Overview

The Chesapeake Bay Remote Sensing Program (CBRSP) has used measurements of ocean color from light aircraft to estimate chlorophyll concentrations in Chesapeake Bay for the past thirteen years. Chlorophyll is a photosynthetic pigment common to microscopic algae - phytoplankton - the main primary producers that comprise the base of the food web in the Bay.  This important pigment imparts color to the water.  Clear blue water has low chlorophyll and reflects strongly at low wavelengths in the visible spectrum, whereas green water has higher chlorophyll and absorbs strongly in the blue, reflecting light at longer wavelengths. Data are collected using airborne radiometric instruments that allow us to measure the reflected light and ascertain concentrations of chlorophyll. This key property is one of the main ingredients we need to estimate primary productivity and to gauge the overall productivity of the ecosystem at higher trophic levels.

Advantages of using remote sensing to measure ocean color

The advantages of using remote sensing to measure ocean color are the rapid coverage of the Bay that is possible using aircraft, and the high spatial and temporal resolution of chlorophyll that can be obtained. CBRSP has made > 300 flights from 1989 to 2002 in a program supported by multiple sponsors, including Maryland Sea Grant, NOAA, NASA, and EPA. Flights are conducted at a frequency of up to twice per week on the main stem of the Bay and monthly on two selected tributaries using ocean-color sensors similar to the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) instrument that is currently in earth orbit. Flight tracks that cover the study sites are flown at an altitude of 500 feet (150 meters), requiring ca. 4-5 h for the main Bay and 1.5 h for the tributaries.

Data and information from the surveys

The 1989-1995 data were generated using the Ocean Data Acquisition System (ODAS), a relatively simple ocean color instrument that was developed in the mid-1980s at NASA Goddard Space Flight Center. ODAS consisted of three radiometers in the blue-green region of the visible spectrum that measured water-leaving radiance at 460, 490 and 520 nm. 1995-96 data were generated by the SeaWiFS Aircraft Simulator (SAS II) that replaced the ODAS instrument. SAS II contained sensors at seven wavebands, including the six visible bands of the SeaWiFS satellite ocean color instrument. The additional bands of SAS II allowed improved recoveries of chlorophyll in highly turbid conditions and at the extremely high concentrations that accompany red tides.  A new version of the SeaWiFS Aircraft Simulator (SAS III) went into service in 1997 and we continue to use this instrument in Bay and tributary flights at this writing. SAS III is a 13-band instrument with improved spectral resolution. All the instruments (ODAS, SAS II, SAS III) are paired with an infrared temperature sensor that we use to measure sea surface temperature during flights. One of our principal sponsors, the NOAA Chesapeake Bay Office, has been the main distributor of imagery derived from the flights in a continuing collaboration.

CISNet tributary flights

In addition to the weekly surveys of main stem of the Bay,  CBRSP has conducted monthly tributary surveys of the Choptank and Patuxent Rivers since 1999. The Choptank and Patuxent River flights began as a part of the Coastal Intensive Site Network (CISNet), a goal of which was to measure chlorophyll in two systems with strongly contrasting watersheds, agriculture in the case of the Choptank, suburban in the case of the Patuxent.