Spectrum of the quantum yield for photochemical mineralization of dissolved organic carbon in a humic lake

We measured photochemical mineralization of dissolved organic carbon in a h umic lake in situ. At a depth of 1 cm, solar radiation mineralized 19 mmol C m(-3) d(-1). The rate of mineralization decreased with increasing depth w ith an attenuation coefficient of 23 m(-1). Consequently most photochemical mineralization in the water column (0.99 mmol C m(-2) d(-1)) took place in the top 10 cm. The rate of photochemical mineralization was also modeled a s a product of three spectra: (1) scalar photon flux density, (2) the appar ent quantum yield (phi(lambda)), and (3) the absorption of chromophoric dis solved organic matter. We described the spectrum for apparent quantum yield as phi(lambda) = c x 10(-d lambda), where c (dimensionless) and d (nm(-1)) are positive constants. Mathematical optimization for the best fit between the measured and the modeled photochemical mineralization resulted in phi( lambda) of 7.52 X 10(-0.0122 lambda). The phi(A) based on the measurements in situ agreed with phi(lambda) determined in a laboratory at 320, 355, and 390 nm. Using the determined phi(lambda), we calculated that UV-B contribu ted 9%, UV-A 68%, and visible light 23% to the photochemical mineralization . Half of total photochemical mineralization was due to wavelengths <360 mm . Our method for the determination of phi(lambda) is applicable in situ, im proves the prediction of photochemical reaction rates in surface waters, an d offers an alternative to the determination of quantum yields at discrete wavelengths.