Temporal Variation in Optical Properties of Chromophoric Dissolved Organic Matter (CDOM) in Southern California Coastal Waters with Nearshore Kelp and Seagrass

Optical properties of chromophoric dissolved organic matter (CDOM) were measured in surf zone waters in diurnal field studies at a Southern California beach with nearshore kelp and seagrass beds and intertidal plant wrack. Absorption coefficients (aCDOM(300 nm)) ranged from 0.35 m21 to 3.7 m21 with short-term variability\u3c1 h, increases at ebb and flood tides and higher values (6 m21) during an offshore storm event. Spectral slopes (S) ranged from 0.0028 nm21 to 0.017 nm21, with higher values after the storm; S was generally inversely correlated with aCDOM(300 nm). 3-D excitation–emission matrix spectra (EEMs) for samples with lower S values had humic-type peaks associated with terrestrial material (A, C), marine microbial material (M) and protein peaks, characteristic of freshly produced organic material. Samples with high S values had no or reduced protein peaks, consistent with aged material. Fluorescent indexes (f450/f500 \u3e2.5, BIX\u3e1.1) were consistent with microbial aquatic sources. Leachates of senescent kelp and seagrass had protein and humic-type EEM peaks. After solar simulator irradiation (4 h), protein peaks rapidly photochemically degraded, humic-type peak C increased in intensity and peak M disappeared. Optical characteristics of kelp leachates were most similar to field samples, consistent with minimal contributions from sea grass, a small component of the biomass at this site. Increases in aCDOM(300 nm) with decreases in S are attributed to the input of freshly produced autochthonous organic material at ebbing and flooding tides, from exudation and microbial processing of senescent plant wrack and nearshore macroalgal vegetation. Other allochthonous sources are hypothesized to be ground water seepage and terrestrial runoff

Production of Acetaldehyde from Ethanol in Coastal Waters

Interest in understanding the cycling of ethanol in the environment has grown as ethanol use as a gasoline additive has increased. The production of acetaldehyde from ethanol was measured in Southern California coastal seawater. The rate of increase of acetaldehyde was positively correlated with the rate constant for ethanol biodegradation and bacteria count and was consistent with two consecutive first-order reactions where acetaldehyde is first biologically produced from ethanol then consumed. Correlation with bacteria counts suggested that acetaldehyde degradation was also a biological process. The rate constants for acetaldehyde production from ethanol and acetaldehyde loss averaged 3.0 ± 3.4 × 10−3 min−1 and 2.3 ± 4.5 × 10−2 min−1 respectively. The branching ratio for acetaldehyde production from ethanol was 0.46 ± 0.26 and estimated acetaldehyde biological production rates ranged from 0.022 to 0.800 nM min−1. With high bacterial counts, biological production rates from ethanol exceeded photochemical production rates from chromophoric dissolved organic matter. Overall, acetaldehyde production rates were larger than biodegradation rates, suggesting these waters are a source of acetaldehyde to the atmosphere. Extrapolation to higher ethanol concentrations associated with spills suggests that the production rate of acetaldehyde will initially increase and then decrease as ethanol concentrations increase

The Aqueous Phase Yield Of Alkyl Nitrates From Roo+No: Implications For Photochemical Production In Seawater

Alkyl nitrates have been observed in remote oceanic regions of the troposphere and in the surface ocean. The mechanism for their production in the oceans is not known. A likely source is the reaction of ROO + NO (where R is an alkyl group). Steady-state laboratory experiments show that alkyl nitrates are produced in the aqueous phase via this reaction, with branching ratios of 0.23 +/- 0.04, 0.67 +/- 0.03, and 0.71 +/- 0.04 for methyl, ethyl, and propyl nitrate respectively. The branching ratios in aqueous solution are significantly higher than in the gas phase. Irradiation of surface seawaters yield rates of alkyl nitrate production on the order of 10(-18) mol cm(-3) s(-1), suggesting that the reaction of ROO and NO is an important source of alkyl nitrates in seawater

Air/Sea Transfer of Highly Soluble Gases Over Coastal Waters

The deposition of soluble trace gases to the sea surface is not well studied due to a lack of flux measurements over the ocean. Here we report simultaneous air/sea eddy covariance flux measurements of water vapor, sulfur dioxide (SO2), and momentum from a coastal North Atlantic pier. Gas transfer velocities were on average about 20% lower for SO2 than for H2O. This difference is attributed to the difference in molecular diffusivity between the two molecules (D SO 2/D H 2O = 0.5), in reasonable agreement with bulk parameterizations in air/sea gas models. This study demonstrates that it is possible to observe the effect of molecular diffusivity on air‐side resistance to gas transfer. The slope of observed relationship between gas transfer velocity and friction velocity is slightly smaller than predicted by gas transfer models, possibly due to wind/wave interactions that are unaccounted for in current models

Optical Characterization and Distribution of Chromophoric Dissolved Organic Matter (CDOM) in Soil Porewater from a Salt Marsh Ecosystem

To characterize chromophoric dissolved organic matter (CDOM) in marsh porewaters and its contribution as a carbon source, optical properties (absorbance, fluorescence indices, 3-dimensional excitation-emission matrices [EEMs]) of soil porewater and surface water were measured in a southern Californian salt marsh. Absorption coefficients and fluorescence intensities were higher in porewater than in overlying surface waters, consistent with higher CDOM concentration at depth. Humic-type peaks A and C were observed in EEMs in all samples, and peak M was observed in surface waters and shallow porewater to -5 cm depth. Fluorescence:absorbance (flu:abs) ratios and spectral slopes (S) decreased across the surface interface, and emission peak maxima were red-shifted—changes that are consistent with increasing molecular weight (MW) and aromaticity in soil porewater due to humification, and lower-MW, less aromatic material in oxic surface waters from oxidative photochemical and biological processing. At lower depths, bands were observed where intensity, flu:abs ratios and S increased; absorption coefficients decreased; emission maxima for humic-type peaks were blue-shifted; and tryptophan-type protein peaks were observed. These changes are consistent with lower-MW and less aromatic material from enhanced microbial activity. Variations in iron concentrations and sulfate depletion with depth were consistent with these bands having different dominant anaerobic microbial metabolic pathways. Overall, optical property trends suggest that soil porewater is a reservoir of CDOM in the salt marsh, with organic material from terrestrial watershed inputs and in situ production from marsh vegetation stored and processed in sediments

Correction to Oceanic Uptake and the Global Atmospheric Acetone Budget

In the paper ‘‘Oceanic uptake and the global atmospheric acetone budget’’ by C. A. Marandino et al. (Geophys. Res. Lett., 32, L15806, doi:10.1029/2005GL023285) it was recently determined that a calculation error was made during flux data processing

Optical Characterization of Chromophoric Dissolved Organic Matter (CDOM) and Fe(II) Concentrations in Soil Porewaters Along a Channel-Bank Transect in a Salt Marsh

Chromophoric dissolved organic matter (CDOM) optical properties were measured in surface and porewaters as a function of depth and distance from the channel in a transect up the bank in a southern California salt marsh. Higher absorbance coefficients and fluorescence intensities in porewaters at depth vs. surface waters and shallower porewaters suggest soil porewater is a reservoir of CDOM in the marsh. Higher values were observed at the marsh sites compared to the channel site, suggesting increased production and storage in the marsh sites, and reduced leaching into overlying surface waters, is occurring. Spectral slope ratios decreased with depth, consistent with more aromatic, higher molecular weight material in the deeper porewaters, possibly due to different bacterial processing in the anaerobic vs. aerobic zones. Fe(II) concentrations, indicative of anaerobic bacterial processing, increased significantly at depth to values \u3e 1000 μM, consistent with active anaerobic microbial processing occurring at depth. The transitions to higher reduced iron concentrations correlated with increased absorbance and fluorescence, suggesting processing by anaerobic iron-reducing bacteria in these deeper zones may not mineralize as much carbon as in the shallower aerobic zones. Alternatively, this may be due to reduction of solid iron oxides coated with organic matter releasing both DOM and Fe(II). The ratio of humic-like fluorescence to the absorption coefficient decreased with increasing iron concentration, possibly due to optical interference by iron species. Taken together, the data indicate that marsh sites in the salt marsh act as a reservoir for higher molecular weight, more aromatic organic matter

Biological Degradation of Acetaldehyde in Southern California Coastal Waters

Oxygenated hydrocarbons are ubiquitous in the atmosphere with levels ranging from low ppt (acetaldehyde) to low ppb (methanol). As an OH sink and an atmospheric HOx and ozone source, oxygenated hydrocarbons have a direct impact on the oxidative capacity of the atmosphere. The oceans are one of the largest sources of uncertainty in current atmospheric budget estimates of these species. A better understanding of the processes that produce and destroy these species in seawater would improve our understanding of the role of the oceans in cycling these species into or out of the atmosphere. We have measured the degradation rate of acetaldehyde in unfiltered and filtered southern California coastal waters. Rates were determined by following the concentrations of D-4 labelled acetaldehyde in spiked (nM levels) seawater in 100ml glass syringes as a function of time. Concentrations were determined by isotope dilution purge and trap gas chromatography mass spectrometry using C-13 labelled acetaldehyde as the internal standard. Degradation rates in 0.2um filtered seawater were not measurable. Degradation rates in unfiltered seawater were first order and ranged from 0.046 to 0.32 hr-1. Bacteria levels were also measured in all samples. Acetaldehyde degradation rates scale with bacteria levels. Variability as a function of time, rainfall and other water quality parameters will be discussed

The Degradation of Acetaldehyde in Estuary Waters in Southern California, USA

Acetaldehyde plays an important role in oxidative cycles in the troposphere. Estimates of its air-water flux are important in global models. Biological degradation is believed to be the dominant loss process in water, but there have been few measurements, none in estuaries. Acetaldehyde degradation rates were measured in surface waters at the inflow to the Upper Newport Back Bay estuary in Orange County, Southern California, USA, over a 6-month period including the rainy winter season. Deuterated acetaldehyde was added to filtered and unfiltered water samples incubated in glass syringes, and its loss analyzed by purge and trap gas chromatography mass spectrometry. Filtered samples showed no significant degradation, suggesting that particle-mediated degradation is the dominant removal process. Correlation between measured degradation rate constants in unfiltered incubations and bacteria counts suggests the loss is due to microorganisms. Degradation in unfiltered samples followed first-order kinetics, with rate constants ranging from 0.0006 to 0.025 min-1 (k; average 0.0043 ± 0.006 min-1). Turnover (1/k) ranged from 40 to 1667 min, consistent with prior studies in coastal waters. Acetaldehyde concentrations in the estuary are estimated to range from 30 to ~500 nM (average ~250 nM). Results suggest the estuary is a source of acetaldehyde to the atmosphere