Carbon Monoxide Photoproduction from Particles and Solutes in the

Article pubs.acs.org/est

Carbon Monoxide Photoproduction from Particles and Solutes in the Delaware Estuary under Contrasting Hydrological Conditions Guisheng Song,†,‡ John D. Richardson,§ James P. Werner,§ Huixiang Xie,*,† and David J. Kieber*,§ †

Institut des Sciences de la Mer de Rimouski, Université du Québec à Rimouski, 310 Allée des Ursulines, Rimouski, Québec G5L 3A1, Canada ‡ College of Marine and Environmental Sciences, Tianjin University of Science and Technology, 29 13th Avenue, TEDA, Tianjin 300457, P.R. China § Department of Chemistry, State University of New York, College of Environmental Science and Forestry, 1 Forestry Drive, Syracuse, New York 13210, United States S Supporting Information *

ABSTRACT: Full-spectrum, ultraviolet (UV), and visible broadband apparent quantum yields (AQYs) for carbon monoxide (CO) photoproduction from chromophoric dissolved organic matter (CDOM) and particulate organic matter (POM) were determined in the Delaware Estuary in two hydrologically contrasting seasons in 2012: an unusually low flow in August and a storm-driven high flow in November. Average AQYs for CDOM and POM in November were 10 and 16 times the corresponding AQYs in August. Maximum AQYs in November occurred in a midestuary particle absorption maximum zone. Although POM AQYs were generally smaller than CDOM AQYs, the ratio of the former to the latter increased substantially from the UV to the visible. In both seasons, UV solar radiation was the primary driver for CO photoproduction from CDOM whereas visible light was the principal contributor to POM-based CO photoproduction. CDOM dominated CO photoproduction in the uppermost water layer while POM prevailed at deeper depths. On a depth-integrated basis, the Delaware Estuary shifted from a CDOM-dominated system in August to a POM-dominated system in November with respect to CO photoproduction. This study reveals that flood events may enhance photochemical cycling of terrigenous organic matter and switch the primary photochemical driver from CDOM to POM.

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INTRODUCTION The ocean is an important source of atmospheric carbon monoxide (CO)1 that regulates the oxidizing capacity of the troposphere through its reaction with the hydroxyl radical.2 Carbon monoxide is used as an energy substrate for a wide range of marine microbes, which oxidize it to carbon dioxide in seawater.3 The rapid microbial consumption of CO and short turnover times4,5 make CO an excellent tracer for tuning upperocean mixing models.6 Despite its microbial consumption and atmospheric efflux, CO is often supersaturated in near-surface waters due to its production from the photochemical oxidation of chromophoric dissolved organic matter (CDOM) in the photic zone, which has long been considered the main source of oceanic CO.4,7,8 Besides CO photoproduction from CDOM, two recent studies have shown that particulate organic matter (POM) photochemistry may also be an important photochemical source of CO in marine waters. Xie and Zafiriou9 assessed the relative contributions of CDOM and POM to CO photoproduction in midlatitude coastal and oligotrophic waters in the NW Atlantic Ocean. Song et al.10 made a similar evaluation for the runoff-impacted Mackenzie estuarine and shelf areas in the Beaufort Sea using a higher-resolution data set that included © 2015 American Chemical Society

spectrally resolved apparent quantum yields (AQY) for the particle-based photoproduction of CO. Both studies demonstrated that the POM term was significant, accounting for 10− 55% of the total CO photoproduction rate. If these results are representative of other oceanic regions, POM photochemistry may provide a sizable new source of CO, tipping the balance of sources and sinks reported by Zafiriou et al.4 and requiring faster microbial oxidation, outgassing, or new sinks to rebalance its budget. Furthermore, the AQY spectra obtained by Song et al.10 revealed that POM was more efficient than CDOM at producing CO at the longer visible wavelengths, and hence POM differed from CDOM in its contribution to the vertical variability in CO concentrations in the water column. Notably, POM-based CO studies have focused on a single season only (i.e., fall in Xie and Zafiriou9 and summer in Song et al.10), making it difficult to assess the seasonality in both the absolute and relative contributions of POM to CO photoproduction. Seasonal variations are expected to be particularly Received: Revised: Accepted: Published: 14048

May 29, October October October

2015 19, 2015 27, 2015 27, 2015 DOI: 10.1021/acs.est.5b02630 Environ. Sci. Technol. 2015, 49, 14048−14056

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Environmental Science & Technology

Figure 1. Distribution of Chl a (A), Kd(PAR) (B), aCDOM(412) (C), ap(412) and aphy(412) (D) versus salinity in August and November 2012 in the Delaware Estuary. For panel C, the dashed and solid lines represent the conservative mixing lines in August and November, respectively.

are given in Toole et al.15 Estuarine water was gravity-filtered through a 20-μm Nitex screen for determination of particlebased absorption spectra and for use in POM-based CO photoproduction experiments. For particulate absorption spectral measurements, 20 μm prefiltered water was passed through a 25-mm-diameter Whatman GF/F filter, and the filter containing the retained particles was stored frozen at −20 °C. As the filter cutoff size for particulate absorption measurement (0.7 μm) was slightly larger than the one for CDOM absorption measurement (0.2 μm), it was assumed that the contribution of the 0.2−0.7 μm colloidal fraction to light absorption, and hence to CO photoproduction as well, was insignificant, as previously demonstrated for water samples from the Mackenzie River Estuary (Song et al., unpublished data). Irradiation. Irradiation experiments were conducted aboard the ship within 30 min of sampling. Water samples were equilibrated with ambient air to lower the initial CO concentration. Samples were then poured into 40-mL Qorpak borosilicate vials filling them with no headspace; vials were capped with green thermoset caps containing Teflon-faced silicone septa. The Qorpak vials were transparent to solar radiation greater than 310 nm, with 50% transmission at 290 nm. All 20-μm filtered water samples were poisoned with potassium cyanide (KCN) at a final concentration of 2.7 mg L−1 in August and 0.27 mg L−1 in November to prevent microbial CO uptake; a higher KCN concentration was used in August due to the high biomass present during that cruise compared to November 2012 (Figure 1A). For each cruise, the same quantity of KCN was added to 0.2-μm filtered samples to minimize systematic biases.9,10 Vials were horizontally immersed in a 3-cm-deep on-deck water bath with a flat black base and filled with continuously flowing ambient nearsurface estuarine water. Samples were irradiated under two spectral treatments: full-spectrum solar radiation and visible

important in temperate and high-latitude estuarine and coastal areas where both the quantity and quality of natural organic matter change due to combined seasonal cycles of in situ biological production, terrestrial runoff, and insolation. This seasonality is expected to be further affected by weather events including droughts or floods. In 2012, the Delaware River basin experienced a very dry summer and received historically high rainfall in late October brought by Superstorm Sandy, resulting in unusually low flow in the summer and flood water levels in November in the Delaware Estuary. Results are presented here that illustrate how these two weather extremes impacted the CO-based photoreactivity of CDOM and POM in the Delaware Estuary. As CO is an important organic matter photoproduct11,12 and a proxy for other major photoproducts such as CO2 and biologically labile DOC,13,14 results from this study have broader implications for organic matter photochemistry and organic carbon cycling in aquatic environments.

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EXPERIMENTAL SECTION Sampling. Estuarine water samples were collected at a depth of ≤1 m along the main axis of the Delaware Estuary in mid-August and mid-November 2012 (Supporting Information (SI) Figure S1) using 12-L Niskin bottles attached to a sampling rosette. For chlorophyll a (Chl a), samples (∼100 mL) were filtered under low vacuum through a 25-mmdiameter GF/F filter, and the filter with retained particles was stored frozen at −20 °C until Chl a extraction and analysis in the home laboratory. Water was gravity-filtered through a precleaned Whatman Polycap filtration capsule containing a 0.8-μm glass-fiber prefilter followed by a 0.2-μm Nylon membrane filter for determination of CDOM absorption spectra and for use in the CDOM-based CO photoproduction experiments. CDOM samples were collected into precleaned 120-mL Qorpak bottles and stored in the dark until analysis. Details regarding glassware and filter precleaning procedures 14049

DOI: 10.1021/acs.est.5b02630 Environ. Sci. Technol. 2015, 49, 14048−14056

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Environmental Science & Technology radiation (≥ ∼400 nm) obtained by filtering out 99% of UV with UF3-Plexiglas. The 0.2-μm and 20-μm filtered samples were exposed to full-spectrum and spectrally filtered solar radiation in duplicate side-by-side in the water bath. Parallel dark incubations were performed with Qorpak vials wrapped in several layers of aluminum foil to monitor potential thermal CO production. The temperature varied by less than 0.5 °C in the shallow water bath during each irradiation. Analysis. CO concentrations were measured using the headspace method outlined by Xie et al.16 The spectral irradiance (290−600 nm) reaching the ship’s deck was determined approximately every 10 min with a NIST (traceable)-calibrated OL-754 spectroradiometer and converted to those entering the Qorpak vials by correcting for the Qorpak vial transmittance. Nitrate actinometry was also used to quantify the time-integrated spectral irradiance (i.e., photon exposure) between 311 and 333 nm entering the irradiation vials exposed to full-spectrum solar radiation.17 The two methods agreed within 2% over the 311−333 nm waveband. The in situ temperature and salinity were also determined during sampling with a CTD (conductivity, temperature, depth) sensor mounted to the sampling rosette. The attenuation coefficient of photosynthetically active radiation (PAR, 400−700 nm) in the water column, Kd(PAR) (m−1), was measured using a Biospherical PNF-210 radiometer. Chl a concentrations and wavelength (λ)-dependent absorption coefficients of CDOM (aCDOM(λ)), particles (ap(λ)), and phytoplankton (aphy(λ)) were determined back in the home laboratory using published techniques. See SI Section S1 for analytical details. CO AQY Calculation. Broadband CO AQYs were obtained by dividing the moles of CO photochemically produced by the moles of photons absorbed by CDOM or particles integrated over 290−600 nm for the full-spectrum treatment and 400− 600 nm for the visible-light spectral treatment, assuming no CO photoproduction at wavelengths greater than 600 nm.10 The amount of photochemically formed CO was taken as the difference in CO concentration between before and after exposure to sunlight, corrected for the cyanide effect (SI Section S2) and any thermal CO production. The total moles of absorbed photons were computed according to Hu et al.18 Calculations confirmed that the number of photons absorbed by CDOM in the 0.2-μm-filtered sample was less than 2% higher than that in the parallel 20-μm-filtered particlecontaining sample, enabling the photons absorbed and hence CO produced by particles to be calculated as the differences between the photons absorbed and CO produced in the 20μm-filtered sample and the corresponding 0.2-μm-filtered sample. Broadband CO AQYs in the UV between 290 and 400 nm were obtained by subtracting results obtained in the visible-light treatments from results in the full-spectrum treatments. Note that light scattering in the water bath or Qorpak irradiation vials posed a negligible effect on the number of photons absorbed within each vial (SI Section S3).

approximately 75% greater than the 5-year-averaged (2007− 2011) monthly mean discharge in November (USGS, http:// www.usgs.gov). Salinity varied from 0.5 to 31.2 in August and from 0.1 to 30.0 in November along the sampling transects. The surface water temperature remained relatively constant throughout the estuary in August (range: 27.6−29.5 °C), except for the three highest salinity stations (Sta. A26−28, Figure S1) where temperatures were lower (22.7−25.3 °C) due to the influence of cooler coastal Atlantic seawater. The surface water temperature in November ranged from 9.4 to 11.6 °C, and was on average approximately 18 °C lower than in August (SI Table S1). In August, Chl a concentrations exhibited considerable patchiness from the head of the estuary to the sea, with slightly higher concentrations in the middle section of the estuary (salinity: 5−20) (Figure 1A). In November, concentrations of Chl a were lower compared to August and fairly uniform throughout the estuary, ranging from 1.91 to 3.54 μg L−1. Seasonally, the mean Chl a concentration in August was nearly 3.5 times higher than that in November. The Kd(PAR) generally decreased seaward in both seasons but several peaks were noted, especially in November, at low and medium salinities (Figure 1B). The Kd(PAR) was much higher in November (range: 1.1−10.1 m−1; mean, 3.74 m−1) than in August (range: 0.42−3.7 m−1; mean, 1.6 m−1). Total suspended matter in the Delaware Estuary and other river plumes has been demonstrated to be linearly correlated to Kd at visible wavelengths.19,20 The higher Kd(PAR) in November was thus at least partly due to a greater abundance of particles in that season, a residual of Superstorm Sandy. CDOM and Particle Absorption Coefficients. Unless otherwise stated, the optical properties of CDOM and particles are reported at 412 nm, as this wavelength is commonly used to remotely quantify ocean reflectance with color sensors such as MODIS. The estuarine mixing behavior of CDOM, as represented by aCDOM(412), was characterized by a convex curve in August and a striking concave curve in November (Figure 1C), demonstrating a net addition of CDOM in August and a net removal of CDOM within the estuary in November across the freshwater−saltwater salinity gradient. This nonconservative mixing behavior contrasts the conservative mixing generally observed for CDOM in the Delaware Estuary21 including August and November 2011 (D. Kieber, unpublished data). The processes giving rise to the nonconservative CDOM mixing behavior seen in the present study are unknown, although they may have been linked to the unusual hydrological conditions during the August and November 2012 cruises. A decrease in the influx of terrigenous CDOM during the lowflow period in the summer would increase the relative proportion of CDOM derived from biological activity within the estuary. In November, the high river runoff led to the freshwater CDOM endmember being nearly three times that in August (e.g., aCDOM(412): 3.24 vs 1.18 m−1). Although there was more CDOM during November at salinities 10 were similar during the two cruises (Figure 1C). This pattern resulted in a concave mixing pattern, suggesting a removal of CDOM at the lower salinities in November possibly due to intensified flocculation driven by an unusually high particle load, a residual of Superstorm Sandy. Alternatively, the curved line could result from a nonsteady state of the CDOM source caused by a rapid increase of the freshwater CDOM endmember during the freshet. A modeling study by Bowers

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RESULTS AND DISCUSSION General Hydrological Features of the Delaware Estuary. The unusually dry summer in 2012 resulted in a monthly mean freshwater discharge in August (119 m3 s−1) that was approximately 40% lower than the August mean discharge averaged from 2007 to 2011. In contrast, the freshet from 28 October to 5 November delivered by Superstorm Sandy gave rise to a daily mean freshwater flow of 600 m3 s−1, which was 14050


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Figure 2. Broadband AQYs of CO photoproduction from CDOM and POM under full-spectrum, UV, and visible radiation in August (A, C, E) and November (B, D, F). Note that the y-axis scales are different among different panels. Vertical bars denote the range of the AQYs (n = 2).

and Brett22 determined that a curved CDOM−salinity profile is observed when the flushing time of an estuary is comparable to the time scale of the CDOM source variation. This might be the case for the Delaware Estuary during the high-flow season in November when both the flushing time of the estuary and the time scale of the source variation were relatively short, probably on the order of a few weeks, when normally the flushing time of the Delaware is on the order of 80 days.23 On the other hand, Kd(PAR), an indicator of total suspended matter, peaked at a salinity 10 and ∼10% lower in fall at salinities >5, consistent with the nonconservative behavior of CDOM observed during our surveys (Figure 1C). In both seasons, particle and phytoplankton absorption coefficients, as represented by ap(412) and aphy (412), respectively, generally decreased with increasing salinity (Figure 1D), a pattern similar to the distribution of suspended particulate matter observed previously in the Delaware

Estuary24 and that of Kd(PAR) observed during our cruises (Figure 1B). On average, ap(412) in November was 1.8 times that in August while aphy(412) in November was 41% lower. The ratio of aphy(412) to ap(412) generally increased seaward, ranging from 0.08 to 0.56 (mean 0.30) in August and from 0.03 to 0.29 (mean 0.11) in November. The ratio of ap(412) to acdom(412) ranged from 0.5 to 2.0 (mean 1.0) in August and from 0.8 to 2.6 (mean 1.44) in November with the maximum ratios corresponding to the ap maximum zones. Broadband CO AQYs (ΦCO). For simplicity, CO AQYs for CDOM and POM are designated as ΦCO‑CDOM and ΦCO‑POM, respectively; wavelength ranges of full-spectrum, UV, and visible are differentiated by FS, UV, and VIS in parentheses following each AQY symbol, e.g., ΦCO‑CDOM(FS) denoting the full-spectrum broadband CO AQY for CDOM. It is further stipulated that each CO AQY symbol can be used as both a singular and a plural form. In both seasons and regardless of the spectral band, ΦCO‑CDOM and ΦCO‑POM showed no consistent trends with salinity, but the pattern of ΦCO‑POM versus salinity generally followed that observed for ΦCO‑CDOM (Figure 2). Values of ΦCO‑POM(FS) and ΦCO‑POM(UV) were consistently lower than those of their CDOM counterparts, while ΦCO‑POM(VIS) were comparable to ΦCO‑CDOM(VIS) such that the ΦCO‑POM/ Φ CO‑CDOM ratio increased from UV to VIS. Notably, ΦCO‑POM(VIS) exceeded ΦCO‑CDOM(VIS) downstream of Sta. N11 (salinity 14.7) in November, showing that POM can be more photoreactive than CDOM in the VIS, consistent with the finding of Song et al.10 in the Mackenzie River Estuary. In 14051


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(and likely less photobleached) signature than in August. However, it should be noted that the lignin yield was unlikely the dominant factor that controlled the CO AQYs in August and November 2012, since they did not consistently decrease with increasing salinity despite a conspicuous seaward decline in the lignin yield, particularly in November. Moreover, increased freshwater discharge is known to augment concentrations of dissolved trace metals, including the photochemically reactive Fe, Cu, and Mn,31 thereby increasing the efficiency of many photoreactions, including CO photoproduction.32 The significantly lower mean daily solar radiation (290−600 nm) in November (460 W m−2) compared to August (1126 W m−2), which would result in less photobleaching, would also give rise to higher AQYs in November.25 It is worth noting that the difference in CO AQYs between the two seasons would have been even larger if water temperature was taken into account, since the average water temperature in November was ∼18 °C lower than in August. Zhang et al.25 reported that a 20 °C increase in temperature resulted in a 30−70% increase in ΦCO‑CDOM in the St. Lawrence Estuary, with low-salinity samples showing the strongest temperature dependence. The temperature dependence of CO photoproduction from POM has not been studied but could be stronger, given that DOC photoproduction from POM increases with temperature at a rate that is twice that for CDOM-based CO photoproduction.33 In addition to the large seasonal variations in CO AQYs, the ratio of ΦCO‑POM to ΦCO‑CDOM also differed seasonally. The ratio of ΦCO‑POM to ΦCO‑CDOM in November averaged 0.44 (±0.19) under full-spectrum solar radiation, 0.36 (±0.29) under UV, and 1.18 (±0.66) under VIS, all of which were considerably larger than those in August, at 0.29 (±0.09), 0.24 (±0.15), and 0.66 (±0.23), respectively. The freshet thus increased the POM photoreactivity more than the CDOM photoreactivity, particularly in the visible regime. Several studies have reported that the solar-spectrum weighted-mean Φ C O ‑ C D O M , which is equivalent to ΦCO‑CDOM(FS), was linearly, positively correlated with aCDOM,10,34,35 with large increases observed when going from the marine end member to the river. The solar-spectrum weighted-mean ΦCO‑CDOM increased by 328% in the spring and 135% in the summer when transitioning from CDOM-depleted seawater to CDOM-enriched freshwater in the Mackenzie River Estuary,34 by 210% in the St. Lawrence River Estuary, and by 410% in the Tyne River Estuary.35,36 In contrast to these studies, ΦCO‑CDOM(FS) in November was only weakly correlated with aCDOM(412) (r2 = 0.44, n = 7), even after excluding the “outlier” at Sta. N11 in the midestuary particle absorption maximum. Likewise, ΦCO‑CDOM(FS) increased only by ∼20% when going from the high-salinity, low-CDOM waters to the low-salinity, high-CDOM endmember, exhibiting a much weaker dependence of ΦCO‑CDOM on aCDOM than seen in the above-mentioned estuaries. Furthermore, no consistent relationship was observed between ΦCO‑CDOM and aCDOM in August. In contrast, White et al.26 observed a strong, linear relationship between ΦCO‑CDOM and aCDOM (r2 = 0.99 at λ = 330 nm) in the Delaware Estuary in the summer of 2002, with ΦCO‑CDOM at 330 nm increasing by 239% from a salinity of 21 to 0.1. For the White et al. study, CDOM mixed conservatively in the Delaware Estuary at salinities >1 in the summer of 2002,26 contrary to the nonconservative behavior observed in the present study. It needs to be determined if processes causing differences in estuarine CDOM dynamics also affected

November, a pronounced maximum (2−3 times the high salinity background) was observed for both CDOM and POM at a salinity of ∼15. These AQY maxima corresponded to a maximum in particle absorption in November (Figure 1D), which approximately coincided with the historical midestuary turbidity maximum zone (TMZ) in the Delaware Estuary.19 Zhang et al.25 also observed a 30% enhancement of ΦCO‑CDOM in the St. Lawrence Estuary TMZ. The processes responsible for this phenomenon are unclear but could be related to differences in the reactivity and sources of organic matter and/ or photoreactive trace metals (e.g., Mn and Fe) in estuarine TMZs compared to other estuarine regions. In August, Φ C O ‑ C D O M (FS), Φ C O ‑ C D O M (UV), and ΦCO‑CDOM(VIS) averaged (±SD) 1.26 × 10−5 (±3.6 × 10−6), 1.79 × 10−5 (±5.1 × 10−6), and 5.0 × 10−6 (±1.7 × 10−6) mol CO (mol quanta)−1, respectively; corresponding values for POM were 3.6 × 10−6 (±1.5 × 10−6), 4.2 × 10−6 (±2.8 × 10−6), and 3.3 × 10−6 (±1.6 × 10−6) mol CO (mol quanta)−1. To compare our broadband AQYs with the spectrally resolved CO AQYs reported by White et al.26 for the Delaware Estuary (ΦCO‑CDOM) and by Song et al.10 for the Mackenzie River Estuary (ΦCO‑POM), we converted the AQYs from the earlier studies to UV- and VIS-broadband AQYs using the approach described in SI Section S4. Our freshwater ΦCO‑CDOM(UV) and ΦCO‑CDOM(VIS) endmembers in August were 40 and 80% lower, respectively, than those observed in the Delaware Estuary in the summer of 2002 by White et al.26 Downstream of the headwater, the summer average ΦCO‑CDOM(UV) from the present study was 30% lower than that in the summer of 2002, whereas the average ΦCO‑CDOM(VIS) from the present study was three and half times higher. Our freshwater ΦCO‑POM(VIS) endmember in August was four times that for the Mackenzie River Estuary determined in August 2009 by Song et al.,10 although this was not the case for the freshwater ΦCO‑POM(UV) endmember from the present study which was 20% lower than that observed in the Mackenzie. These patterns also held for the saline waters in the two estuaries. Our results indicate that there are strong interannual and geographic variations in both ΦCO‑CDOM and ΦCO‑POM, particularly in the VIS regime. In November, ΦCO‑CDOM and ΦCO‑POM were both higher than in August throughout the estuary (Figure 2). The overall average ΦCO under the full-spectrum treatment in November was ∼10 times that in August for CDOM and ∼16 times for POM. The higher AQYs in November could be attributed to the organic matter brought by the storm-induced freshet in November being more photochemically labile. Typically, rivers that discharge to the western North Atlantic Ocean mainly contain old terrigenous DOM and POM resulting from leaching and erosion of deep soil horizons.27 However, flood events will reduce the residence time of water in surface soils28 and thus flush into rivers more fresh, less photochemically altered, organic matter derived from surface biomass, leaf litter, and organic-rich surface horizons. To evaluate the source character of CDOM, we estimated the dissolved organic carbon (DOC)-normalized lignin yield from the spectral slope coefficient between 275 and 295 nm (S275−295) according to Fichot and Benner29 (SI Section S5). The lignin yield of the freshwater endmember in November was three times that in August and the difference decreased with increasing salinity, with the cruise mean in November 65% higher than in August (SI Figure S2). As lignin is an important chromophore and a biomarker of terrigenous DOC,30 this result indicated that CDOM in November possessed a much stronger terrigenous 14052


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Figure 3. Percent contributions of UV and VIS to just-below-the-surface CO photoproduction rates in the Delaware Estuary from CDOM in August (A) and November (B) and from POM in August (C) and November (D) 2012. Vertical lines denote the range of the contributions (n = 2).

the ΦCO‑CDOM versus aCDOM relationships between the two studies. Furthermore, the poor correlation between ΦCO‑CDOM and aCDOM observed in the present study suggests that correlations between these two variables, as reported previously, may not be a common feature for river-impacted ocean margins, but will likely depend on hydrological conditions, in addition to geographical or seasonal influences. In this regard, a recent study by Powers and Miller37 failed to observe a significant relationship between ΦCO‑CDOM and CDOM optical properties, including aCDOM, in the Northern Gulf of Mexico. In the more biologically productive summer season, ΦCO‑POM(VIS) was significantly correlated with aphy(675) (r2 = 0.61) (SI Figure S3). As the absorption peak at 675 nm is characteristic of Chl a (which was much better defined than the 440 nm peak in our case), the correlation between ΦCO‑POM(VIS) and aphy(675) suggests that pigments played an important role in the POM-based CO production. This is consistent with the visible light-induced photodegradation of phytoplanktonic pigments that has been well documented in the literature.38−40 No significant relationship was seen between ΦCO‑POM(VIS) and aphy(675) in November when phytoplankton biomass was much lower. A regression analysis was also done of ΦCO‑POM against aphy(412)/ap(412) but failed to find any significant correlations between the two variables in both August (r2 = 0.26, p = 0.08) and November (r2 = 0.0002, p = 0.98). Relative Contribution of UV and VIS to CO Photoproduction. The relative contributions of UV and VIS to fullspectrum-based CO photoproduction rates just below the water surface in the Delaware Estuary were assessed directly from ondeck sunlight exposure experiments with 0.2-μm and 20-μm filtered water samples. The UV rates were obtained by subtracting the VIS rates from those for the full spectrum. Measured rates accounted for all factors that affected rates, including organic matter absorption, actinic solar radiation spectral qualities and intensities, and CO photoproduction AQYs. For CDOM, the relative contribution of UV far exceeded that of VIS in all samples, with an average (±SD)

contribution from UV of 83% (±4%) in August and 84% (±4%) in November (Figure 3A, B). In contrast to CDOM, the VIS was the main contributor to the POM-based CO photoproduction rate in 71% of the samples (Figure 3C, D), and its average contribution was 62% in both seasons. The percent VIS contribution was particularly striking for some stations including Sta. A12 and A28 (>98%) and Sta. A3, N2, and N5 (81−88%). The average percent contribution of VIS radiation to the POM-based CO photoproduction in the Delaware Estuary was much higher than that observed in the Mackenzie River Estuary (21%),10 due mainly to a much greater ΦCO‑POM(VIS)/ΦCO‑POM(UV) ratio in the Delaware Estuary (0.67 for both seasons) compared to the Mackenzie River Estuary (0.10). However, results from Song et al.10 are comparable to what we observed for the percent contribution of VIS to CDOM-based CO photoproduction in the Delaware Estuary. Relative Contribution of POM to CO Photoproduction. The percent contribution of POM to total CO photoproduction (i.e., CDOM plus POM) just below the water surface under full-spectrum radiation ranged from 14 to 36% (mean ± SD, 20 ± 6%) in August and from 21 to 60% (mean ± SD, 33 ± 13%) in November. The higher contribution in November was primarily due to the significantly higher ap/aCDOM and ΦCO‑POM/ΦCO‑CDOM ratios in that season as previously discussed. The relative contribution of POM to total CO photoproduction was also assessed on a depth-integrated basis. Depth-integrated CO photoproduction rates in the water column (Pcol, mol m−2 s−1) were calculated between 290 and 600 nm from eq 1, which was modified from Song et al.10 600

Pcol = ΦCO(FS)

∫290

Q 0(λ)(a(λ)/a t(λ))dλ

(1)

where Q0(λ) (mole quanta m−2 s−1 nm−1) is the on-deck spectral irradiance measured with the OL-754 spectroradiometer; at(λ) (m−1) is the sum of aCDOM(λ), ap(λ), and the absorption coefficient of pure water, aw(λ);41,42 ΦCO(FS) and a(λ) (m−1) denote the ΦCO‑POM(FS) and ap(λ) when 14053


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production rate was considerably higher, on average by 17% (±5%) in August and by 23% (±8%) in November, than that observed just below the surface. The greater contribution of POM to the depth-integrated rate resulted from a slower attenuation of VIS radiation with increasing depth compared to UV, as indicated by differences in the 1% light penetration depth (Table 1), and an increase in the ΦCO‑POM/ΦCO‑CDOM ratio from the UV to VIS as previously discussed. Finally, except for Sta. N5, POM-based depth-integrated CO photoproduction rates exceeded CDOM-based rates throughout the Delaware Estuary in November; this was not the case for August other than for the upstream particle absorption maximum Sta. A7. In November, the predominance of POM over CDOM could be attributed to ap outweighing aCDOM (Figure 1C,D), Φ CO‑POM (VIS) that were greater than ΦCO‑CDOM(VIS) (Figure 2F), and VIS irradiances that were much larger than corresponding UV irradiances. In August, the high ap/aCDOM ratio at Sta. A7 (2.0) was likely the main factor affecting depth-integrated rates, since ΦCO‑POM(VIS) was generally lower than ΦCO‑CDOM(VIS) at that location (Figure 2E). Implications for Organic Carbon Cycling. Carbon monoxide is one of the main photoproducts detected in marine waters,12 and it can be detected at very low concentrations with high precision.43 It is for these reasons that CO has served as a general proxy for other photoproducts including CO2 and biolabile carbon that are produced during the exposure of filtered water samples to solar radiation.12 Thus, results obtained for CO can provide insights for organic carbon cycling in general in the dissolved phase. However, the proxy role of CO in POM photochemistry has not been examined. Therefore, it is not known whether trends and ratios observed in the dissolved phase can translate into the particulate phase,9 and hence the contribution of particle photochemistry is still unknown to marine carbon cycling.12 Nonetheless, the POM results may be even more important, since POM is subject not only to photomineralization but to photodissolution as well, producing dissolved organic matter.44−47 The results obtained from the present study may thus have broader significance beyond CO photoproduction and cycling in estuarine waters. The seasonality in CO-based photoreactivity of CDOM and POM observed in this study highlights the need to assess how meteorological extremes, which are predicted to take place more frequently due to global climate change,48 impact organic carbon cycling in aquatic environments. Given that ΦCO‑CDOM and ΦCO‑POM in the Delaware Estuary during an unusually highflow period in November were >10-fold higher than those during an unusually low-flow period in August, flood events are expected to accelerate photochemical cycling of terrigenous organic matter. However, the shorter residence time of CDOM and POM and the rapid attenuation of UV and PAR in the water column associated with these flood events would temper this effect in estuaries. Much of the increased rate of photochemical cycling is expected to take place in the more optically transparent coastal waters where photolysis of the reactive, flood-derived POM and CDOM would occur deeper in the water column due to deeper penetration of solar radiation, especially in the UV. The increasing ΦCO‑POM/ΦCO‑CDOM ratio from the UV to VIS suggests that a CDOM-dominated photochemical system can transition to a POM-dominated system in environments where ap is comparable to or greater than acdom. Likewise, in POM-

calculating POM-based CO photoproduction rates and ΦCO‑CDOM+POM(FS) and aCDOM(λ) plus ap(λ) when calculating total CO photoproduction rates due to CDOM and POM. Solar radiation reflected by the air−water interface and backscattered from the water column were not considered, since our focus was on the ratio of the POM-based to total CO production and not individual fluxes. The uncertainty associated with using the broadband CO AQY in eq 1, which neglected its spectral dependence, was evaluated using published CO AQY spectra.10 Results indicated that the particle contributions to total CO photoproduction were underestimated by 0−6% (SI Section S6 and Table S2), validating the method adopted in this study. Evaluations were made for all sampling stations (Table 1) and several important findings were evident. First, the percent contribution of POM to depth-integrated CO photochemical production rates was greater in November than in August, consistent with the just-below-the-surface results. Second, the percent POM contribution to the total depth-integrated Table 1. Percent Contributions of POM to Total CO Photoproduction Rates Just Below the Water Surface and Depth-Integrated to the 1% Light Level for 600-nm Solar Radiation POM contribution (%) station

water depth (m)

A1 A3 A7 A10 A11 A12 A13 A16 A17 A20 A21 A23 A28

14.1 9.4 12.5 15.2 12.3 9.8 9.3 13.1 14.7 13.5 10.9 13.4 14.1

N2 N5 N6 N11 N13 N14 N15 N22

11.1 18.9 14.8 14.2 14.2 13.7 15.7 17.0

surface

water column

August Cruise 19 39 20 46 36 55 19 36 25 42 14 34 16 24 15 29 20 31 14 33 20 33 25 37 19 40 November Cruise 23 53 21 48 40 51 60 77 24 52 31 53 36 51 31 62

1% penetration depth (m)a 320 nm 400 nm

600 nm

0.47 0.41 0.40 0.49 0.50 0.57 0.59 0.59 0.78 0.76 1.37 1.21 2.87

1.47 1.23 1.13 1.54 1.59 1.87 1.99 1.90 2.41 2.65 4.15 3.57 10.3

9.74 8.29 7.63 10.0 9.77 >9.8b >9.3b 11.0 12.3 >13.5b >10.9b >13.4b >14.1b

0.22 0.27 0.41 0.39 0.80 0.76 0.76 1.27

0.55 0.78 1.26 0.99 2.79 2.54 2.59 3.94

4.55 6.41 8.79 7.09 13.5 12.7 13.2 14.0

a

Transmission of solar radiation in the water column is assumed to follow the model of Ez(λ) = E0(λ) × exp(−Kd(λ) × z), where E0(λ) and Ez(λ) are downwelling irradiances at the surface and depth z (m), respectively, and Kd(λ) denotes the diffusion attenuation coefficient (m−1). The 1% light penetration depth is defined as the depth at which Ez(λ)/E0(λ) × 100 = 1%. The upper limit of the 1% light penetration depth was estimated by approximating Kd(λ) as the sum of aCDOM(λ) and ap(λ), and ignoring in situ particle scattering which was not measured. bThe depth corresponds to the total water-column depth at these stations, since the 1% penetration depth exceeded the total water-column depth. 14054


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Nature et technologies (FRQNT). Comments from three anonymous reviewers greatly improved the manuscript.

rich waters, the relatively rapid attenuation of UV radiation in the water column will result in a transition from a CDOMdominated system for CO photoproduction in near surface waters to POM- and VIS-dominated CO photoproduction deeper in the water column, as seen in the Delaware Estuary wherein the water column is characterized photochemically as a continuum between dissolved solutes and solids. The greater contribution of POM observed in November in the Delaware Estuary demonstrates that flooding can shift a CDOM- to a POM-dominated photochemical environment by disproportionally increasing the photoreactivity of POM and the particle absorption in comparison to CDOM. The highly elevated ΦCO‑CDOM and ΦCO‑POM observed in the midestuary particle absorption maximum in November illustrates the extent to which estuarine processes can modify the photoreactivity of dissolved and particulate organic matter. Moreover, the predominance of POM-based over CDOMdriven CO photoproduction in the particle absorption maximum during the low-flow period in August indicates that POM photochemistry can outweigh its CDOM equivalent in highly turbid waters even if AQYs of POM photoprocesses are much lower than that for CDOM. This conclusion is consistent with the suggestion that DOC production from the photodissolution of POM exceeds photomineralization of CDOM in particle-laden estuarine and coastal waters.33 As turbid water bodies are widespread, the implication of this phenomenon for organic matter cycling warrants further investigation.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b02630. Additional information on sampling details, analytical methods, effect of cyanide on CO photoproduction, calculation of absorbed photons, conversion of spectrally resolved CO AQYs to broadband CO AQYs, calculation of lignin yields, correlation between ΦCO‑POM(VIS) and aphy(675), and uncertainty analysis of POM contributions to depth-integrated CO photoproduction (PDF).

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Phone: 1 418 723-1986 × 1767; fax: 1 418 724-1842; e-mail: [email protected]. *Phone: 1 315 470-6951; fax: 1 315 470-6856; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank David Kirchman, the Chief Scientist on the Delaware Estuary cruises, for providing the Chl a, Kd(PAR), and DOC data, Simon Bélanger for using the PerkinElmer Lambda 850 Spectrometer, and the crew of the R.V. Sharp cruises for their logistical support. We gratefully acknowledge support from National Science Foundation grant OCE-1029569 (D.J.K.), a State University of New York, College of Environmental Science and Forestry Honor’s Fellowship (J.D.R.), and a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant (H.X.). G.S. was supported by graduate scholarships from the Institut des Sciences de la Mer de Rimouski (ISMER) and Fonds de recherche du Québec − 14055


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