Chromophoric Dissolved Organic Matter Photobleaching Research Articles

Photochemical production and consumption of ammonium in a temperate river–sea system

We carried out 12 irradiation experiments under artificial light using samples collected from two UK upland temperate rivers, the Tyne and the Tay, and from the Tyne estuary and adjacent coastal North Sea waters. During these experiments, we followed concentration changes of ammonium (NH 4 +) and spectral changes of chromophoric dissolved organic matter (CDOM). In all experiments CDOM absorbance followed pseudo-first-order loss kinetics and photochemical NH 4 + production occurred consistently. However, the patterns of NH 4 + concentration changes with time were complex. Experiments in which irradiated waters were sampled at approximately 10 min intervals revealed an initial lag phase of ∼ 30 to 120 min during which NH 4 + concentrations did not change significantly. This lag phase was followed by a net photoproduction phase, and finally an apparent NH 4 + consumption phase. Such behaviour has not previously been reported and appeared to be independent of sample origin, initial NH 4 + concentration and initial CDOM absorption coefficient at 350 nm ( a 350). The observed behaviour could be described by a 3-stage irreversible reaction model: A → B → NH 4 + → D with first-order rate constants ka, kb and kc for the 3 consecutive reaction steps. Both ka and kc were significantly correlated ( p < 0.05) to the logarithm of the product of a 350 and salinity over the CDOM spectral slope ( S 290–350) through multiple linear regression analysis, suggesting that dissolved organic matter (DOM) properties associated with CDOM absorbance characteristics play an important role in this photochemical process. NH 4 + photoproduction rates, calculated over the lag- and production-phases, were significantly correlated with the initial a 350 for the River Tyne (including tributaries) samples, but not when samples from the estuary, coastal North Sea and River Tay were included. We also tested for possible correlations between CDOM photobleaching and spectral properties, but found no significant correlations between NH 4 + photoproduction rates and the initial spectral slope of CDOM or the CDOM photooxidation rate constants at 285, 310, 350 and 390 nm.

Photochemistry of Cu complexed with chromophoric dissolved organic matter: implications for Cu speciation in rainwater

Significant quenching of fluorescence by Cu in rainwater samples from southeastern North Carolina demonstrates that chromophoric dissolved organic matter (CDOM) is an effective ligand for Cu in rainwater. A strong inverse correlation between the decrease in fluorescence upon Cu addition and CDOM abundance suggests the presence of excess binding sites for Cu in high CDOM samples. Electroanalytical studies indicate that CDOM extracted from C18 cartridges formed Cu complexes with concentrations and conditional stability constants similar to ligands found in ambient rainwater. When authentic rainwater samples were photolyzed with simulated sunlight both photoproduction and photodestruction of ligands were observed, suggesting the photochemical response of Cu-complexing ligands in rainwater is the result of two competing reactions. The rate of CDOM photobleaching was directly related to changes in strong ligands (KCuL ∼ 1015) whereas weaker ligands (KCuL < 1013) were not correlated, suggesting the photolabile CDOM resides in the strong ligand class. A photolysis study comparing filtered and unfiltered rainwater samples indicated that Cu-complexing ligands adsorbed onto or otherwise associated with particles are photodegraded much more rapidly than dissolved ligands. Photolysis with UV radiation appears to be most effective at engendering changes in Cu ligands, however a significant photochemical response was also observed when samples were exposed to photosynthetically active radiation with wavelengths greater than 400 nm. Results from this study demonstrate that complexation of Cu by CDOM has important ramifications for controlling both the speciation of the metal and the reactivity of CDOM in rainwater.

Factors Affecting the Efficiency of Carbon Monoxide Photoproduction in the St. Lawrence Estuarine System (Canada)

This study examined the effects of water temperature and the origin (terrestrial vs marine) and light history of chromophoric dissolved organic matter (CDOM) on the apparent quantum yields of carbon monoxide (CO) photoproduction for water samples collected along a salinity gradient (salinity range: 0-33) in the St. Lawrence estuarine system (Canada). The solar insolation-weighted mean apparent quantum yield of CO (phico) decreased as much as fourfold with increasing salinity and showed a strong positive correlation with the dissolved organic carbon-specific absorption coefficient at 254 nm. This suggests that terrestrial CDOM is more efficient at photochemically producing CO than is marine algae-derived CDOM and that aromatic moieties are likely involved in this photoprocess. CDOM photobleaching, mainly at the very early stage, dramatically decreased phico (by up to 6.4 times) for low-salinity samples, but photobleaching had little effect on the most marine sample. For a 20 degree C increase in temperature, phico increased by approximately 70% for low-salinity samples and 30-40% for saline samples. This study demonstrates that water temperature, as well as the CDOM's origin and light history, strongly affect the efficiency of CO photoproduction. These factors should be taken into account in modeling the photochemical fluxes of CO and other related CDOM photoproducts on varying spatiotemporal scales.

Photochemical production of ammonium in the oligotrophic Cyprus Gyre (Eastern Mediterranean)

Abstract. We investigated the photoproduction of ammonium (NH4+) in surface waters of the Cyprus gyre in the central Eastern Mediterranean in May 2002, in 8 on deck irradiations with freshly collected, filtered samples. NH4+ photoproduction (photoammonification) increased with time-integrated irradiance during the course of irradiations. Photoammonification rates around local noon were 0.4–2.9 nmol L−1 h−1. Normalised to time integrated irradiance, these rates were 0.9–3.8 pmol L−1 h−1/(W m−2) and were significantly correlated with Chromophoric Dissolved Organic Matter (CDOM) absorbance at 300 nm normalised to Dissolved Organic Carbon (DOC). These results are consistent with the notion that successive CDOM photobleaching in the surface mixed layer results in decreased DOC-normalised light absorbance concurrent with decreased dissolved organic matter reactivity with regard to photochemical NH4+ release. Combining our experimental data with estimates of annual solar irradiance and water column light attenuation yields an annual photoammonification rate for the Cyprus Gyre of 40±17 mmol m−2 a−1, equivalent to ~12±5% of the previously estimated annual nitrogen requirement of new production and in the same order of magnitude as atmospheric N deposition in this region. Based on this analysis, NH4+ photoproduction makes a small, but significant contribution to the nitrogen budget of the euphotic zone in the oligotrophic Cyprus Gyre.

Photochemical production of hydrogen peroxide and methylhydroperoxide in coastal waters

Hydrogen peroxide (H 2O 2) has been observed in significant concentrations in many natural waters. Because hydrogen peroxide can act as an oxidant and reductant, it participates in an extensive suite of reactions in surface waters. Hydrogen peroxide is produced as a secondary photochemical product of chromophoric dissolved organic matter (CDOM) photolysis. Apparent quantum yields for the photochemical production of hydrogen peroxide were determined in laboratory irradiations of filtered surface waters from several locations in the Chesapeake Bay and in Arctic coastal waters with varying levels of CDOM. The apparent quantum yield for H 2O 2 decreases by about an order of magnitude from 280 nm to 500 nm, and the majority of H 2O 2 production occurs at wavelengths less than 340 nm. The apparent quantum yield for H 2O 2 production at 290 nm ranged from 4.2 × 10 − 4 to 2.1 × 10 − 6 mol H 2O 2 (mol photons) − 1 from freshwater to marine waters. A linear relationship was found between the production of H 2O 2 and change in CDOM absorbance characterized as photobleaching (loss of absorbance). No significant relationship was observed between DOC concentration and peroxide production. Methylhydroperoxide (CH 3O 2H) was the only short chain peroxide produced during the irradiations, and its production is at least an order magnitude less than that of hydrogen peroxide. Peroxide production was greatest in waters containing significant amounts of terrigenous C in the form of humic substances. Surface waters whose synchronous fluorescence spectra indicated the presence of polyaromatic and/or extensive conjugated compounds exhibited the greatest peroxide production. CDOM photobleaching is not significantly linked to apparent quantum yields for peroxide production.

Spatial and seasonal distribution of chromophoric dissolved organic matter and dissolved organic carbon in the Middle Atlantic Bight

Spatial and seasonal distributions of chromophoric dissolved organic matter (CDOM) absorption, fluorescence (excited at 355 nm; F n) and dissolved organic carbon (DOC) concentration in coastal waters of the Middle Atlantic Bight (MAB) were acquired. A linear relationship was observed between CDOM absorption at 355 nm [ a CDOM(355)] and F n on the shelf of the MAB ( a CDOM(355)≤1 m −1), but this relationship curved downward for higher a CDOM(355) and F n values in the Chesapeake Bay (CB) and the Delaware Bay (DB) source waters ( a CDOM(355)≥1 m −1), suggesting the presence of different end members with higher F n/ a CDOM(355) ratios, the occurrence of a photochemical/biological processing of the CDOM as it moves down the estuaries or the involvement of both of these processes. Strong evidence was acquired for the photobleaching of CDOM in the surface waters of the MAB during summertime stratification, as indicated by the highly nonlinear dependence of a CDOM(355) and F n on salinity in surface waters, the concomitant increase of the CDOM spectral slope parameter ( S) and the much lower ratio of a CDOM to DOC ( a CDOM/DOC) observed in surface waters. The a CDOM/DOC ratio decreased by over an order of magnitude with increasing salinity, owing primarily to two reasons: (1) the substantial photobleaching of CDOM in the surface waters of the shelf and (2) the very different content of CDOM and DOC in the freshwater and oceanic end members. These results indicate that CDOM represents only a small portion of the DOC pool in offshore waters and that the sources and sinks of CDOM and DOC are uncoupled despite the often-observed correlation between CDOM and DOC in the coastal environment.

Calculation of spectral weighting functions for the solar photobleaching ofchromophoric dissolved organic matter in temperate lakes

The effect of solar radiation on the dissolved absorption coefficient (λCDOM[λ]), 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 µm 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 αCDOM[λ] (the αCDOM[λ] 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 (&lt;20% of total decrease in the absorption coefficient) compared to UVA 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 (&lt;10%).

Hydrologic control and diurnal photobleaching of CDOM in a subarctic lake

Changes in the concentration of chromophoric dissolved organic matter (CDOM) were determined over short (hours to days) and medium (weeks) time scales during summer in a shallow, polymictic lake in subarctic Quebec, Canada. CDOM fluorescence (F CDOM ) decreased linearly from mid-June until early August, consistent with the zero-order kinetics expected for photodegradation of organic matter in a regularly mixed lake in which most of the photochemically-active radiation is absorbed by CDOM. A major rainfall event then resulted in a sharp increase in F CDOM back to levels recorded at the start of the study. These changes showed that CDOM concentrations were controlled by the balance between catchment inputs and subsequent photodegradation processes within the lake. Shorter term changes in CDOM concentration were monitored by measuring the penetration of ultraviolet radiation (UVR) using a Satlantic TACCS system moored near the center of the lake and set to monitor down-welling 380 nm irradiance at 10 min intervals. The diffuse attenuation coefficient K d (380) decreased by 12 % during the deployment indicating increased penetration of UVR and the change paralleled the drop in F CDOM over the same period. Many of the daily records showed a significant decline in K d (380) in the near-surface waters over the course of the morning and early afternoon indicating diurnal photobleaching of CDOM, and an increase later in the day consistent with the breakdown of the diurnal thermocline and entrainment of deeper waters. The results illustrate the short and medium term dynamics of CDOM, the importance of diurnal stratification and mixing for photochemical processes, and the variability that aquatic organisms must experience in UV exposure in their natural environment.

Modeled Effects of Dissolved Organic Carbon and Solar Spectra on Photobleaching in Lake Ecosystems

Dissolved organic matter (DOM) contains molecules that absorb light at various wavelengths. This chromophoric DOM (CDOM) influences the transmission of both visible and ultraviolet energy through water. The absorption of light by CDOM often causes structural changes that reduce its capacity to further absorb light, a process termed ‘photobleaching‘. A model was designed to assess photobleaching through the entire water column of lake ecosystems. The model uses lake morphometry and dissolved organic carbon (DOC) concentration in conjunction with a defined solar spectrum and experimentally measured photobleaching rates to compute the total water columm photobleaching. The model was initially applied to a theoretical ‘average‘ lake using solar spectra for both the north (N) and south (S) temperate western hemispheres and variable DOC from 0.3 to 30 mg L 21 . The consequences of varying waveband-specific photobleaching coefficients and lake morphometry were explored in a second set of simulations. Finally, the model was also applied to four temperate northern lakes for which we had prior measurements of CDOM photobleaching rates. The model demonstrates that all three wavebands of solar radiation (UVB, UVA, and PAR) contribute significantly to total water column photobleaching, with UVA being most important. The relative contributions of the three wavebands were invariant for DOC more than 3 mg L 21 . Total water column photobleaching at 440 nm was three to five times faster under the UV-enriched solar spectrum of the southern hemisphere. Increasing the lake’s mean depth (from 0.37 to 9.39 m) resulted in five- or 15-fold slower rates of total water column photobleaching for DOC concentrations of 1 or 10 mg L 21 , respectively. Varying the waveband-specific photobleaching coefficients by 10-fold resulted in a similar change in total water column photobleaching rates. Applying the model to four specific lakes revealed that photobleaching for the entire water column would reduce CDOM light absorption by 50% in 18‐44 days under summer conditions.

Interactions of ultraviolet‐B radiation, mixing, and biological activity on photobleaching of natural chromophoric dissolved organic matter: A mesocosm study

A natural planktonic assemblage from the St. Lawrence Estuary was isolated in eight 1,500‐liter outdoor meso‐cosms and subjected to combinations of fast or slow mixing regimes with natural solar radiation or natural solar radiation artificially enhanced with ultraviolet‐B (UVB, 280–320 nm) radiation. The interdependent evolution of dissolved organic carbon (DOC), absorption by chromophoric dissolved organic matter (CDOM), chlorophyll a (Chl a), particulate organic carbon (POC), and bacterial abundance in the mesocosms was followed over a 10‐d period. There was a net increase of Chl a, POC, and DOC in all systems over time; however, the slower mixing treatments had less accumulation than the systems with faster mixing. All systems displayed weak correlations of DOC with POC and Chl a. A significant effect of enhanced UVB radiation on concentrations of these bulk properties was not observed in any of the mesocosms. A strong correlation of CDOM absorbance loss (photobleaching) with absorbed radiation dose was observed in all treatments, with the fast mixing systems having larger absorbance losses and faster loss rates. Photobleaching was wavelength dependent, resulting in an increase in the spectral slope of CDOM absorption over time. Thus, although CDOM photobleaching may result in deeper penetration of light at all wavelengths, the ratios of UVB to ultraviolet‐A (UVA) and photosynthetically active radiation (PAR) are reduced. The effect of enhanced UVB radiation was unexpected, with no proportional increases in CDOM photo‐bleaching in the #x002BUVB treatments. Comparisons of the different treatments indicate that interactions of biological activity, mixing, and the in situ light field can influence CDOM absorbance properties and/or photoreactivity and that there is a possible role for UVB in the production of CDOM.