Modeled Effects of Dissolved Organic Carbon and Solar Spectra on Photobleaching in Lake Ecosystems |
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Authors: | Isabel Reche Michael L Pace Jonathan J Cole |
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Institution: | (1) Institute of Ecosystem Studies, Box AB, Millbrook, New York 12545-0129, USA, US |
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Abstract: | 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−1. 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−1. 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−1, 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.
Received 17 November 1998; accepted 27 June 2000. |
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Keywords: | : CDOM photobleaching solar wavebands ultraviolet radiation lake ecosystems dissolved organic carbon |
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