Computation of photosynthetically usable radiation in turbid waters

Abstract

This paper describes how the depth-dependent photosynthetically available radiation (PUR) can be derived for waters with various types of suspended particles and turbidity values. Using a Mie-scattering routine, we compute the wavelength-dependent scattering phase functions and scattering cross-sections for a suspension of particles in water following the Junge-type particle size distributions. The particles are assumed to compose of various proportions of silt, clay and sand particles. The inherent optical properties are then calculated. These optical properties are used to simulate the downwelling spectral irradiance and photon flux density at various depths, by using the "Hydrolight" radiative transfer software package. PUR is calculated by weighting the downwelling photon flux density with a normalized chlorophyll absorption spectrum from the PROSPECT package and then integrated over the visible wavelength region. Our results indicate that the PUR generally decreases exponentially with depth for all suspension types and turbidities considered. In addition, we demonstrate that in clear waters, it typically takes more than 10m for the PUR to decrease to one-tenth of its just-below-surface values. In more turbid waters, however, it takes less than 2m for the PUR to show the same decrease.