Primary Productivity
Our present assessment of global primary productivity is based on an aggregation
of haphazardly located and timed ship observations scanning many decades. It is
impossible to resolve time-dependent global changes from this data set, and
current estimates of mean annual productivity vary by more than a factor of two.
However, satellite observations eliminate some of the important sampling
problems inherent in traditional ship-based oceanography, such as the long
interval (compared to typical ocean-dynamical timescales) between observations
at different locations. For example, a two-minute satellite scene comtains
2 million pixels that cover an area of 2 million square kilometers; it woould
take more than 11 years for a ship traveling at 20 km/hr to sample each pixel.
The use of satellite ocean-color imagery to estimate primary-productivity for
relatively large areas of the ocean is therefore an attractive alternative. In
this approach, satellite estimates of near-surface chlorophyll provide an
initial estimate of phytoplankton biomass (in units of carbon or nitrogen) as
input to a mathematical model of primary productivity. The model may also
require measurements or estimates of other variables affecting primary
productivity, such as water temperature, sunlight intensity, and nutrient
supply. Model predictions of daily primary productivity are then used to
calculate the potential cumulative increase in phytoplankton biomass from the
time of the initial biomass estimate until the next available satellite image.
Work in progress shows good agreement between field observations and model
output for at least some regions. However, major issues need to be resolved
before biological oceanographers will accept primary production estimates based
only on satellite measurements and mathematical models. For example, CZCS
imagery yields chlorophyll estimates only for the upper layers, whereas primary
productivity extends deeper into the water column. Nevertheless, many
oceanographers now believe that satellite ocean color measurements must become
an essential component of future ocean field experiments.
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Atmos Composition
