) we have found common, ubiquitous fluorescing components which correlate most strongly with lignin phenol concentrations in several estuarine and coastal environments. Optical proxies for lignin were computed for the following regions: Mackenzie River Estuary, Atchafalaya River Estuary (ARE), Charleston Harbor, Chesapeake Bay, and Neuse River Estuary (NRE) (all in North America). The slope of linear regression models relating CDOM absorption at 350 nm (a 350) to DOC and to lignin, varied 5–10-fold among systems. Where seasonal observations were available from a region, there were distinct seasonal differences in equation parameters for these optical proxies. The variability appeared to be due primarily to river flow into these estuaries and secondarily to biogeochemical cycling of DOM within them. Despite the variability, overall models using single linear regression were developed that related dissolved organic carbon (DOC) concentration to CDOM (DOC 2 = 40 ± 2 × a + ± 350 138 16; R = 0.77; N = 130) and lignin (8) to CDOM (= 2.03 ± 0.07 × a .47 2 − 0 ± = 8 350 0.59; R 0.87; N = 130). This wide variability suggested that local or regional optical models should be developed for predicting terrestrial DOM flux into coastal oceans and taken into account when upscaling to remote sensing observations and calibrations.

Research paper thumbnail of The use of PARAFAC components in studying headwater catchments: a stream and soil study

Research paper thumbnail of Photobleaching of Dissolved Organic Material from a Tidal Marsh-Estuarine System of the Chesapeake Bay†

Photochemistry and Photobiology, 2007

Wetlands and tidal marshes in the Rhode River estuary of the Chesapeake Bay act as important sour... more Wetlands and tidal marshes in the Rhode River estuary of the Chesapeake Bay act as important sources of dissolved organic carbon and strongly absorbing dissolved organic matter (DOM) for adjacent estuarine waters. The effects of solar exposure on the photochemical degradation of colored DOM (CDOM) were examined for material derived from different sources (estuarine and freshwater parts of the Rhode River, sub-watershed stream, marshes) in this estuarine ecosystem. Consistent with changes in fluorescence emission, absorption loss upon exposure to different portions of the solar spectrum (i.e. different long-pass cut-off filters) occurred across the entire spectrum but the wavelength of maximum photobleaching decreased as the cut-off wavelength of the filter decreased. Our results illustrate that solar exposure can cause either an increase or a decrease in the CDOM absorption spectral slope, S CDOM , depending on the spectral quality of irradiation and, thus, on the parameters (e.g. atmospheric composition, concentration of UV-absorbing water constituents) that affect the spectral characteristics of the light to which CDOM is exposed. We derived a simple spectral model for describing the effects of solar exposure on CDOM optical quality. The model accurately, and consistently, predicted the observed dependence of CDOM photobleaching on the spectral quality of solar exposure.

Research paper thumbnail of Photoreactivity of chromophoric dissolved organic matter transported by the Mackenzie River to the Beaufort Sea

Marine Chemistry, 2009

The photoreactivity of chromophoric dissolved organic matter (CDOM) transported to Arctic shelf e... more The photoreactivity of chromophoric dissolved organic matter (CDOM) transported to Arctic shelf environments by rivers has only recently been studied and its quantitative role in Arctic shelf biogeochemistry has received little attention. Sunlight exposure experiments were performed on CDOM collected over a three year period (2002 to 2004) from river, estuary, shelf, and gulf regions of the Western Canadian Arctic. Decreases in CDOM absorption, synchronous fluorescence (SF), and dissolved organic carbon (DOC) concentration were followed after 3 days of exposure, and in two experiments, six optical cutoff filters were used to incrementally remove ultraviolet radiation incident on the samples. Apparent quantum yields for CDOM photobleaching (AQY ble ) and for DOC photomineralization (AQY min ) were computed, as were two AQY spectra (ϕ ble and ϕ min ) for the Mackenzie River and a sample from the Mackenzie Shelf. The photoreactivity of Mackenzie River CDOM was highest after break-up and peak discharge and lowest in late summer. The half-lives of CDOM and DOC were estimated at 3.7 days and 4.8 days, respectively, when Mackenzie River water was exposed to full sunlight. Photobleaching of Mackenzie River CDOM fluorescence after most UV-B wavelengths were removed increased the correlation between the river and offshore waters in the Beaufort Sea. When light attenuation from particle-and CDOMrich river water was considered for the Mackenzie Shelf, our photodegradation models estimated around 10% loss of absorption and b 1% DOC loss, suggesting that sunlight exposure does not substantially degrade CDOM on Arctic shelves.

Research paper thumbnail of Calculation of spectral weighting functions for the solar photobleaching of chromophoric dissolved organic matter in temperate lakes

Limnology and Oceanography, 2001

The effect of solar radiation on the dissolved absorption coefficient (a CDOM []), which reflects... more The effect of solar radiation on the dissolved absorption coefficient (a CDOM []), which reflects the concentration of chromophoric dissolved organic matter (CDOM), was investigated in several lakes near Bariloche, Argentina and in northeastern Pennsylvania, USA. Samples of 0.2 m filtered lake water were exposed in quartz tubes to different portions of the solar spectrum using optical cutoff filters to remove parts of the ultraviolet (UV) region of the solar spectrum. Changes in the spectral absorption in these samples and the absorbed incident energy were used to calculate spectral weighting functions (SWFs) for the photobleaching (PB) of CDOM. PB was measured as the loss of a CDOM () (the a CDOM [] was averaged from 280 to 500 nm) per unit absorbed energy. CDOM from humic and clear lakes, as well as from a Sphagnum bog and an algal culture, was used in the experiments covering a wide range of carbon sources. We used an iterative, nonlinear optimization method to fit the measured results to a simple exponential function in order to generate each SWF. Comparing individual SWFs calculated for various CDOM sources, we computed a summary SWF from the experiments using epilimnial CDOM from our study lakes. Our summary SWF was able to explain 80-90% of the observed variance in our exposure experiments, and we were able to predict PB results obtained for other Argentine lakes (mean error 14.5%). Finally, we calculated that the effect of UV-B radiation on PB was small (Ͻ20% of total decrease in the absorption coefficient) compared to UV-A and blue light radiation. This suggested that increased UV-B radiation due to stratospheric ozone depletion would not greatly increase the photobleaching of whole water column CDOM in Patagonian lakes (Ͻ10%).

Research paper thumbnail of Bacterial production and microbial food web structure in a large arctic river and the coastal Arctic Ocean

Journal of Marine Systems, 2008

Globally significant quantities of organic carbon are stored in northern permafrost soils, but li... more Globally significant quantities of organic carbon are stored in northern permafrost soils, but little is known about how this carbon is processed by microbial communities once it enters rivers and is transported to the coastal Arctic Ocean. As part of the Arctic River-Delta Experiment (ARDEX), we measured environmental and microbiological variables along a 300 km transect in the Mackenzie River and coastal Beaufort Sea, in July-August 2004. Surface bacterial concentrations averaged 6.7 × 10 5 cells mL − 1 with no significant differences between sampling zones. Picocyanobacteria were abundant in the river, and mostly observed as cell colonies. Their concentrations in the surface waters decreased across the salinity gradient, dropping from 51,000 (river) to 30 (sea) cells mL − 1 . There were accompanying shifts in protist community structure, from diatoms, cryptophytes, heterotrophic protists and chrysophytes in the river, to dinoflagellates, prymnesiophytes, chrysophytes, prasinophytes, diatoms and heterotrophic protists in the Beaufort Sea.

Research paper thumbnail of Comparison of the optical properties of dissolved organic matter in two river-influenced coastal regions of the Canadian Arctic

Estuarine, Coastal and Shelf Science, 2007

The optical characteristics of coloured dissolved organic matter (CDOM) were analyzed in the Grea... more The optical characteristics of coloured dissolved organic matter (CDOM) were analyzed in the Great Whale River and adjacent Hudson Bay (55 N, 77 W) in the eastern Canadian Low Arctic, and in the Mackenzie River and adjacent Beaufort Sea in the western Canadian High Arctic (70 N, 133 W). Sampling was during ice-free open water conditions. Both rivers contained high concentrations of dissolved organic carbon (3 and 6 mg DOC l À1 in the Great Whale River and Mackenzie River, respectively) and CDOM (a 320 of 11 and 14 m À1 ), resulting in a substantial load of organic matter to their coastal seas. There were pronounced differences in the CDOM characteristics of the two rivers, notably in their synchronous fluorescence scans (SFS). The latter showed that the Mackenzie River was depleted in humic materials, implying a more mature catchment relative to the younger, more recently glaciated Great Whale River system. SFS spectra had a similar shape across the freshwatere saltwater transition zone of the Great Whale plume, and DOC was linearly related to salinity implying conservative mixing and no loss by flocculation or biological processes across the salt front. In contrast, there were major differences in SFS spectral shape from the Mackenzie River to the freshwater-influenced coastal ocean, with a marked decrease in the relative importance of fulvic and humic acid materials. The SFS spectra for the coastal Beaufort Sea in SeptembereOctober strongly resembled those recorded for the Mackenzie River during the high discharge, CDOM-rich, snowmelt period in June, but with some loss of autochthonous materials. These results are consistent with differences in freshwater residence time between the Mackenzie River and Great Whale River coastal ocean systems. Models of arctic continental shelf responses to present and future climate regimes will need to consider these striking regional differences in the organic matter content, biogeochemistry and optics between waters from different catchments and different inshore hydrodynamic regimes.

Research paper thumbnail of Effects of Solar Radiation on the Optical Quality of Dissolved Organic Material in a Tidal Marsh Estuarine Ecosystem

Photochemical alteration of colored dissolved organic matter (CDOM) during exposure to solar UV a... more Photochemical alteration of colored dissolved organic matter (CDOM) during exposure to solar UV and Visible radiation generates a variety of photoproducts, including reactive oxygen species, atmospherically important trace gases, and microbially labile carbonyl compounds. Sunlight-induced changes in CDOM chemical structure are reflected in changes of its optical properties that provide a first order measure of the photoreactivity of CDOM in

Research paper thumbnail of Chemical Analysis of CDOM Sources in Danish Coastal Waters of the Baltic Sea-North Sea Mixing Zone

... Data synthesis and analysis is 80% completed. ... optical algorithms derived for river-domina... more ... Data synthesis and analysis is 80% completed. ... optical algorithms derived for river-dominated margins will not likely operate in complex coastal environments such as those found in Danish coastal waters. ... ASLO/AGU Ocean Sciences Meeting 2008, Orlando, Florida, USA. ...

Research paper thumbnail of Dissolved and particulate organic matter fluorescence in coastal river systems in relation to land use and carbon transfer

Research paper thumbnail of Photochemical changes in the dissolved organic matter of temperate lakes: implications for organic carbon cycling and lake transparency

Research paper thumbnail of Bacterial Metabolism, Aromatic Biodegradation, and Lignin Biogeochemistry in Sediment Cores from Pearl Harbor, Hawaii

Approved for public release; distribution is unlimited. ... Chemical Dynamics and Diagnostics Bra... more Approved for public release; distribution is unlimited. ... Chemical Dynamics and Diagnostics Branch Chemistry Division ... Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std. Z39.18 ... Public reporting burden for this collection of information is estimated to average 1 hour ...

Research paper thumbnail of HE ZAGARESE, DP MORRIS, BR HARGREAVES, AND WE CRAVERO. 2001. Calculation of spectral weighting functions for the solar photobleaching of chromophoric dissolved organic matter in temperate lakes

Research paper thumbnail of Chemical and optical changes in freshwater dissolved organic matter exposed to solar radiation

We studied the chemical and optical changes in the dissolved organic matter (DOM) from two freshw... more We studied the chemical and optical changes in the dissolved organic matter (DOM) from two freshwater lakes and a Sphagnum bog after exposure to solar radiation. Stable carbon isotopes and solid-state 13 C-NMR spectra of DOM were used together with optical and chemical data to interpret results from experimental exposures of DOM to sunlight and from seasonal observations of two lakes in northeastern Pennsylvania. Solar photochemical oxidation of humic-rich bog DOM to smaller LMW compounds and to DIC was inferred from losses of UV absorbance, optical indices of molecular weight and changes in DOM chemistry. Experimentally, we observed a 1.2‰ enrichment in δ 13 C and a 47% loss in aromatic C functionality in bog DOM samples exposed to solar UVR. Similar results were observed in the surface waters of both lakes. In late summer hypolimnetic water in humic Lake Lacawac, we observed 3 to 4.5‰ enrichments in δ 13 C and a 30% increase in aromatic C relative to early spring values during spring mixing. These changes coincided with increases in molecular weight and UV absorbance. Anaerobic conditions of the hypolimnion in Lake Lacawac suggest that microbial metabolism may be turning over allochthonous C introduced during spring mixing, as well as autochthonous C. This metabolic activity produces HMW DOM during the summer, which is photochemically labile and isotopically distinct from allochthonous DOM or autochthonous DOM. These results suggest both photooxidation of allochthonous DOM in the epilimnion and autotrophic production of DOM by bacteria in the hypolimnion cause seasonal trends in the UV absorbance of lakes.

Research paper thumbnail of The record of global change in mid-Cretaceous (Barremian-Albian) sections from the Sierra Madre, northeastern Mexico

Our current understanding of mid-Cretaceous global change is largely based on investigations of p... more Our current understanding of mid-Cretaceous global change is largely based on investigations of pelagic sections from southern Europe and deep sea drilling sites. Much less information exists from other continents and from hemipelagic sections deposited on continental margins. This investigation seeks to broaden our understanding of mid-Cretaceous global change by focusing on the record from hemipelagic sections deposited along the continental margin of northeastern Mexico. The major goals are to compare the record, timing, and extent of the Oceanic Anoxic Events (OAEs) in Mexico and other areas, and to determine the relationship between these events and the global burial of organic material using carbon isotopes.

Research paper thumbnail of Evaluating PAH Biodegradation Relative to Total Bacterial Carbon Demand in Coastal Ecosystems: Are PAHs Truly Recalcitrant?

Various techniques have been used to evaluate microbial metabolic activity in natural environment... more Various techniques have been used to evaluate microbial metabolic activity in natural environments. In recent years, tracer additions of radiolabeled substrates coupled with short term, environmentally-relevant incubation experiments have become standard for assessing overall microbial growth rates as well as utilization rates for specific components of the bioavailable carbon pool. Using these techniques, microbial activities have been determined over a wide range of spatial and temporal scales in a variety of estuarine settings, providing valuable insight into carbon cycling in these dynamic systems. In this paper, we focus on patterns of microbial carbon consumption in estuarine sediments and relate these to the utilization of polyaromatic hydrocarbons (PAHs) as a recalcitrant, yet microbially available component of the carbon pool. Drawing on extensive field studies and those published in the literature, we relate microbial metabolic activities to the utilization of specific carbon species, discuss the partitioning of carbon between bacterial production and respiration (utilization efficiency), address the substrate-activity relationship for utilization of recalcitrant organic carbon and identify seasonal effects on the utilization of natural and anthropogenic carbon pools. These studies elucidate the advantages, disadvantages and caveats of relating microbial consumption of bulk labile organic carbon to the utilization of specific components and improve our ability to predict the fate of organic contaminants in estuarine systems.

Research paper thumbnail of Seasonal variation in the quality of dissolved and particulate organic matter exchanged between a salt marsh and its adjacent estuary

Journal of Geophysical Research: Biogeosciences, 2015

Fluorescence was used to examine the quality of dissolved and particulate organic matter (DOM and... more Fluorescence was used to examine the quality of dissolved and particulate organic matter (DOM and POM) exchanging between a tidal creek in a created salt marsh and its adjacent estuary in eastern North Carolina, USA. Samples from the creek were collected hourly over four tidal cycles in May, July, August, and October 2011. Absorbance and fluorescence of chromophoric DOM (CDOM) and of base-extracted POM (BEPOM) served as the tracers for organic matter quality while dissolved organic carbon (DOC) and base-extracted particulate organic carbon (BEPOC) were used to compute fluxes. Fluorescence was modeled using parallel factor analysis (PARAFAC) and principle components analysis (PCA) of the PARAFAC results. Of nine PARAFAC components (C) modeled, C3 represented recalcitrant DOM and C4 represented fresher soil-derived source DOM. Component 1 represented detrital POM, and C6 represented planktonic POM. Based on mass balance, recalcitrant DOC export was 86 g C m À2 yr À1 and labile DOC export was 49 g C m À2 yr À1 ; no planktonic DOC was exported. The marsh also exported 41 g C m À2 yr À1 of detrital terrestrial POC, which likely originated from lands adjacent to the North River estuary. Planktonic POC export from the marsh was 6 g C m À2 yr À1 . Assuming the exported organic matter was oxidized to CO 2 and scaled up to global salt marsh area, respiration of salt marsh DOC and POC transported to estuaries could amount to a global CO 2 flux of 11 Tg C yr À1 , roughly 4% of the recently estimated CO 2 release for marshes and estuaries globally.

Research paper thumbnail of Microbial activities and dissolved organic matter dynamics in oil-contaminated surface seawater from the Deepwater Horizon oil spill site

The Deepwater Horizon oil spill triggered a complex cascade of microbial responses that reshaped ... more The Deepwater Horizon oil spill triggered a complex cascade of microbial responses that reshaped the dynamics of heterotrophic carbon degradation and the turnover of dissolved organic carbon (DOC) in oil contaminated waters. Our results from 21-day laboratory incubations in rotating glass bottles (roller bottles) demonstrate that microbial dynamics and carbon flux in oil-contaminated surface water sampled near the spill site two weeks after the onset of the blowout were greatly affected by activities of microbes associated with macroscopic oil aggregates. Roller bottles with oil-amended water showed rapid formation of oil aggregates that were similar in size and appearance compared to oil aggregates observed in surface waters near the spill site. Oil aggregates that formed in roller bottles were densely colonized by heterotrophic bacteria, exhibiting high rates of enzymatic activity (lipase hydrolysis) indicative of oil degradation. Ambient waters surrounding aggregates also showed enhanced microbial activities not directly associated with primary oil-degradation (bglucosidase; peptidase), as well as a twofold increase in DOC. Concurrent changes in fluorescence properties of colored dissolved organic matter (CDOM) suggest an increase in oil-derived, aromatic hydrocarbons in the DOC pool. Thus our data indicate that oil aggregates mediate, by two distinct mechanisms, the transfer of hydrocarbons to the deep sea: a microbially-derived flux of oil-derived DOC from sinking oil aggregates into the ambient water column, and rapid sedimentation of the oil aggregates themselves, serving as vehicles for oily particulate matter as well as oil aggregateassociated microbial communities.

Research paper thumbnail of Runoff-mediated seasonal oscillation in the dynamics of dissolved organic matter in different branches of a large bifurcated estuary-The Changjiang Estuary

Journal of Geophysical Research: Biogeosciences, 2014

, 897 ͑2000͔͒ extended the standard SeaWiFS atmospheric-correction algorithm for use over turbid ... more , 897 ͑2000͔͒ extended the standard SeaWiFS atmospheric-correction algorithm for use over turbid coastal and inland waters. However, Ruddick's method is based on the assumption of a spatially homogeneous constant ratio for the water-leaving reflectances normalized by the sun-sea atmospheric transmittance at 765 and 865 nm. Such first-order b b ͞a model-based assumption can result in an inaccuracy for highly turbid water. Using the first-and second-order b b ͑͞a ϩ b b ͒ models as well as the second-order b b ͞a model ͑which, more realistically, do not assume spatial homogeneity ratio͒, we suggest using the modified assumption, R ͑8͒Ϫ1 ϭ ␣ 0 R ͑7͒Ϫ1 ϩ ͑l 1 Q͒ Ϫ1 ͑1 Ϫ ␣ 0 ͒, instead of Ruddick's assumption, in SeaWiFS atmospheric-correction algorithms for highly turbid waters.

Research paper thumbnail of Seasonal variation in the quality of dissolved and particulate organic matter exchanged between a salt marsh and its adjacent estuary

Fluorescence was used to examine the quality of dissolved and particulate organic matter (DOM and... more Fluorescence was used to examine the quality of dissolved and particulate organic matter (DOM and POM) exchanging between a tidal creek in a created salt marsh and its adjacent estuary in eastern North Carolina, USA. Samples from the creek were collected hourly over four tidal cycles in May, July, August, and October 2011. Absorbance and fluorescence of chromophoric DOM (CDOM) and of base-extracted POM (BEPOM) served as the tracers for organic matter quality while dissolved organic carbon (DOC) and base-extracted particulate organic carbon (BEPOC) were used to compute fluxes. Fluorescence was modeled using parallel factor analysis (PARAFAC) and principle components analysis (PCA) of the PARAFAC results. Of nine PARAFAC components (C) modeled, C3 represented recalcitrant DOM and C4 represented fresher soil-derived source DOM. Component 1 represented detrital POM, and C6 represented planktonic POM. Based on mass balance, recalcitrant DOC export was 86 g C m À2 yr À1 and labile DOC export was 49 g C m À2 yr À1 ; no planktonic DOC was exported. The marsh also exported 41 g C m À2 yr À1 of detrital terrestrial POC, which likely originated from lands adjacent to the North River estuary. Planktonic POC export from the marsh was 6 g C m À2 yr À1. Assuming the exported organic matter was oxidized to CO 2 and scaled up to global salt marsh area, respiration of salt marsh DOC and POC transported to estuaries could amount to a global CO 2 flux of 11 Tg C yr À1 , roughly 4% of the recently estimated CO 2 release for marshes and estuaries globally.

Research paper thumbnail of Optical Proxies for Terrestrial Dissolved Organic Matter in Estuaries and Coastal Waters

Dissolved organic matter (DOM) absorbance and fluorescence were used as optical proxies to track ... more Dissolved organic matter (DOM) absorbance and fluorescence were used as optical proxies to track terrestrial DOM fluxes through estuaries and coastal waters by comparing models developed for several coastal ecosystems. Key to using these optical properties is validating and calibrating them with chemical measurements, such as lignin-derived phenols—a proxy to quantify terrestrial DOM. Utilizing parallel factor analysis (PARAFAC), and comparing models statistically using the OpenFluor database (http://www.openfluor.org) we have found common, ubiquitous fluorescing components which correlate most strongly with lignin phenol concentrations in several estuarine and coastal environments. Optical proxies for lignin were computed for the following regions: Mackenzie River Estuary, Atchafalaya River Estuary (ARE), Charleston Harbor, Chesapeake Bay, and Neuse River Estuary (NRE) (all in North America). The slope of linear regression models relating CDOM absorption at 350 nm (a 350) to DOC and to lignin, varied 5–10-fold among systems. Where seasonal observations were available from a region, there were distinct seasonal differences in equation parameters for these optical proxies. The variability appeared to be due primarily to river flow into these estuaries and secondarily to biogeochemical cycling of DOM within them. Despite the variability, overall models using single linear regression were developed that related dissolved organic carbon (DOC) concentration to CDOM (DOC 2 = 40 ± 2 × a + ± 350 138 16; R = 0.77; N = 130) and lignin (8) to CDOM (= 2.03 ± 0.07 × a .47 2 − 0 ± = 8 350 0.59; R 0.87; N = 130). This wide variability suggested that local or regional optical models should be developed for predicting terrestrial DOM flux into coastal oceans and taken into account when upscaling to remote sensing observations and calibrations.

Research paper thumbnail of The use of PARAFAC components in studying headwater catchments: a stream and soil study

Research paper thumbnail of Photobleaching of Dissolved Organic Material from a Tidal Marsh-Estuarine System of the Chesapeake Bay†

Photochemistry and Photobiology, 2007

Wetlands and tidal marshes in the Rhode River estuary of the Chesapeake Bay act as important sour... more Wetlands and tidal marshes in the Rhode River estuary of the Chesapeake Bay act as important sources of dissolved organic carbon and strongly absorbing dissolved organic matter (DOM) for adjacent estuarine waters. The effects of solar exposure on the photochemical degradation of colored DOM (CDOM) were examined for material derived from different sources (estuarine and freshwater parts of the Rhode River, sub-watershed stream, marshes) in this estuarine ecosystem. Consistent with changes in fluorescence emission, absorption loss upon exposure to different portions of the solar spectrum (i.e. different long-pass cut-off filters) occurred across the entire spectrum but the wavelength of maximum photobleaching decreased as the cut-off wavelength of the filter decreased. Our results illustrate that solar exposure can cause either an increase or a decrease in the CDOM absorption spectral slope, S CDOM , depending on the spectral quality of irradiation and, thus, on the parameters (e.g. atmospheric composition, concentration of UV-absorbing water constituents) that affect the spectral characteristics of the light to which CDOM is exposed. We derived a simple spectral model for describing the effects of solar exposure on CDOM optical quality. The model accurately, and consistently, predicted the observed dependence of CDOM photobleaching on the spectral quality of solar exposure.

Research paper thumbnail of Photoreactivity of chromophoric dissolved organic matter transported by the Mackenzie River to the Beaufort Sea

Marine Chemistry, 2009

The photoreactivity of chromophoric dissolved organic matter (CDOM) transported to Arctic shelf e... more The photoreactivity of chromophoric dissolved organic matter (CDOM) transported to Arctic shelf environments by rivers has only recently been studied and its quantitative role in Arctic shelf biogeochemistry has received little attention. Sunlight exposure experiments were performed on CDOM collected over a three year period (2002 to 2004) from river, estuary, shelf, and gulf regions of the Western Canadian Arctic. Decreases in CDOM absorption, synchronous fluorescence (SF), and dissolved organic carbon (DOC) concentration were followed after 3 days of exposure, and in two experiments, six optical cutoff filters were used to incrementally remove ultraviolet radiation incident on the samples. Apparent quantum yields for CDOM photobleaching (AQY ble ) and for DOC photomineralization (AQY min ) were computed, as were two AQY spectra (ϕ ble and ϕ min ) for the Mackenzie River and a sample from the Mackenzie Shelf. The photoreactivity of Mackenzie River CDOM was highest after break-up and peak discharge and lowest in late summer. The half-lives of CDOM and DOC were estimated at 3.7 days and 4.8 days, respectively, when Mackenzie River water was exposed to full sunlight. Photobleaching of Mackenzie River CDOM fluorescence after most UV-B wavelengths were removed increased the correlation between the river and offshore waters in the Beaufort Sea. When light attenuation from particle-and CDOMrich river water was considered for the Mackenzie Shelf, our photodegradation models estimated around 10% loss of absorption and b 1% DOC loss, suggesting that sunlight exposure does not substantially degrade CDOM on Arctic shelves.

Research paper thumbnail of Calculation of spectral weighting functions for the solar photobleaching of chromophoric dissolved organic matter in temperate lakes

Limnology and Oceanography, 2001

The effect of solar radiation on the dissolved absorption coefficient (a CDOM []), which reflects... more The effect of solar radiation on the dissolved absorption coefficient (a CDOM []), which reflects the concentration of chromophoric dissolved organic matter (CDOM), was investigated in several lakes near Bariloche, Argentina and in northeastern Pennsylvania, USA. Samples of 0.2 m filtered lake water were exposed in quartz tubes to different portions of the solar spectrum using optical cutoff filters to remove parts of the ultraviolet (UV) region of the solar spectrum. Changes in the spectral absorption in these samples and the absorbed incident energy were used to calculate spectral weighting functions (SWFs) for the photobleaching (PB) of CDOM. PB was measured as the loss of a CDOM () (the a CDOM [] was averaged from 280 to 500 nm) per unit absorbed energy. CDOM from humic and clear lakes, as well as from a Sphagnum bog and an algal culture, was used in the experiments covering a wide range of carbon sources. We used an iterative, nonlinear optimization method to fit the measured results to a simple exponential function in order to generate each SWF. Comparing individual SWFs calculated for various CDOM sources, we computed a summary SWF from the experiments using epilimnial CDOM from our study lakes. Our summary SWF was able to explain 80-90% of the observed variance in our exposure experiments, and we were able to predict PB results obtained for other Argentine lakes (mean error 14.5%). Finally, we calculated that the effect of UV-B radiation on PB was small (Ͻ20% of total decrease in the absorption coefficient) compared to UV-A and blue light radiation. This suggested that increased UV-B radiation due to stratospheric ozone depletion would not greatly increase the photobleaching of whole water column CDOM in Patagonian lakes (Ͻ10%).

Research paper thumbnail of Bacterial production and microbial food web structure in a large arctic river and the coastal Arctic Ocean

Journal of Marine Systems, 2008

Globally significant quantities of organic carbon are stored in northern permafrost soils, but li... more Globally significant quantities of organic carbon are stored in northern permafrost soils, but little is known about how this carbon is processed by microbial communities once it enters rivers and is transported to the coastal Arctic Ocean. As part of the Arctic River-Delta Experiment (ARDEX), we measured environmental and microbiological variables along a 300 km transect in the Mackenzie River and coastal Beaufort Sea, in July-August 2004. Surface bacterial concentrations averaged 6.7 × 10 5 cells mL − 1 with no significant differences between sampling zones. Picocyanobacteria were abundant in the river, and mostly observed as cell colonies. Their concentrations in the surface waters decreased across the salinity gradient, dropping from 51,000 (river) to 30 (sea) cells mL − 1 . There were accompanying shifts in protist community structure, from diatoms, cryptophytes, heterotrophic protists and chrysophytes in the river, to dinoflagellates, prymnesiophytes, chrysophytes, prasinophytes, diatoms and heterotrophic protists in the Beaufort Sea.

Research paper thumbnail of Comparison of the optical properties of dissolved organic matter in two river-influenced coastal regions of the Canadian Arctic

Estuarine, Coastal and Shelf Science, 2007

The optical characteristics of coloured dissolved organic matter (CDOM) were analyzed in the Grea... more The optical characteristics of coloured dissolved organic matter (CDOM) were analyzed in the Great Whale River and adjacent Hudson Bay (55 N, 77 W) in the eastern Canadian Low Arctic, and in the Mackenzie River and adjacent Beaufort Sea in the western Canadian High Arctic (70 N, 133 W). Sampling was during ice-free open water conditions. Both rivers contained high concentrations of dissolved organic carbon (3 and 6 mg DOC l À1 in the Great Whale River and Mackenzie River, respectively) and CDOM (a 320 of 11 and 14 m À1 ), resulting in a substantial load of organic matter to their coastal seas. There were pronounced differences in the CDOM characteristics of the two rivers, notably in their synchronous fluorescence scans (SFS). The latter showed that the Mackenzie River was depleted in humic materials, implying a more mature catchment relative to the younger, more recently glaciated Great Whale River system. SFS spectra had a similar shape across the freshwatere saltwater transition zone of the Great Whale plume, and DOC was linearly related to salinity implying conservative mixing and no loss by flocculation or biological processes across the salt front. In contrast, there were major differences in SFS spectral shape from the Mackenzie River to the freshwater-influenced coastal ocean, with a marked decrease in the relative importance of fulvic and humic acid materials. The SFS spectra for the coastal Beaufort Sea in SeptembereOctober strongly resembled those recorded for the Mackenzie River during the high discharge, CDOM-rich, snowmelt period in June, but with some loss of autochthonous materials. These results are consistent with differences in freshwater residence time between the Mackenzie River and Great Whale River coastal ocean systems. Models of arctic continental shelf responses to present and future climate regimes will need to consider these striking regional differences in the organic matter content, biogeochemistry and optics between waters from different catchments and different inshore hydrodynamic regimes.

Research paper thumbnail of Effects of Solar Radiation on the Optical Quality of Dissolved Organic Material in a Tidal Marsh Estuarine Ecosystem

Photochemical alteration of colored dissolved organic matter (CDOM) during exposure to solar UV a... more Photochemical alteration of colored dissolved organic matter (CDOM) during exposure to solar UV and Visible radiation generates a variety of photoproducts, including reactive oxygen species, atmospherically important trace gases, and microbially labile carbonyl compounds. Sunlight-induced changes in CDOM chemical structure are reflected in changes of its optical properties that provide a first order measure of the photoreactivity of CDOM in

Research paper thumbnail of Chemical Analysis of CDOM Sources in Danish Coastal Waters of the Baltic Sea-North Sea Mixing Zone

... Data synthesis and analysis is 80% completed. ... optical algorithms derived for river-domina... more ... Data synthesis and analysis is 80% completed. ... optical algorithms derived for river-dominated margins will not likely operate in complex coastal environments such as those found in Danish coastal waters. ... ASLO/AGU Ocean Sciences Meeting 2008, Orlando, Florida, USA. ...

Research paper thumbnail of Dissolved and particulate organic matter fluorescence in coastal river systems in relation to land use and carbon transfer

Research paper thumbnail of Photochemical changes in the dissolved organic matter of temperate lakes: implications for organic carbon cycling and lake transparency

Research paper thumbnail of Bacterial Metabolism, Aromatic Biodegradation, and Lignin Biogeochemistry in Sediment Cores from Pearl Harbor, Hawaii

Approved for public release; distribution is unlimited. ... Chemical Dynamics and Diagnostics Bra... more Approved for public release; distribution is unlimited. ... Chemical Dynamics and Diagnostics Branch Chemistry Division ... Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std. Z39.18 ... Public reporting burden for this collection of information is estimated to average 1 hour ...

Research paper thumbnail of HE ZAGARESE, DP MORRIS, BR HARGREAVES, AND WE CRAVERO. 2001. Calculation of spectral weighting functions for the solar photobleaching of chromophoric dissolved organic matter in temperate lakes

Research paper thumbnail of Chemical and optical changes in freshwater dissolved organic matter exposed to solar radiation

We studied the chemical and optical changes in the dissolved organic matter (DOM) from two freshw... more We studied the chemical and optical changes in the dissolved organic matter (DOM) from two freshwater lakes and a Sphagnum bog after exposure to solar radiation. Stable carbon isotopes and solid-state 13 C-NMR spectra of DOM were used together with optical and chemical data to interpret results from experimental exposures of DOM to sunlight and from seasonal observations of two lakes in northeastern Pennsylvania. Solar photochemical oxidation of humic-rich bog DOM to smaller LMW compounds and to DIC was inferred from losses of UV absorbance, optical indices of molecular weight and changes in DOM chemistry. Experimentally, we observed a 1.2‰ enrichment in δ 13 C and a 47% loss in aromatic C functionality in bog DOM samples exposed to solar UVR. Similar results were observed in the surface waters of both lakes. In late summer hypolimnetic water in humic Lake Lacawac, we observed 3 to 4.5‰ enrichments in δ 13 C and a 30% increase in aromatic C relative to early spring values during spring mixing. These changes coincided with increases in molecular weight and UV absorbance. Anaerobic conditions of the hypolimnion in Lake Lacawac suggest that microbial metabolism may be turning over allochthonous C introduced during spring mixing, as well as autochthonous C. This metabolic activity produces HMW DOM during the summer, which is photochemically labile and isotopically distinct from allochthonous DOM or autochthonous DOM. These results suggest both photooxidation of allochthonous DOM in the epilimnion and autotrophic production of DOM by bacteria in the hypolimnion cause seasonal trends in the UV absorbance of lakes.

Research paper thumbnail of The record of global change in mid-Cretaceous (Barremian-Albian) sections from the Sierra Madre, northeastern Mexico

Our current understanding of mid-Cretaceous global change is largely based on investigations of p... more Our current understanding of mid-Cretaceous global change is largely based on investigations of pelagic sections from southern Europe and deep sea drilling sites. Much less information exists from other continents and from hemipelagic sections deposited on continental margins. This investigation seeks to broaden our understanding of mid-Cretaceous global change by focusing on the record from hemipelagic sections deposited along the continental margin of northeastern Mexico. The major goals are to compare the record, timing, and extent of the Oceanic Anoxic Events (OAEs) in Mexico and other areas, and to determine the relationship between these events and the global burial of organic material using carbon isotopes.

Research paper thumbnail of Evaluating PAH Biodegradation Relative to Total Bacterial Carbon Demand in Coastal Ecosystems: Are PAHs Truly Recalcitrant?

Various techniques have been used to evaluate microbial metabolic activity in natural environment... more Various techniques have been used to evaluate microbial metabolic activity in natural environments. In recent years, tracer additions of radiolabeled substrates coupled with short term, environmentally-relevant incubation experiments have become standard for assessing overall microbial growth rates as well as utilization rates for specific components of the bioavailable carbon pool. Using these techniques, microbial activities have been determined over a wide range of spatial and temporal scales in a variety of estuarine settings, providing valuable insight into carbon cycling in these dynamic systems. In this paper, we focus on patterns of microbial carbon consumption in estuarine sediments and relate these to the utilization of polyaromatic hydrocarbons (PAHs) as a recalcitrant, yet microbially available component of the carbon pool. Drawing on extensive field studies and those published in the literature, we relate microbial metabolic activities to the utilization of specific carbon species, discuss the partitioning of carbon between bacterial production and respiration (utilization efficiency), address the substrate-activity relationship for utilization of recalcitrant organic carbon and identify seasonal effects on the utilization of natural and anthropogenic carbon pools. These studies elucidate the advantages, disadvantages and caveats of relating microbial consumption of bulk labile organic carbon to the utilization of specific components and improve our ability to predict the fate of organic contaminants in estuarine systems.

Research paper thumbnail of Seasonal variation in the quality of dissolved and particulate organic matter exchanged between a salt marsh and its adjacent estuary

Journal of Geophysical Research: Biogeosciences, 2015

Fluorescence was used to examine the quality of dissolved and particulate organic matter (DOM and... more Fluorescence was used to examine the quality of dissolved and particulate organic matter (DOM and POM) exchanging between a tidal creek in a created salt marsh and its adjacent estuary in eastern North Carolina, USA. Samples from the creek were collected hourly over four tidal cycles in May, July, August, and October 2011. Absorbance and fluorescence of chromophoric DOM (CDOM) and of base-extracted POM (BEPOM) served as the tracers for organic matter quality while dissolved organic carbon (DOC) and base-extracted particulate organic carbon (BEPOC) were used to compute fluxes. Fluorescence was modeled using parallel factor analysis (PARAFAC) and principle components analysis (PCA) of the PARAFAC results. Of nine PARAFAC components (C) modeled, C3 represented recalcitrant DOM and C4 represented fresher soil-derived source DOM. Component 1 represented detrital POM, and C6 represented planktonic POM. Based on mass balance, recalcitrant DOC export was 86 g C m À2 yr À1 and labile DOC export was 49 g C m À2 yr À1 ; no planktonic DOC was exported. The marsh also exported 41 g C m À2 yr À1 of detrital terrestrial POC, which likely originated from lands adjacent to the North River estuary. Planktonic POC export from the marsh was 6 g C m À2 yr À1 . Assuming the exported organic matter was oxidized to CO 2 and scaled up to global salt marsh area, respiration of salt marsh DOC and POC transported to estuaries could amount to a global CO 2 flux of 11 Tg C yr À1 , roughly 4% of the recently estimated CO 2 release for marshes and estuaries globally.

Research paper thumbnail of Microbial activities and dissolved organic matter dynamics in oil-contaminated surface seawater from the Deepwater Horizon oil spill site

The Deepwater Horizon oil spill triggered a complex cascade of microbial responses that reshaped ... more The Deepwater Horizon oil spill triggered a complex cascade of microbial responses that reshaped the dynamics of heterotrophic carbon degradation and the turnover of dissolved organic carbon (DOC) in oil contaminated waters. Our results from 21-day laboratory incubations in rotating glass bottles (roller bottles) demonstrate that microbial dynamics and carbon flux in oil-contaminated surface water sampled near the spill site two weeks after the onset of the blowout were greatly affected by activities of microbes associated with macroscopic oil aggregates. Roller bottles with oil-amended water showed rapid formation of oil aggregates that were similar in size and appearance compared to oil aggregates observed in surface waters near the spill site. Oil aggregates that formed in roller bottles were densely colonized by heterotrophic bacteria, exhibiting high rates of enzymatic activity (lipase hydrolysis) indicative of oil degradation. Ambient waters surrounding aggregates also showed enhanced microbial activities not directly associated with primary oil-degradation (bglucosidase; peptidase), as well as a twofold increase in DOC. Concurrent changes in fluorescence properties of colored dissolved organic matter (CDOM) suggest an increase in oil-derived, aromatic hydrocarbons in the DOC pool. Thus our data indicate that oil aggregates mediate, by two distinct mechanisms, the transfer of hydrocarbons to the deep sea: a microbially-derived flux of oil-derived DOC from sinking oil aggregates into the ambient water column, and rapid sedimentation of the oil aggregates themselves, serving as vehicles for oily particulate matter as well as oil aggregateassociated microbial communities.

Research paper thumbnail of Runoff-mediated seasonal oscillation in the dynamics of dissolved organic matter in different branches of a large bifurcated estuary-The Changjiang Estuary

Journal of Geophysical Research: Biogeosciences, 2014

, 897 ͑2000͔͒ extended the standard SeaWiFS atmospheric-correction algorithm for use over turbid ... more , 897 ͑2000͔͒ extended the standard SeaWiFS atmospheric-correction algorithm for use over turbid coastal and inland waters. However, Ruddick's method is based on the assumption of a spatially homogeneous constant ratio for the water-leaving reflectances normalized by the sun-sea atmospheric transmittance at 765 and 865 nm. Such first-order b b ͞a model-based assumption can result in an inaccuracy for highly turbid water. Using the first-and second-order b b ͑͞a ϩ b b ͒ models as well as the second-order b b ͞a model ͑which, more realistically, do not assume spatial homogeneity ratio͒, we suggest using the modified assumption, R ͑8͒Ϫ1 ϭ ␣ 0 R ͑7͒Ϫ1 ϩ ͑l 1 Q͒ Ϫ1 ͑1 Ϫ ␣ 0 ͒, instead of Ruddick's assumption, in SeaWiFS atmospheric-correction algorithms for highly turbid waters.