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OCEAN COLOR FROM SPACE: 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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Last updated: May 08, 2012 02:06 PM ET