Carbon cycling in the upper waters of the Sargasso Sea: II. Numerical simulation of apparent and inherent optical properties

A mathematical framework to incorporate spectral apparent and inherent opti cal properties into a one-dimensional ecological simulation (EcoSim) of the Sargasso Sea is developed. The simulation includes equations for spectral algal particulate absorption, in the form of 4 functional groups of phytopl ankton, colored degradational matter (CDM), in the form of 2 classes of col ored dissolved organic carbon (CDOC), and downwelling diffuse attenuation c oefficients (K-d's) at a 5 nm resolution. Particulate absorption responds t o changes in phytoplankton species biomass, pigmentation, and photo-adaptat ion. CDM absorption responds to concentration changes of labile and relict CDOC. K-d's respond to changes in spectral total absorption, backscattering , and the average cosine of downwelling photons. The spectral bio-optical outputs provide an additional means of validating an ecological simulation. The vertical and seasonal changes in the diffuse attenuation coefficient at wavelengths of 412, 442, 467, 487, 522, and 567 nm compare well with in situ measurements. There appears to be an underesti mation of CDM by the EcoSim that is reflected in the simulated absorption a nd attenuation at 412 nm. The particulate absorption slope between 412 and 487 nm suggests an overestimation of chlorophyll b and photoprotective caro tenoid concentrations from Prochlorococus functional groups. Simulated K-d( 522) and K-d(567) appear to be higher than observations and may result from the exclusion of the inelastic scattering process (i.e. Raman scattering a nd CDM fluorescence). Results suggest that CDM absorption does not co-vary with particulate absorption. Comparison with a six year time-series of CZCS data suggests that CDM interference of the estimated CZCS chlorophyll a ma y have been prevalent in the late spring and the early fall. (C) 1999 Elsev ier Science Ltd. All rights reserved.