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

Abstract

A mathematical framework to incorporate spectral apparent and inherent optical 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 phytoplankton, colored degradational matter (CDM), in the form of 2 classes of colored dissolved organic carbon (CDOC), and downwelling diffuse attenuation coefficients ( K d ’s) at a 5 nm resolution. Particulate absorption responds to changes in phytoplankton species biomass, pigmentation, and photo-adaptation. 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 underestimation of CDM by the EcoSim that is reflected in the simulated absorption and attenuation at 412 nm. The particulate absorption slope between 412 and 487 nm suggests an overestimation of chlorophyll b and photoprotective carotenoid concentrations from Prochlorococcus 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 and 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 may have been pre-valent in the late spring and the early fall.