Scale closure in upper ocean optical properties : from single particles to ocean color

Abstract

Predictions of chlorophyll concentration from satellite ocean color are an indicator of phytoplankton primary productivity, with implications for foodweb structure, fisheries, and the global carbon cycle. Current models describing the relationship between optical properties and chlorophyll do not account for much of the optical variability observed in natural waters, because of the presence of seawater constituents that do not covary with phytoplankton pigment concentration. In an attempt to better understand variability in these models, the contributions of seawater constituents to ocean optical properties were investigated. A combination of Mie theory and flow cytometry was used to determine the diameter, complex refractive index (n+n'i), and optical cross-sections of individual particles, based on a method developed in the laboratory using phytoplankton cultures. Individual particle measurements were used to interpret variability in concurrently measured bulk optical properties in New England continental shelf waters in two seasons. The summed contribution to scattering of individual particles in the size range of 0.1-50 μm accounted for approximately the entire scattering coefficient measured independently using bulk methods. In surface waters in both seasons, the large diameters and n' of eukaryotic phytoplankton caused them to be the main particle contributors to both absorption and scattering. Minerals were the main contributor to backscattering, bb, in the spring, whereas in the summer both minerals and detritus contributed to bb. Synechococcus and heterotrophic bacteria were less important optically, contributing ≤11% each to attenuation in either season. The role of seawater constituents in determining remote sensing reflectance, Rrs, was determined using radiative transfer theory. Seasonal differences in the spectral shape of Rrs were contributed to approximately equally by eukaryotic phytoplankton absorption, dissolved absorption, and non-phytoplankton bb. A higher inverse wavelength dependence of non-phytoplankton bb in the summer was caused by the contribution of small detritus, in contrast to larger minerals in the spring. Measurements of bb and Rrs were compared to values from bio-optical models based on chlorophyll concentration. Differences in measured and modeled bb and Rrs were caused by higher dissolved absorption and higher backscattering efficiencies and scattering by non-phytoplankton than were assumed by the model.