Calculation of spectral weighting functions for the solar photobleaching of chromophoric dissolved organic matter in temperate lakes (original) (raw)
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Modeled Effects of Dissolved Organic Carbon and Solar Spectra on Photobleaching in Lake Ecosystems
Ecosystems, 2000
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 Ϫ1. 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 Ϫ1. 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 Ϫ1 , 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.
The attenuation of solar UV radiation in lakes and the role of dissolved organic carbon
Limnology and …, 1995
Diffuse attenuation coefficients (&) for solar UV radiation (UVR) (305, 320, 340, 380 nm, and PAR) were measured in the mixed layer of 65 lake sites in Alaska, Colorado, and Pennsylvania and the Bariloche region of Argentina. Integrated mixed layer samples of lake water were concurrently collected, and a multivariate approach was used to model Kd with a number of optical and chemical variables.
Replicated mesocosm study on the role of natural ultraviolet radiation in high CDOM, shallow lakes
The role of ultraviolet radiation on shallow, high CDOM (colored dissolved organic matter) lakes was investigated during two consecutive summers (1999 and 2000) in replicated mesocosms (rectangular fiberglass tanks). Each tank (volume: 300 L; depth: 40 cm) was covered with a layer (∼3 cm) of sediment from lake El Toro (40Њ 14Ј S; 70Њ 22Ј W) and filled with filtered water. The experimental design consisted of two treatments: full natural radiation (UV-exposed) and natural radiation without ultraviolet radiation (UV-shielded). UV-exposed and UV-shielded treatments differed in most studied variables as revealed by repeated measures ANOVA. UV-exposed tanks displayed lower CDOM levels (dissolved absorbance) of lower average molecular size (absorbance ratio between 250 and 365 nm), higher bacterial biomass, and lower chlorophyll a concentration. The effect on consumers (rotifers and crustaceans) was less noticeable. The results are consistent with UV stimulation of bacteria production mediated by higher rates of CDOM photobleaching, and the photoinhibition of planktonic algae. Thus, a major effect of UVR in shallow, high CDOM ecosystems appears to be the stimulation of heterotrophic pathways and a simultaneous inhibition of photoautotrophs.
Limnology, 2007
The increase of ultraviolet radiation (UVR, 280-400 nm) caused by stratospheric ozone depletion has profound effects on aquatic ecosystems. High-altitude lakes in the Yunnan Plateau are exposed to high intensities of UVR and contain low concentrations of chromophoric dissolved organic matter (CDOM). Thirty-eight lakes in the Yunnan Plateau with elevations from 1291 to 3809 m above sea level were investigated to study CDOM concentrations and possible effects of UVR on the lake ecosystem. The attenuation of UVR in the Yunnan Plateau lakes was calculated from the absorption coeffi cient of CDOM based on an empirical relationship from lakes in the Alps and Pyrenees mountains. Absorption coeffi cients [a(λ)] at 320 nm [a(320)] ranged from 0.52 to 14.05 m −1 (mean ± standard deviation, 4.40 ± 3.85 m −1 ) and at 380 nm [a(380)] from 0.05 to 4.51 m −1 (1.40 ± 1.30 m −1 ). The exponential slope coeffi cient for the relationship of wavelength to a(λ) ranged from 16.2 to 41.4 µm −1 (21.74 ± 4.93 µm −1 ) over the 280-400 nm interval. Normalized fl uorescence emission (NFLU) at 450 nm from an excitation wavelength of 355 nm, F n (355), averaged 7.93 ± 3.22 NFLU. A signifi cant positive relationship was found between a(355) and F n (355). The estimated diffuse attenuation coeffi cients of UV-B (320 nm) and UV-A (380 nm) ranged from 0.55 to 15.77 m −1 and from 0.24 to 6.73 m −1 ; the corresponding 1% attenuation depths ranged from 0.29 to 8.44 m and from 0.68 to 19.12 m. Twenty-fi ve of 38 lakes had 1% UV-B attenuation depths of 1.5 m or more. The median 1% attenuation depth was 28.8% of the sampling depth for UV-B radiation and 60% for UV-A. In addition to CDOM, chlorophyll a (Chla) and total suspended matter (TSM) also may contribute to attenuation of UVR.
Écoscience, 2015
High-latitude lakes usually have a high penetration of light, due to their low productivity and low concentration of dissolved organic matter (DOM), but large variations in lake optical properties can be found within and between regions. We investigated the underwater light regimes in relation to DOM in 18 oligotrophic, high-latitude lakes across mountain birch woodland, shrub tundra and barren tundra in NW Finnish Lapland. DOM variability was measured by quantification of organic carbon and analysis of UV-visible absorbance and fluorescence spectra. In 12 out of 18 lakes > 1% of PAR reached the lake bottom while UV radiation exposure was more variable with 1% UVB depth ranging from 0.1 to > 12 m. Lakes located in barren tundra had highest transparency, lowest DOC concentration and lowest chromophoric DOM (CDOM) absorption (mean values: Kd PAR 0.3 m-1 , DOC 2.1 mg l-1 , a440 0.4 m-1), while lakes in shrub tundra and mountain birch forest were in general less transparent although still clear with a mean DOC concentration of 4.7 mg l-1 and CDOM absorption (a440) of 1.4 m-1. Solar attenuation and lake transparency were correlated with CDOM absorption (a440), but the relationship was affected by the quality of organic matter and the concentration of DOC. Our survey emphasizes the importance of catchment type on DOM characteristics and lake optics and suggest that changes in vegetation zones will alter the overall aquatic light milieu in oligotrophic high-latitude lakes. We predict that even small changes in CDOM quality may largely change the UV radiation exposure of the studied high latitude lakes with likely consequences on biota while changes in PAR may have smaller biological effects in these shallow lakes that are already illuminated to the bottom even in the darkest systems.
Limnology and Oceanography, 2000
High-altitude lakes are exposed to high fluence rates of solar ultraviolet radiation (UVR; 290-400 nm) and contain low concentrations of dissolved organic carbon (DOC). While in most lowland lakes, DOC can be used to predict UV transparency with sufficient accuracy, current models fail to estimate UVR in clear alpine lakes. In these lakes, phytoplankton may contribute significantly to the UV attenuation either as particles or as a source of chromophoric dissolved organic matter (CDOM) with distinctive properties. We investigated a series of 26 lakes in the Alps and Pyrenees, situated at elevations ranging from 422 to 2,799 m above sea level and having DOC concentrations ranging from 0.2 to 3.5 mg L Ϫ1 . CDOM, as measured by the absorptivity of filtered lake water, explained most of the variability in the attenuation of underwater UVR among lakes (r 2 ϭ 0.94, P Ͻ 0.001). However, within-lake variation in the UV attenuation revealed a significant contribution from phytoplankton in deeper waters (UV attenuation increasing with chlorophyll a concentration; r 2 ϭ 0.97, P ϭ 0.002), only apparent when DOC concentrations were low (ϳ0.3 mg L Ϫ1 ). The DOC-specific absorptivity (a g *) was also important for characterizing the optical conditions in this series of lakes. Epilimnetic values of a g * were significantly lower in lakes located at high elevations (with low allochthonous CDOM inputs from the catchment), compared to lakes surrounded by trees and meadows. Moreover, a g * was generally lower in surface waters than in deeper water layers, suggesting the influence of photobleaching on UV transparency. The slope S of the exponential regression between CDOM absorptivity and wavelength did not show clear patterns, such as found in marine systems, and often presented lower values in the epilimnetic waters (in association with lower a g *). Collectively, our results suggest that in transparent alpine lakes, the dynamics of the CDOM pool and phytoplankton production will have a strong effect on temporal changes in UV underwater attenuation. Solar ultraviolet-B radiation (UVB; 290-320 nm) has increased during the last 15 yr over many Earth's locations as a consequence of the degradation of the stratospheric ozone layer. Beside Antarctica, where the increment is notorious,
Biogeosciences Discussions, 2016
The development and validation of remote sensing-based approaches for the retrieval of CDOM concentrations requires a comprehensive understanding of the sources and magnitude of variability in the optical properties of dissolved material within lakes. In this study, spatial and seasonal variability in concentration and composition of CDOM and the origin of its variation was studied in Lake Balaton (Hungary), a large temperate shallow lake in central Europe. In addition, we investigated the effect of photobleaching on the optical properties of CDOM through in-lake incubation experiments. There was marked variability throughout the year in CDOM absorption in Lake Balaton (aCDOM (440) = 0.06–9.01 m−1). The highest values were consistently observed at the mouth of the main inflow (River Zala), which drains humic-rich material from the adjoining Kis-Balaton wetland, but CDOM absorption decreased rapidly towards the east where it was consistently lower and...
2010
1] In 2004, ultraviolet-B (UVB) and ultraviolet-A (UVA) attenuation were measured in Mackenzie Delta lakes spanning gradients in water renewal rate, dissolved organic carbon (DOC) concentration, and dissolved organic matter (DOM) composition. DOM compositions (ratio of chromophoric DOM (CDOM) to non-chromophoric DOM) in Delta lakes are complex, evolving seasonally via flooding, dilution, macrophyte production, photobleaching, and bacterial metabolism. Attenuation was more strongly related to CDOM absorption coefficients (a 330 ; UVB r 2 = 0.69, p < 0.0001; UVA r 2 = 0.58, p < 0.0001) than to DOC concentrations due to variable DOM compositions. Attenuation in one set of lakes was well related (linear models) to a 330 and total suspended solids (UVB R 2 = 0.80, p < 0.0001; UVA R 2 = 0.81, p < 0.0001). When these models were applied to other Delta lakes, however, attenuation of UVB was overestimated in 17 of 19 cases and attenuation of UVA was overestimated in all 18 cases. This bias indicates that models are not transferrable among Delta lakes, and likely cannot be applied in other circumpolar delta lakes with similarly complex DOM compositions. Although attenuation is high in Delta lakes (K d UVB 17.1-33.4 m −1 ; K d UVA 7.7-19.2 m −1 ), plankton and photoreactive solutes may be exposed to high levels of ultraviolet radiation (UVR) because Delta lakes are shallow, UVB and UVA penetrate the top 19% and 31% of water columns, respectively, and day lengths are extended during open water. Thus, climate change effects on DOM compositions may significantly alter in situ UVR environments in circumpolar delta lakes.
Chemical and optical changes in freshwater dissolved organic matter exposed to solar radiation
2001
We studied the chemical and optical changes in the dissolved organic matter (DOM) from two freshwater lakes and a Sphagnum bog after exposure to solar radiation. Stable carbon isotopes and solid-state 13 C-NMR spectra of DOM were used together with optical and chemical data to interpret results from experimental exposures of DOM to sunlight and from seasonal observations of two lakes in northeastern Pennsylvania. Solar photochemical oxidation of humic-rich bog DOM to smaller LMW compounds and to DIC was inferred from losses of UV absorbance, optical indices of molecular weight and changes in DOM chemistry. Experimentally, we observed a 1.2‰ enrichment in δ 13 C and a 47% loss in aromatic C functionality in bog DOM samples exposed to solar UVR. Similar results were observed in the surface waters of both lakes. In late summer hypolimnetic water in humic Lake Lacawac, we observed 3 to 4.5‰ enrichments in δ 13 C and a 30% increase in aromatic C relative to early spring values during spring mixing. These changes coincided with increases in molecular weight and UV absorbance. Anaerobic conditions of the hypolimnion in Lake Lacawac suggest that microbial metabolism may be turning over allochthonous C introduced during spring mixing, as well as autochthonous C. This metabolic activity produces HMW DOM during the summer, which is photochemically labile and isotopically distinct from allochthonous DOM or autochthonous DOM. These results suggest both photooxidation of allochthonous DOM in the epilimnion and autotrophic production of DOM by bacteria in the hypolimnion cause seasonal trends in the UV absorbance of lakes.
PLoS ONE, 2013
CDOM biogeochemical cycle is driven by several physical and biological processes such as river input, biogeneration and photobleaching that act as primary sinks and sources of CDOM. Watershed-derived allochthonous (WDA) and phytoplankton-derived autochthonous (PDA) CDOM were exposed to 9 days of natural solar radiation to assess the photobleaching response of different CDOM sources, using absorption and fluorescence (excitation-emission matrix) spectroscopy. Our results showed a marked decrease in total dissolved nitrogen (TDN) concentration under natural sunlight exposure for both WDA and PDA CDOM, indicating photoproduction of ammonium from TDN. In contrast, photobleaching caused a marked increase in total dissolved phosphorus (TDP) concentration for both WDA and PDA CDOM. Thus TDN:TDP ratios decreased significantly both for WDA and PDA CDOM, which partially explained the seasonal dynamic of TDN:TDP ratio in Lake Taihu. Photobleaching rate of CDOM absorption a(254), was 0.032 m/MJ for WDA CDOM and 0.051 m/MJ for PDA CDOM from days 0-9, indicating that phototransformations were initially more rapid for the newly produced CDOM from phytoplankton than for the river CDOM. Extrapolation of these values to the field indicated that 3.9%-5.1% CDOM at the water surface was photobleached and mineralized every day in summer in Lake Taihu. Photobleaching caused the increase of spectral slope, spectral slope ratio and molecular size, indicating the CDOM mean molecular weight decrease which was favorable to further microbial degradation of mineralization. Three fluorescent components were validated in parallel factor analysis models calculated separately for WDA and PDA CDOM. Our study suggests that the humic-like fluorescence materials could be rapidly and easily photobleached for WDA and PDA CDOM, but the protein-like fluorescence materials was not photobleached and even increased from the transformation of the humic-like fluorescence substance to the protein-like fluorescence substance. Photobleaching was an important driver of CDOM and nutrients biogeochemistry in lake water.