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This study evaluated impacts of the BP Deepwater Horizon ( DWH ) oil spill on petroleum hydrocarbons in surface waters of the Louisiana continental shelf in northern Gulf of Mexico. Surface water (~top 5 cm) without visible oil was collected from three cruises in May 2010 during the oil spill, August 2010 after the well was capped, and May 2011 one year after the spill. Concentrations of total dissolved n -alkanes (C 9 –C 35 ) in surface seawater were more than an order of magnitude higher in May 2010 than August 2010 and May 2011, indicating contamination by the DWH oil spill. This conclusion was further supported by more abundant smaller n -alkanes (C 9 –C 13 ), together with pristane and phytane, in May than August 2010 samples. In contrast, even carbon-numbered dissolved n -alkanes (C 14 –C 20 ) dominated the May 2011 samples, and this distribution pattern of dissolved n -alkanes is the first documentation for water samples in the northern Gulf of Mexico. However, this pattern was not observed in May 2011 suspended particles except for Sta. OSS. This decoupling between dissolved and particle compositions suggests that either these even carbon-numbered n -alkanes originated from bacteria rather than algae, or that the alkanes in the shelf were transported from elsewhere. Concentrations of polycyclic aromatic hydrocarbons (PAHs) in suspended particles were 5 times higher on average in May 2010 (83–252 ng L −1 ) than May 2011 (7.2–83 ng L −1 ), also indicating contamination by the DWH oil spill. Application of a biomarker ratio of 17α(H),21β(H)-30-norhopane over 17α(H),21β(H)-hopane confirmed that suspended particles from at least two stations were contaminated by the DWH oil spill in May 2010. Taken together, these results showed that surface waters of the sampling area in May 2010 were contaminated by the oil spill, but also that rapid weathering and/or physical dilution quickly reduced hydrocarbon levels by August 2010.

Solar radiation-enhanced dissolution (photodissolution) of particulate organic matter (POM) has the potential to provide a substantial pool of dissolved organic matter (DOM) to surrounding waters in a variety of aquatic systems. This study focused on the magnitude of photodissolution in south Texas estuaries and the influence of environmental factors such as sediment desiccation and water chemistry on the production of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) via photodissolution. Substantial photoproduction of DOM from POM was observed during 24-h irradiations, with the potential to augment DOC and TDN loads in Texas estuarine waters by as much as 85 and 75 %, respectively. Excitation-emission matrix (EEM) fluorescence analysis revealed that fluorescent DOM (FDOM) produced during photodissolution is primarily humic-like. Experimental evidence suggested that photochemical degradation of POM was a major pathway for DOM photoproduction, at least a strong supplement to microbial-mediated degradation and/or stimulation of benthic primary production by benthic phytoplankton or algae. Sediment dry-wet cycles strongly influenced photodissolution as sediment desiccation augmented the photoproduction of DOC and TDN in organic-poor water. The organic content of ambient water by itself did not significantly affect the magnitude of DOM photoproduction, but the combined influence of water organic content and sediment desiccation played a substantial role in controlling the extent of photodissolution. Thus, photodissolution is likely an important source of DOC and TDN to shallow Texas estuarine waters especially for habitats where organic-rich sediments undergo dry-wet cycles.

[1] Rising CO2 concentration in the atmosphere, global climate change, and the sustainability of the Earth's biosphere are great societal concerns for the 21st century. Global climate change has, in part, resulted in a higher frequency of flooding events, which allow for greater exchange between soil/plant litter and aquatic carbon pools. Here we demonstrate that the summer 2011 flood in the Mississippi River basin, caused by extreme precipitation events, resulted in a “flushing” of terrestrially derived dissolved organic carbon (TDOC) to the northern Gulf of Mexico. Data from the lower Atchafalaya and Mississippi rivers showed that the DOC flux to the northern Gulf of Mexico during this flood was significantly higher than in previous years. We also show that consumption of radiocarbon-modern TDOC by bacteria in floodwaters in the lower Atchafalaya River and along the adjacent shelf contributed to northern Gulf shelf waters changing from a net sink to a net source of CO2 to the atmosphere in June and August 2011. This work shows that enhanced flooding, which may or may not be caused by climate change, can result in rapid losses of stored carbon in soils to the atmosphere via processes in aquatic ecosystems.

The distribution and photoreactivity of chromophoric dissolved organic matter (CDOM) in the northern Gulf of Mexico along the Louisiana coastal shelf were examined during three cruises in summer 2007, fall 2007, and summer 2008. The influence of the Mississippi River plume was clearly evident as CDOM levels (defined as a305) and dissolved organic carbon (DOC) concentrations were well-correlated with salinity during all cruises. Elevated CDOM and CDOM:DOC ratios of surface samples collected offshore of Atchafalaya Bay and the Breton–Chandeleur Sound complex indicated emanations of organic-rich waters from coastal wetlands are also an important source to nearshore shelf waters. Generally, CDOM and DOC levels were highest in surface waters and decreased with depth, but during summer 2007 and summer 2008, CDOM levels in near-bottom samples were occasionally higher than at mid-depths without concomitant increases in DOC. CDOM photobleaching was measured during 24 irradiations using a SunTest XLSþ solar simulator with photobleaching rate coefficients (k305) ranging from 0.011 to 0.32 h � 1 . For fall 2007 and summer 2008, higher k305 values were generally observed in samples with higher initial CDOM levels. However, samples collected during summer 2007 did not exhibit a similar pattern nor were there differences in photobleaching rates between surface and bottom samples. Spectral slope coefficients (S275–295 or S350–400) and DOC levels were largely unchanged after 24 h irradiations. Modeled CDOM photobleaching for northern Gulf of Mexico mid-shelf waters predicts that during the summer when solar irradiance is high and the water column becomes stratified, nearly 90% of the CDOM in the upper 1 m may be lost to photobleaching, with losses up to 20% possible even at 10 m depth.

Chromophoric dissolved organic matter (CDOM) strongly absorbs solar radiation in the blue-green and serves as the primary attenuator of water column ultraviolet radiation (UV-R). CDOM interferes with remote sensing of ocean chlorophyll and can control UV-R-induced damage to light-sensitive organisms including corals. We used laboratory incubations to evaluate CDOM production from senescing Rhizophora mangle (red mangrove) leaf litter (yellow, orange, and brown) and floating Sargassum colonies. Mangroves exist at the land–ocean interface near coral reefs in sub-tropical and tropical regions while floating Sargassum colonies tend to congregate in sub-tropical ocean gyres. CDOM production (∼ 48 h) from mangrove leaves collected during a dry period in June 2004 (0.17 ± 0.11 m − 1 g − 1 l h − 1 ) was lower than production from leaves collected during a wet period in September 2003 (0.57 ± 0.42 m − 1 g − 1 l h − 1 ) suggesting that CDOM production from leaf litter fluctuates in response to environmental factors. CDOM production was greatest for the mid-senescence orange leaves and lowest for the severely senesced brown leaves in both experiments. Along the sub-tropical Florida Keys coral reef ecosystem, the primary source of CDOM is discharge from the shallow seagrass-dominated Florida Bay as evidenced by a strong correlation between field CDOM measurements and previously reported Florida Bay discharge volumes. However, field observations provide evidence that large expanses of red mangroves throughout the Keys could be important CDOM sources to the region's coral reefs during periods of reduced Florida Bay discharge. Floating Sargassum colonies also readily produced CDOM in laboratory incubations, but at much more variable rates than mangrove leaves. However, our calculations indicate that large mats of floating Sargassum could provide important CDOM quantities to oligotrophic oceanic waters including the Gulf of Mexico and North Atlantic.

This study examined freshwater discharge of dissolved organic matter (DOM) to the shallow Lavaca–Matagorda (LM) Bay estuarine system along the central Texas coast and investigated whether chromophoric DOM (CDOM) photochemical reactions have the potential to stimulate microbial activity within LM estuarine waters. Dissolved organic carbon (DOC) concentrations ranged from 3 to 10 mg C l−1 and CDOM levels (reported as a 305) ranged from 8 to 77 m−1 during April and July, 2007, when the LM system was experiencing very high freshwater inputs. DOC and CDOM levels were well-correlated with salinities > 3, but exhibited considerable variability at salinities

The optical properties and geochemical cycling of chromophoric dissolved organic matter (CDOM) are altered by its sorption to freshwater and estuarine sediments. Measured partition coefficients ( K p ) of Satilla River (Georgia) and Cape Fear River estuary (North Carolina) CDOM ranged from 19 to 233 L kg −1 when model freshwater and organic-rich estuarine sediments were added to solution (concentrations of 0.1, 1, or 10 g L −1 ), with the largest K p values measured in solutions with the lowest sediment concentrations. Sorption of Satilla River CDOM was augmented upon raising the ionic strength of solution by mixing with natural seawater, likely due to the ‘salting out effect’ of Ca +2 and Mg +2 ions. For turbid estuarine systems with particle loads of ∼100 mg L −1 , we estimate that ∼1–2% of the CDOM pool sorbs to settling particles, facilitating the transfer to a potentially large colored particulate organic matter (CPOM) reservoir within sediments. Our results also indicate that >30% of the colored organic matter pool within sediment pore waters (sediment concentration > 10 g L −1 ) may exist as CPOM. Spectral slope coefficients (300–700 nm) of initial CDOM samples increased as much as 20% after mixing with 10 g L −1 sediment and 5% after mixing with 1 g L −1 sediment indicating that sorption to particles has the potential to significantly alter the optical properties of CDOM in the water column of turbid shallow environments or in areas of high benthic exchange.

In the summer of 1996, southeastern North Carolina, United States, was struck by two hurricanes, with the second (Hurricane Fran) doing considerably more damage than the first (Hurricane Bertha). The Cape Fear watershed, largest in North Carolina, suffered from severe water quality problems for weeks following Fran, including a massive fish kill in the Northeast Cape Fear River. Post-hurricane flooding caused inputs of riparian swamp water to river channels, and sewage treatment plant and pump station power failures caused diversions of millions of liters of raw and partially treated human waste into rivers. Additionally, several swine waste lagoons were breached, overtopped, or inundated, discharging large quantities of concentrated organic waste into the system, particularly into the Northeast Cape Fear River. Dissolved oxygen (DO) decreased to 2 mg/L in the mainstem Cape Fear River, and fell to zero in the Northeast Cape Fear River for >3 wk. Biochemical oxygen demand in the Northeast Cape Fear River w...

During July 1995, a poultry waste lagoon ruptured in Duplin County, North Carolina, sending 32.6 million L of chicken waste effluent into a nearby creek and the Northeast Cape Fear River. In August 1995 a breach of a hog waste lagoon released approximately 7.6 million L of waste into a system of blackwater creeks in Brunswick County, North Carolina. The poultry waste spill occurred under high rainfall-high river flow conditions, while the swine waste lagoon spill occurred during dry conditions. Both spills caused high turbidity and low dissolved oxygen (DO) in receiving waters, and DO levels in the Northeast Cape Fear River displayed a sag curve 10 d after the poultry waste spill, reaching a minimum of 1.0 mg L -1 90 km downstream. Both spills delivered high N loads to receiving waters (maxima of 92.1 mg L -1 from the poultry spill and 47.0 mg L -1 from the swine waste spill). Phosphorous concentrations reached 6.0 and 11.5 mg L -1 in receiving waters of the poultry and swine waste spills, respectively. Dense phytoplankton blooms (> 100 μg chlorophyll a L -1 ) were measured in the blackwater creeks after the swine spill. High fecal coliform concentrations were delivered by both spills, and concentrations of the pathogenic bacterium Clostridium perfringens ranged up to 40 000 colony-forming units (CFU) mL -1 in the receiving waters after the poultry spill. In 1995 and 1996 more than 30 animal waste lagoon breaches, overtoppings, and inundations were reported in North Carolina, demonstrating the major pollution potential of these treatment systems.

[1] The hurricane flux of rain and river water dissolved organic carbon (DOC) to Long Bay located on the southeastern coast of the United States was determined for four hurricanes that made landfall in the Cape Fear region of North Carolina. Riverine flux of DOC following hurricanes Fran (1996) and Floyd (1999) represented one third and one half of the entire annual river flux of DOC to Long Bay, respectively. The majority of this DOC was recalcitrant and not available for biological consumption. The high flux of DOC from hurricane Floyd resulted from extremely high precipitation amounts (in excess of 50 cm) associated with the hurricane and subsequent flooding. High riverine DOC fluxes were observed following hurricane Fran but not hurricanes Bertha (1996) and Bonnie (1998). The westerly path of Fran deposited rain inland along the Cape Fear River watershed, causing high river flow conditions, while Bonnie and Bertha took an eastern path, resulting in a minimal effect to the Cape Fear River flow rates. The rainwater flux of total DOC to Long Bay from the four hurricanes was not as dramatic as that observed for riverine fluxes. However, unlike river water DOC that is refractory, rainwater DOC is highly labile. Rainwater from the four hurricanes in this study deposited 2–5 times the DOC deposited in an average storm. This represented a flux of 3–9% of the entire annual budget of bioavailable DOC to Long Bay being deposited over a 1 or 2 day period, likely spurring short-term secondary productivity following the hurricanes.

[1] This study presents the first comparison of riverine and rainwater dissolved organic carbon (DOC) flux to a coastal region, and the first examination of rainwater DOC bioavailability. The bioavailability of DOC from these two sources was determined because its impact on coastal waters depends on whether DOC is labile or refractory. The study site, Long Bay, off the North Carolina and South Carolina coasts in the southeastern United States, receives external sources of DOC from the Cape Fear River system and rainwater. Conservative mixing of estuarine DOC, and a consistent DOC value for the river sources entering the Cape Fear Estuary (Cape Fear and Black rivers combined: 937 ± 178 μM, n = 12; Northeast Cape Fear River: 1266 ± 196 μM, n = 8) allowed for a calculation of DOC flux using river DOC values and U.S.G.S. river flow data. Rainwater DOC flux was determined by a deposition calculation at a rainwater collection station in Wilmington, N. C., 40 km from Long Bay. Riverine DOC flux was determined to be (77 × 109 gCyr−1) or roughly 4x the rainwater flux (21 × 109 gCyr−1) on an annual basis. The bioavailability of the riverine DOC (9.0 ± 4.5%) was much less than that of rainwater (63 ± 14%). Therefore, taking into account only bioavailable DOC, rainwater provides more labile DOC (13 × 109 gCyr−1) to Long Bay than riverwater (7 × 109 gCyr−1) meaning that rainwater is important for fueling secondary productivity whereas river DOC is important for long-term storage of carbon.

Coastal areas in the United States and many other countries are considered to be desirable regions to live and recreate. However, as human use of coastal land and water increases, so does the incidence of aquatic-borne disease from contact with contaminated water and eating contaminated shellfish. Movement of humans into coastal areas both greatly increases the number of sources of microbial pathogens and radically alters the landscape through increased construction activity and paving of former natural areas. On a regional scale, increases in human population over a 14-year period in coastal North Carolina were strongly correlated with increases in shellfish bed closures due to high fecal coliform bacterial counts. On a watershed scale, an analysis of several tidal creeks found strong correlations between mean estuarine fecal coliform bacterial counts and watershed population, percent developed area and especially with percent impervious surface coverage. Conversion of natural landscapes to impervious surfaces (roads, drives, sidewalks, parking lots and roofs) removes the land’s natural filtration capability, allows for increased concentration of pollutants at the land’s surface and provides a means of rapid conveyance of pollutants to downstream waterways. An analysis of rural watersheds in the Coastal Plain found that stream fecal coliform counts and turbidity were both strongly correlated with rainfall in the previous 24 h in watersheds containing extensive industrial swine and poultry operations, as well as watersheds containing more traditional agriculture and cattle husbandry. In contrast, in watersheds rich in swamp wetlands these relationships were not significant, even in watersheds containing extensive animal production. Based on these findings, we suggest that waterborne microbial pathogen abundance can be minimized in urbanizing coastal areas through reduced use of impervious surfaces and maximal use of natural or constructed wetlands for passive stormwater runoff treatment. In animal husbandry areas, retention of natural wetlands and management practices designed to minimize sediment runoff can likely reduce inputs of pathogenic microbes into streams.

Phytoplankton nutrient limitation experiments were performed from 1994 to 1996 at three stations in the Cape Fear River Estuary, a riverine system originating in the North Carolina piedmont. Nutrient addition bioassays were conducted by spiking triplicate cubitainers with various nutrient combinations and determining algal response by analyzing chlorophyll a production and 14C uptake daily for 3 d. Ambient chlorophyll a, nutrient concentration, and associated physical data were collected throughout the estuary as well. At a turbid, nutrient-rich oligohaline station, significant responses to nutrient additions were rare, with light the likely principal factor limiting phytoplankton production. During summer at a mesohaline station, phytoplankton community displayed significant nitrogen (N) limitation, while both phosphorus (P) and N were occasionally limiting in spring with some N+P co-limitation. Light was apparently limiting during fall and winter when the water was turid and nutrient-rich, as well as during other months of heavy rainfall and runoff. A polyhaline station in the lower estuary had clearer water and displayed significant responses to nutrient additions during all enrichment experiments. At this site N limitation occurred in summer and fall, and P limitation (with strong N+P co-limitation) occurred in winter and spring. The data suggest there are two patterns controlling phytoplankton productivity in the Cape Fear system: 1) a longitudinal pattern of decreasing light limitation and increasing nutrient sensitivity along the salinity gradient, and 2) a seasonal alternation of N limitation, light limitation, and P limitation in the middle-to-lower estuary. Statistical analyses indicated upper watershed precipitation events led to increased flow, turbidity, light attenuation, and nutrient loading, and decreased chlorophyll a and nutrient limitation potential in the estuary. Periods of low rainfall and river flow led to reduced estuarine turbidity, higher chlorophyll a, lower ambient nutrients, and more pronounced nutrient limitation.