The Role of Photochemical Degradation of Dissolved Organic Carbon in Regulating the UV Transparency of Three Lakes on the Pocono Plateau

The role of photochemical degradation of dissolved organic carbon (DOC) on UV transparency (280-400 nm) was investigated in three Pocono Plateau lakes. Diffuse attenuation coefficients (KdUV) in the epilimnia of these lakes varied seasonally (39-81% decline from maximum Kd320mm), with minimum KdUV)...

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Bibliographic Details
Published in:Limnology and oceanography 1997-03, Vol.42 (2), p.239-249
Main Authors: Morris, Donald P., Hargreaves, Bruce R.
Format: Article
Language:English
Subjects:
Absorptivity
Bottles
Earth sciences
Earth, ocean, space
Exact sciences and technology
Freshwater
Freshwater ecosystems
Geochemistry
Hydrology
Hydrology. Hydrogeology
Irradiance
Lakes
Lentic systems
Marine
Mineralogy
Optical properties
Silicates
Solar radiation
Sunlight
Water geochemistry
Watersheds
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Summary:The role of photochemical degradation of dissolved organic carbon (DOC) on UV transparency (280-400 nm) was investigated in three Pocono Plateau lakes. Diffuse attenuation coefficients (KdUV) in the epilimnia of these lakes varied seasonally (39-81% decline from maximum Kd320mm), with minimum KdUV) values occurring near summer solstice. Declines in KdUVcorresponded to reductions in UV absorbance by dissolved material (adUV), caused by declines in both DOC concentration and UV absorptivity (ad:[DOC]). The seasonal decline in KdUVwas also accompanied by substantial increases in an absorption coefficient ratio (ad250nm:ad365nm) and decreases in spectral slopes (S). Experimental studies of photochemical degradation were also performed using filtered lake water and natural sunlight. Exposure to solar radiation produced a number of changes in optical parameters and DOC that resembled those observed in the water colum: adUV(-35 to -52%), UV absorptivity (-31 to -48%), ad250nm:ad365nm(0 to +39%), spectral slope, S (0 to -27%), and DOC concentration 0 to -20%). Change in adUVwere correlated with absorbed UV dose. The derived rate constants were used in a model to estimate the contribution of photochemical degradation of DOC to water column declines in KdUV. Results suggest that rates of photochemical degradation were sufficient to account for the summer reduction in KdUVobserved in the three lakes.
ISSN:0024-3590
1939-5590
DOI:10.4319/lo.1997.42.2.0239