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Volume 3, Number 2
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In Plane Sight

DeHaviland Beaver
diagram of how the data is gathered by the ODAS sensor using light reflected back from the water
Reacting to the sun's radiant energy, aircraft-borne sensors quantify chlorophyll by reading the changing intensity and color signature of light as it reflects out of the waters below. The single-engine DeHavilland Beaver shown above gathered data up and down the Bay for six years using the Ocean Data Acquisition System (ODAS), which was among the first generation of such equipment.

Small airplanes provide just the right scale of resolution for capturing the big picture of phytoplankton abundance and change through time in the Chesapeake Bay.

While shipboard sampling is critical to ground-truth measurements taken from the sky, providing information about species composition and primary production, these surveys are too patchy and infrequent to synthesize information over the whole estuary. Measurements from satellites like SeaWiFS occur regularly, providing a complete picture of the Bay every two days, but the information is too coarse to resolve the spatial scale of plankton distributions in a coastal system like the Bay.

For Larry Harding, the airplane has become both his scientific and personal modus operandi. He has applied aircraft-mounted remote sensing technology on spatial scales over which an aircraft flies, parceling the Bay into a set of 7000 pixels that can be reconstructed to visualize the estuary at a resolution of one square kilometer - a scale fine enough to see subtle changes in plankton abundance but broad enough to capture the full extent of a bloom.

The Ocean Data Acquisition System (ODAS) was the first generation of aircraft remote sensors that Harding used in Chesapeake Bay and he successfully collected data with it from 1989-1995. ODAS was comprised of 3 radiometers, instruments that quantify the intensity of electromagnetic radiation in three different wavelengths in the blue to green range of the visible light spectrum. Chlorophyll absorbs light in the blue part of the color spectrum, but not in the green. When there is more chlorophyll in the water, more blue light is absorbed, so less is reflected. This is why the phytoplankton-rich Bay appears greenish while the deep ocean, where primary production is scarce, is blue to the eye. By calculating a ratio of blue to green light reflected, scientists can calculate how much chlorophyll is in the water.

With a sensor that points straight up out of the top of the airplane, ODAS, and the generation of tools to follow, also collect data on the total amount of sunlight on a given day (irradiance) and have an instrument to measure the temperature of the water (SST-Sea Surface Temperature).

Harding began using a new set of aircraft remote sensing instruments in 1995. The major innovation of these devices has been an increase in the number of wavelengths of light sampled. The new scanner, the SeaWiFS Aircraft Simulator, paralleled the SeaWiFS satellite scanner, measuring the same 8 wavelengths so that the data could be compared. One of the advantages of data collected at low aircraft altitudes is that there is much less interference from the atmosphere. The newest model of this simulator (SAS III) now measures 13 wavelengths of light, including and adding to those sampled by the SeaWiFS satellite scanner. Sampling additional wavelengths improves Harding's ability to resolve chlorophyll abundance in highly turbid conditions or at the extremely high concentrations that accompany seasonal blooms of diatoms and dinoflagellates.

Harding's group currently makes up to two-dozen aircraft overflights of the whole Bay in a given year and he has made a series of monthly flights over the Choptank and Patuxent rivers over the past four years, through a separate program called the Coastal Intensive Site Network (CISNet). Combined with regular water samples that span the length and width of the whole Bay and ocean color images from several satellite sensors, Harding has amassed a long-term data set of the physical and biological parameters that make the Bay tick. As shifts occur in the Bay's plankton composition over space and time, Harding's sensors are almost certain to see them.

- Erica Goldman

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