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Abstract: We investigated chromophoric dissolved organic matter (CDOM) dynamics in response to the severe drought conditions of 2009 in the Aransas–Copano (AC) Bay complex and adjacent nearshore Gulf of Mexico (GoM) along the southern Texas (USA) coast. Surface water absorption coefficients (a 305 in m−1) were measured daily at the University of Texas at Austin Marine Science Institute (UTMSI) pier located at the interface between the AC complex and the GoM. From June to August 2009, a 305 averaged 0.81 ± 0.31 m−1 compared to 1.20 ± 0.70 m−1 during summer of 2008 (June–August) and 1.84 ± 0.34 m−1 during summer 2007 (non-drought year). Despite negligible freshwater input to the AC system for most of 2009, mean a 305 values for AC Bay sites were similar in 2008 and 2009, ranging from 3.24 ± 0.60 m−1 in the lower estuary to 6.22 ± 0.55 m−1 in the upper estuary during summer 2009, and 3.15 ± 0.38 m−1 to 6.81 ± 0.73 m−1 during summer 2008 (no data for 2007). CDOM photobleaching experiments were performed using a SunTest XLS + solar simulator with an irradiation spectrum that closely matches solar UV at subtropical latitudes. AC Bay samples exhibited a 305 photobleaching half-lives of 33–48 h, while GoM samples exhibited a 305 photobleaching half-lives of 36–89 h. We estimate that summertime photobleaching may reduce CDOM levels by >50% in the AC bay complex before discharge into the GoM and by up to 25% more throughout the nearshore waters along the western GoM shelf. [Copyright &y& Elsevier]

Abstract: 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 a 305) 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 XLS+ solar simulator with photobleaching rate coefficients (k 305) ranging from 0.011 to 0.32h−1. For fall 2007 and summer 2008, higher k 305 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 (S 275–295 or S 350–400) and DOC levels were largely unchanged after 24h 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 1m may be lost to photobleaching, with losses up to 20% possible even at 10m depth. [Copyright &y& Elsevier]

Abstract: 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 (∼48h) from mangrove leaves collected during a dry period in June 2004 (0.17±0.11m− 1 g− 1 lh− 1) was lower than production from leaves collected during a wet period in September 2003 (0.57±0.42m− 1 g− 1 lh− 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. [Copyright &y& Elsevier]

Abstract: We examined the photoreactivity of chromophoric dissolved organic matter (CDOM) derived from Rhizophora mangle (red mangrove) leaf litter and floating Sargassum colonies as these marine plants can be important contributors to coastal and open ocean CDOM pools, respectively. Mangrove and Sargassum CDOM readily degraded when exposed to simulated solar irradiance (CPS SunTest solar simulator exposures). CDOM produced from brown mangrove leaves (representative of substantial senescence) exhibited shorter photobleaching half-lives (a 305 t 1/2 <50h) than CDOM produced from yellow and orange (early and mid senescence) leaves (a 305 t 1/2 ∼60–90h). Mangrove CDOM photobleaching rates were higher in the mid-UVA (a 350) than in the UVB (a 305) spectral region. Photobleaching half-lives of Sargassum CDOM were mostly <40h and more consistent across UVB (a 305) and UVA (a 350) wavelengths. Sargassum CDOM photomineralized during simulated solar irradiation producing DIC at rates exceeding 2500nmolm l–1 h–1, indicating that regions of the surface ocean with large concentrations of this plant may provide a strong CO2 source to the atmosphere. Sargassum CDOM photoreactions also produced CO more efficiently than terrestrial CDOM and much more efficiently than ambient CDOM in the open ocean. Thus, biological production of CDOM may be the rate-limiting step for photoproduction of DIC from Sargassum and other sources in the open ocean. An examination of CDOM photobleaching in Florida Keys coastal waters indicates that one month of summertime solar radiation may substantially increase UVB and UVA exposure to corals in shallow waters (<4m), especially along the offshore reef tract. But, our results also indicate that when ambient CDOM levels are high, the corals are well-buffered against increases in ultraviolet radiation (UV-R) exposure even after periods of extended CDOM photobleaching. [Copyright &y& Elsevier]

Abstract: 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 233Lkg−1 when model freshwater and organic-rich estuarine sediments were added to solution (concentrations of 0.1, 1, or 10gL−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 ∼100mgL−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>10gL−1) may exist as CPOM. Spectral slope coefficients (300–700nm) of initial CDOM samples increased as much as 20% after mixing with 10gL−1 sediment and 5% after mixing with 1gL−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. [Copyright &y& Elsevier]