From Cells to the Ocean: Satellite Ocean Color

Abstract

Variations in the color of the ocean as seen from space are principally due to variations in the concentration and optical properties of biogenic materials, dissolved and particulate, in the upper ocean. From 1978 to 1986 the NIMBUS-7 Coastal Zone Color Scanner observed these variations over the global ocean; the wealth of data that has resulted is just now being appreciated. The resultant ability to observe the ocean from a biological perspective over synoptic scales revolutionized the field.

The satellite observations require algorithms to interpret the received signal in terms of meaningful geophysical quantities or processes. Most of the signal results from the atmosphere and corrections to permit analysis of the ocean signal is non-trivial. Assuming that this can be done with acceptable accuracy, it is still necessary to relate the observations of radiance leaving the surface of the ocean to more useful variables such as the concentration of chlorophyll in the sea surface or the primary productivity of the ocean. The link between the observations and the desired retrieval are the so-called bio-optical algorithms.

The relationship between water-leaving radiance and chlorophyll in the water column is not simple because most of the chlorophyll is contained within phytoplankton particles of varying dimension and with varying internal concentration of chlorophyll. The presence of ancillary and detrital pigments, in addition to chlorophyll, and the presence of non-chlorophyllous particles further complicate the issue. Two approaches to the bio-optical algorithms have been taken to resolve these problems. The first attempts to describe empirically the variability in particle size and composition in terms of coefficients of statistical relationships between water-leaving radiance and chlorophyll concentration. Primary production is described in a similar fashion. The second, not mutually exclusive, relies on first principles of radiative transfer and the physiology of phytoplankton coupled with a detailed understanding of the nature and size distribution of the particle population and their optical properties. One of the challenges of the latter path is to reproduce the water-leaving signal given the solar input and detailed knowledge of the particle population from analysis of individual cells. The inverse problem, infering the individual cell characteristics from the water-leaving radiance signal, is equally or more challenging.

Here, the bases for the remote measurement of ocean color from space, and the algorithms used in the estimation of chlorophyll concentration and primary productivity from these remote measurements will be discussed with a view towards interpretation based on the characteristics of the ensemble of individual particles in the upper ocean.