Modeled effects of dissolved organic carbon and solar spectra on photobleaching in lake ecosystems

Dissolved organic matter (DOM) contains molecules that absorb light at vari ous wavelengths. This chromophoric DOM (CDOM) influences the transmission o f both visible and ultraviolet energy through water. The absorption of ligh t by CDOM often causes structural changes that reduce its capacity to furth er 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) concentr ation in conjunction with a defined solar spectrum and experimentally measu red 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 waveba nd-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 photo bleaching rates. The model demonstrates that all three wavebands of solar r adiation (UVB, WA, and PAR) contribute significantly to total water column photobleaching, with WA 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 U V-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 coefficient s by 10-fold resulted in a similar change in total water column photobleach ing rates. Applying the model to four specific lakes revealed that photoble aching for the entire water column would reduce CDOM light absorption by 50 % in 18-44 days under summer conditions.