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

Anssi V. Vahatalo, Mirja Salkinoja-Salonen, Petteri Taalas, Kalevi Salonen

Limnol. Oceanogr., 45(3), 2000, 664-676 | DOI: 10.4319/lo.2000.45.3.0664

ABSTRACT: We measured photochemical mineralization of dissolved organic carbon in a humic 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 with 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 as a product of three spectra: (1) scalar photon flux density, (2) the apparent quantum yield, and (3) the absorption of chromophoric dissolved organic matter. We described the spectrum for apparent quantum yield as Φλ = c x 10-dl, 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 Φλ of 7.52 x 10-0.0122l . The Φλ based on the measurements in situ agreed with Φλ determined in a laboratory at 320, 355, and 390 nm. Using the determined Φλ, we calculated that UV-B contributed 9%, UV-A 68%, and visible light 23% to the photochemical mineralization. Half of total photochemical mineralization was due to wavelengths <360 nm. Our method for the determination of Φλ is applicable in situ, improves the prediction of photochemical reaction rates in surface waters, and offers an alternative to the determination of quantum yields at discrete wavelengths.

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