Abstract
An analytic model (AM) was developed to solve the radiative transfer equation for the natural fluorescence of phytoplankton in a horizontally homogeneous ocean with depth-dependent inherent optical properties using the quasi-single scattering approximation. A standard Monte Carlo simulation code (MC) for the underwater light field was modified to include fluorescent emission. Using the MC results for the incident scalar irradiance $E\sb{o}(z,\lambda\sb{e}),$ given the absorption coefficient $a(z,\lambda\sb{e})$ at an exciting wavelength $\lambda\sb{e},$ and the downwelling irradiance attenuation coefficient $K\sb{d}(z,\lambda\sb{f})$ at the fluorescent wavelength $\lambda\sb{f}$ as experimental data, the AM-computed values for fluorescent upwelling and downwelling irradiances, $E\sb{u}(z,\lambda\sb{e},\lambda\sb{f})$ and $E\sb{d}(z,\lambda\sb{e},\lambda\sb{f}),$ and upwelling and downwelling scalar irradiances, $E\sb{ou}(z,\lambda\sb{e},\lambda\sb{f})$ and $E\sb{od}(z,\lambda\sb{e},\lambda\sb{f}),$ reproduce the MC simulations at all depths. Using this model, the validity of the mean chlorophyll a absorption approximation has been examined and the vertical distribution of the ratio of total fluorescent nadir radiance $L\sb{ut}(z,685)$ at $\lambda$ = 685 nm to photosynthetically available radiation $E\sb{o}(z,PAR)$ has been calculated. The results were compared with observed data to estimate the fluorescence quantum efficiency of chlorophyll a for several stations with different chlorophyll a concentration profiles.