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Photochemical degradation of dissolved organic substances in freshwater environment

ecology

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Photochemical degradation of dissolved organic substances in freshwater environment

Photochemical degradation of dissolved organic substances in freshwater environment

Abstract: The carbon cycle in water systems is largely influenced by the photochemical degradation of dissolved organic substances. The aim of this study was to measure the rate of the photochemical degradation of DOC in Lake Balaton. Water samples, collected in June 2007 in the mouth of River Zala and in different basins of the lake (Keszthelyi, Szigligeti, Szemesi and Siófoki -at Tihany and at Balatonfűzfő- basins), were irradiated with the average radiation (UV-B, UV-A and PAR) of a sunny summer day in a sun simulator for 28 days. Samples were taken on day 3, day 7, day 14, day 21, and day 28. Light absorbance (Pt-colour was determined) and fluorescence intensity were measured. The concentration of dissolved organic carbon (DOC) as well as the quantity, degradation rate and half-life of photolitically degradable DOC was measured. The proportion of the DOC contributable to humic substances was measured both at the beginning and at the end of the experiment. The photolysis ran its curse in 21 days. The concentration of photolitically degradable DOC was 3.4 mg l-1 (24%) at the mouth of River Zala, which was an order of magnitude lower (0.2 mg l-1 – 1,6 %) in the Siófok basin. The interval of the degradation coefficient ranged between 0.2 and 0.01 day-1, which equal 3 and 59 days of half-life, respectively. The brown-coloured “Zala-water” discoloured to the Pt-colour value of the Keszthely basin, while the fluorescent spectra showed that the Zala water turned into “Siófok water” as a result of photolysis. Our results suggest that not only the DOC concentration decreases but the quality of organic substances is also transformed during photolysis, whereby they become more resistant to light degradation during residence in the lake.

Key words: Lake Balaton, DOC, photolysis, degradation coefficient, half-life

Introduction

In water systems, so in Lake Balaton to, the amount of organic substances is one of the major characteristic of the water quality. In lakes the dissolved organic carbon (DOC) arises from two well determined sources, from the primary production in the lake (allochton), and from the catchment area. The DOC affects a large scale of physical, chemical and biological processes. The light absorption of dissolved organic carbon strongly influence the penetration depth of photosynthetically active radiation (PAR) and ultraviolet radiation (Morris et al. 1995), they vigorously absorb the short wavelength light so they change the underwater light clime (Kirk, 1976; Bricaud et al. 1981), also have influence on the temperature (heat household???)of the lake (Snucins & Gunn, 2000) The DOC is interfering with the dissolved nutrition and metals hereby can change his concentrations and biological availability (Perdue 1998, Shaw et al. 2000). The DOC is composed mostly of acid compounds, therefore they can affect the pH of the lake and in lakes with low pH can act like buffers (Driscoll et al., 1994).

In Lake Balaton the major part of the allochtons comes with the Zala River (V.-Balogh et al., 2007), were the part of the humic substances is large up to 75% (V.-Balogh et al., 2003), these substances decompose by adsorption, microbial and photochemical degradation. In course of the photochemical degradation that molecules decay which ones can absorb the UV radiation, in consequence of this the penetration depth of the light in the water is increasing. This progression is more accentuated in the UV-B (280-320 nm) region, which wavelength which wavelength region is the most harmful for the biological systems (Karentz et al. 1994). With tapering of the ozone layer the rate of UV radiation increasing therefore the harmful effects is increasing to. The fotolyzis increase the biological availability of the substances (Strome & Miller, 1978), also generate reactive oxygen radicals (O2-, H2O2), which’s directly – damaging the biological membranes- or indirectly –by modifying the redox potential of the metals- (Cooper et al., 1989) influence the biological systems.

From the above follows that the photochemical degradation of the dissolved organic substances can modify remarkably the carbon cycle of the waters. The aim of this study was to measure the rate of the photochemical degradation of DOC in Lake Balaton

Methods

Water samples, collected in June 2007 in the mouth of River Zala and in different basins of the lake (Keszthelyi, Szigligeti, Szemesi and Siófoki -at Tihany and at Balatonfűzfő- basins), were used for the photochemical degradation study. The water samples were filtered (precombusted at 450 °C, GF-5 glass filters, 0.45 µm pore size) and 280ml from these were placed in clean quartz tubes (height= 35 cm, interior diameter=3,5 cm) , three same sample from each location. The water samples were exposed to artificial PAR and UV (UV-A, UV-B) radiation in a Sun simulator for 28 days, the dark treatment was wrapped in aluminum foil. On the star of the studies the oxygen concentration of the samples were set at 100%. The intensity of the irradiation in the Sun simulator was: UV-B: 0,291 mW cm-2, UV-A: 0,403 mW cm-2, PAR: 695 mmol m-2 sec-1, this is equal with the average radiation (UV-B, UV-A and PAR) of a sunny summer day. To measure the intensity of the PAR and UV radiation were used LI-COR respectively VLX 3W type radiometer. The presents of the microbes was checked with Nikon Optoiphot epifluorescent microscope. From the quartz tubes samples were taken on the beginning of the study, the day 3, day 7, day 14, day 21, and day 28. The DOC concentration was measured with Elementar High TOC organic carbon analyzer, also we determined the concentration of the photochemical degradable dissolved organic carbon (FDOC), the dissolution speed and the half time period of DOC, whit the following formulas: FDOC=DOC0-DOCt, where DOC0= DOC concentration at the beginning of the study, DOCt= DOC concentration after t incubation period, dissolution speed of FDOC: FDOCt=FDOC0ekt, where FDOCt = concentration of FDOC t incubation period, FDOC0 = the total concentration of FDOC after the treatment, k= decomposition coefficient, t = time in days. FDOC half time period = ln2 k-1. The absorbance of samples determined using Simadzu UV 160 A spectrophotometer, we calculated the color intensity (Pt-units) of humic substances using the absorbance at 440nm (Cuthbert & del Giorgio, 1992)

Results and discussion

At the start of the study the DOC concentration in the water samples was among 8,7 mg l-1 (Siófoki-basin) and 14,4 mg l-1 (mouth of River Zala), in the dark controls the DOC concentration doesn’t change. The DOC concentration decreased exponentially to the 21. day of the study and after that did not change. The amount of the photochemicaly degradable DOC differed significantly in the different sites, only 0,2 mg l-1 was in the Siófoki-basin, more than sixfold (1,2 -1,4 mg l-1) was decaying on the middle and western area of the lake, and with an order of magnitude bigger quantity , 3,4 mg l-1 was decaying in the mouth of River Zala (1. table). This DOC quantity 1.6 % was in the Siófoki-basin and 24 % in the Zala estuary. If we demonstrate the decay rate by the half-life period, then considerable difference can be observed from the different sites, the half-life period was the smallest in the Keszthelyi-basin and in the Zala estuary, which one increases gradually and significantly as we recede from the incoming area. The half-life period was more than 17 times bigger in the Siófoki-basin, than at the incoming of the lake (We mention that the half-life period is far underestimated under continuous irradiance). Due to irradiance the DOC decays most quickly in the Zala River and the Keszthelyi-basin, the slowest decaying was detected in the Siófoki-basin.

In the experimental samples the bacteria did not proliferate, so the microbial decomposition did not influence the experimental results. The number of the bacteria was increasing slightly in the in dark control samples, but this did not cause demonstrable decrease in the DOC concentration.

Table 1. The amount and decomposition speed of photochemical degradable dissolved organic carbon from the mouth of River Zala and in different basins of the lake

Sample

Amount of FDOC

(t=21 nap)

FDOC bomlási sebesség

(t=21 nap)

Decreased DOC

(mg l-1)

Decreased DOC

(%)

k

Day-1)

Halftime period (irradiated day)

Mouth of River Zala

3,40

23,55

0,2027

3.42

Keszthelyi Basin

1,25

8,66

0,1727

3.03

Szigligeti Basin

1,41

9,76

0,0671

10.39

Szemesi Basin



1,30

9,00

0,0584

11.84

Siófoki Basin (Tihany)

0,22

1,52

0,0135

51.34

Siófoki Basin (Balatonfűzfő)

0,24

1,66

0,0118

58.74

Due to the irradiation the composition of the dissolved organic matters changed to. In the samples from the mouth of River Zala the concentrations of the humic acids (HA) decreased with 58%, the concentration of fulvic acids (FA) decreased with 31%, while the concentrations of non humic substances (NHS) increased with 1.6%. On the opposite pole of the lake, at Balatonfüzfő this change was less demonstrable, the concentration of the HA decreased with 10,5 %, the concentration of the FA decreased with 8 % , while the proportion of the NHS increased with 6,5 %.

The qualitative differences along the longitudinal axis of the lake were observable well on the beginning of the experiment already. The characteristics of the water samples differed drastically on the beginning of the experiment already. The water samples differed drastically on the beginning of the experiment in their absorbance (figure 1.), the absorbance of the Zala’s water was multiple of the Siófoki-basins. The absorption of the water samples decreased continuously to the 21. day due to the irradiation, after the 21.day did not change significantly. The decreasing measure of the absorbance in the Siófoki-basin was 60%, in the Zala water this was 60-80%.

1. figure

The changes in the absorbance of the dissolved organic substances during the irradiation study, mouth of River Zala (A), Siófoki-basin at Balatonfűzfő (B)

The brown colored water from the mouth of River Zala decreased from the value 89.57 mg Pt l-1 to 14,89 mg Pt l-1.  This is below the value 16.57 mg Pt l-1 which belongs to the water collected from Keszthely. There for, regarding the color of the water, the brown colored water from mouth of River Zala became water from Keszthely.

2. figure

The changing in the fluorescent spectra of the dissolved organic substances during the irradiation study, mouth of River Zala (A), Siófoki-basin at Balatonfűzfő (B)

Conclusion

Our results suggest that not only the DOC concentration decreases but the quality of organic substances is also transformed during photolysis, whereby they become more resistant to light degradation during residence in the lake. The quantity of the fluorophores and chromophores decreased, while the transparency of the water increased.

Acknowledgement

This study was made possible trough financial support of NKFP 3B022_04 BALÖKO and OTKA K 63296.

Összefoglaló

Az oldott szervesanyagok fotokémiai bomlása jelentős mértékben befolyásolja vízi rendszerekben a szénforgalmat, ezért tűztük célul, hogy meghatározzuk a DOC fotokémiai bomlásának sebességét a Balatonban. A Zala torkolatban és a Balaton különböző medencéiben (Keszthelyi-medence, Szigligeti-medence, Szemesi-medence, és a Siófoki-medencében, Tihanynál és Balatonfűzfőnél) 2007 júliusában vett vízmintákat Nap-szimulátorban egy átlagos nyári nap sugárzásának (UV-B, UV-A és PAR) tettük ki 28 napon át. A 3., 7., 14., 21. és 28. napon vettünk mintát. Mértük a fényabszorbanciát (meghatároztuk a Pt-szín) és a fluoreszcencia intenzitást. Mértük az oldott szerves szén (DOC) koncentrációt, meghatároztuk a fotolitikusan bontható oldott szerves szén mennyiségét, bomlási sebességét és felezési idejét. A kísérlet elején és végén meghatároztuk a huminanyagoknak tulajdonítható DOC részesedést. A fotolízis 21 nap alatt lezajlott. A fotolitikusan bontható DOC koncentráció 0,2 mg l-1 (1,6%) volt a Siófoki-medencében, ennél nagyságrenddel több (3,4 mg l-1 - 24%) bomlott a Zala folyó torkolatában. A bomlási koefficiens intervallum 0,2 és 0,01 nap-1 közé esett, előbbi 3 nap, utóbbi 59 nap felezési időnek felel meg folyamatos besugárzás mellett. A barna Zalavíz a fotolízis következtében a Keszthelyi Pt-szín értékre fakult, míg a fluoreszcens spektrumok alapján a Zalavizből úgymond “Siófoki-víz” lett. Eredményeink rámutatnak, hogy a fotolízis során nemcsak a DOC koncentráció csökken, hanem a szervesanyagok minősége is átalakul, melynek eredményeként perzisztensebbé válnak a fénybontással szemben a tóban való tartózkodás során.

References

Bricaud, A., A. Morel & L. Prieur (1981) Absorption by dissolved organic matter of the sea (yellow substance) int he UV and visible domains. Limnol. Oceanogr. 26: 43-53.

Cooper, W. J., R. G. Zika, R. G. Petasne & A. M. Ficher (1989) Sunlight induced photochemistry of humic substances in natural waters: major reactive species. In: Suffet, I. H. & P. MacCarthy (Eds) Aquatic humic substances. Influence on fate and treatment of pollutants. America Chemical Society, Washington DC 333-362.

Cuthbert, I. D. & P. del Giorgio (1992) Toward a standard method of measuring color in freshwater. Limnol. Oceanogr. 37: 1319-1326

Drischoll, C. T., M. D. Lehtinen & T. J. Sullivan (1994) Modeling the acid-base chemistry of organic solutes in Adirondack, New York, lakes. Water Resour. Res. 30: 297-306.

Karentz, D., Bothwell, M.L., Coffin, R.B., Hanson, A., Herndl, G.J., Kilham, S.S., Lesser, M.P., Lindell, M., Moeller, R.E., Morris, D.P., Neale, P.J., Sanders, R.W., Weiler, C.S. & R.G. Wetzel, (1994) Impact of UV-B radiation on pelagic freshwater ecosystems: Report of the working group on bacteria and phytoplankton. Arch. Hydrobiol. Ergeb. Limnol. 43: 1-226.

Kirk, J. T. O. (1976) Yellow substance (glebsoff) and its contribution to the attenuation of photosynthetically active radiation in the aquatic environment., In: Allard, B. et al. (Eds.) Humic substances in the aquatic and terrestrial environment. Springer, 369-390.

Morris, D. P., H. Zagarese, C. E. Williamson, E. G. Belserio, B. R. Hargreaves, B. Modenutti, R. Moeller & C. Queimalinos (1995) The attenuation of solar UV radiation in lakes and the role of dissolved organic carbon. Limnol. Oceanogr. 40: 1381-1391.

Perdue, E. M. (1998) Chemical composition, structure, and metal binding properties. In: Hessen, D. O. & L. Tranvik (Eds.) Aquatic humic substances. Ecology and biogeochemistry. Springer-Verlag, Berlin Heidelberg, p. 41-61.

Shaw, P. J:, R. I. Jones & H. De Haan (2000) The influence of humic substances on the molecular weight distributions of phosphate and iron in epilimnetic lake waters Freshw. Biol. 45: 383-393.

Snucins, E. & J. Gunn (2000) Interannual variation in the thermal structure of clear and colored lakes. Limnol. Oceanogr. 45: 1639-1646.

Standard Methods (1995) Eaton, A. D., L. S. Clesceri & A. E. Greenberg (Eds.) 19th Edition American Public Health Association, Washington.

Srtome, D. J., & M. C. Miller (1998) Photolytic changes in dissolved humic substances. Verh. Internat. Verein. Limnol. 20: 1248-1254.

Tóth N. & V.-Balogh K. (2006) Meteorológiai és hidrológiai tényezők hatása a szerves szén frakciók koncentrációjának időbeli változására a Zala folyó torkolatában. Hidrológiai Közlöny 86: 130-132.

V.-Balogh, K. L. Vörös, N. Tóth & M. Bokros (2003) Changes of organic matter quality along the longitudinal axis of a large shallow lake (Lake Balaton). Hydrobiologia 506-509: 67-74.

V.-Balogh K., Tóth N., Somogyi B. & Vörös L. (2007) A Balaton biológiailag hozzáférhető szerves szén terhelése, Hidrológiai Közlöny 87: 147-149.

1. figure

The changes in the absorbance of the dissolved organic substances during the irradiation study, mouth of River Zala (A), Siófoki-basin at Balatonfűzfő (B)

2. figure

The changing in the fluorescent spectra of the dissolved organic substances during the irradiation study, mouth of River Zala (A), Siófoki-basin at Balatonfűzfő (B)

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