333
determination of chlorophyll, these efforts span the range from purely empirical relationships
to estimates constructed from principles of optics and algal physiology. Given an exact
estimation of chlorophyll, sea-surface radiation and the attenuation coefficient (not possible
from space observations), it appears that some of these relationships may account for as much
as 75 % of the variability in primary production on a global scale; when the uncertainties in
the chlorophyll concentration, solar flux and attenuation is propagated through, the error
explained is much less, perhaps 20-30% (see Kuring et al., 1990). Nonetheless, this
represents a significant improvement over current ship-based estimates where the bias due to
undersampling at the large scale is enormous.
Alternatively, one could in principle eliminate the intermediate step in the calculation of
chlorophyll, and attempt to estimate the light absorbed by viable photosynthetic organisms
directly (Perry, pers. commun). Given an estimate of the quantum efficiency of
photosynthesis, the photosynthetic rate could be directly determined. Problems with this
approach lie in ambiguity introduced through the backscattering coefficient as in the estimation
of chlorophyll, in estimating the portion of the ocean particle absorption that is due to
photosynthetically competent pigments rather than detritus or photoprotective pigments, and
achieving robust estimates of quantum efficiency.
Another alternative of great current interest is the prediction of the photosynthetic rate from
a remote measurement of the upwelled radiance associated with red fluoresence which results
from absorption of solar energy by photosynthetic pigments (e.g. Gower and Borstad, 1981).
The fluoresced radiance seen by a remote sensor is emitted from the upper ocean by biogenic
particles, it is less dependent on the backscattering coefficient, and on the concentration of
dissolved organic matter. The estimation of photosynthetic rate depends on a predictable ratio
of the quantum yield for fluorescence to that of photosynthesis defined on the basis of constant
irradiance and on the bulk phytoplankton assemblage. It is not clear at present that this
condition is met to a desired degree over the ocean provinces of interest and remains an active
area of research.
determination of chlorophyll, these efforts span the range from purely empirical relationships
to estimates constructed from principles of optics and algal physiology. Given an exact
estimation of chlorophyll, sea-surface radiation and the attenuation coefficient (not possible
from space observations), it appears that some of these relationships may account for as much
as 75 % of the variability in primary production on a global scale; when the uncertainties in
the chlorophyll concentration, solar flux and attenuation is propagated through, the error
explained is much less, perhaps 20-30% (see Kuring et al., 1990). Nonetheless, this
represents a significant improvement over current ship-based estimates where the bias due to
undersampling at the large scale is enormous.
Alternatively, one could in principle eliminate the intermediate step in the calculation of
chlorophyll, and attempt to estimate the light absorbed by viable photosynthetic organisms
directly (Perry, pers. commun). Given an estimate of the quantum efficiency of
photosynthesis, the photosynthetic rate could be directly determined. Problems with this
approach lie in ambiguity introduced through the backscattering coefficient as in the estimation
of chlorophyll, in estimating the portion of the ocean particle absorption that is due to
photosynthetically competent pigments rather than detritus or photoprotective pigments, and
achieving robust estimates of quantum efficiency.
Another alternative of great current interest is the prediction of the photosynthetic rate from
a remote measurement of the upwelled radiance associated with red fluoresence which results
from absorption of solar energy by photosynthetic pigments (e.g. Gower and Borstad, 1981).
The fluoresced radiance seen by a remote sensor is emitted from the upper ocean by biogenic
particles, it is less dependent on the backscattering coefficient, and on the concentration of
dissolved organic matter. The estimation of photosynthetic rate depends on a predictable ratio
of the quantum yield for fluorescence to that of photosynthesis defined on the basis of constant
irradiance and on the bulk phytoplankton assemblage. It is not clear at present that this
condition is met to a desired degree over the ocean provinces of interest and remains an active
area of research.