Modis Satellite Chlorophyll-a
There has been a long history of monitoring Chlorophyll-a in ocean surface waters, beginning with the Coastal Zone Color Scanner (CZCS) in 1978 - 1986. Since then, ocean color has been acquired through the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS; 1997 through the presen t), which is housed on the SeaStar satellite and from the Moderate Resolution Imaging Spectroradiometer (MODIS) Terra (2000 through the pre sent) and Aqua (2002 through the present) platforms. The entire planet is viewed by the satellites three times a day for chlorophyll: once by SeaWiFS, once by MODIS-Terra, and once by MODIS-Aqua, so you have three opportunities to view the ocean every day, where is not cloudy. The ocean color sensors cannot see through clouds.

All of these sensors have viewed the surface of the ocean by seeing the same colors as a human eye would see. Chlorophyll appears green to our eyes, so when we are looking for chlorophyll in the ocean, we look for waters that are more green than the dark blue color an ocean wi thout chlorophyll would have. By mapping where the waters are greener, we can tell where chlorophyll is present in the surface waters.

Ocean color sensors can only see down into the water as far as you could see if you looked off of the side of a ship, so water clarity is a factor when knowing how deep the satellite can see below the surface. Also, there are things that can trick the sensors, such as sediment . When you look at where a river flows into the ocean, you will likely see what looks like a big chlorophyll plume, but it is actually sus pended sediment in the water column, so you have to be careful when you interpret ocean color images to make sure that what you are seeing is really chlorophyll.

Another challenge with ocean color is that it is hard to obtain accurate chlorophyll measurements near the coastline. This can be equated to when you have been in a very bright or dark room and then you switch to the opposite scenario. Your eyes need time to adjust to the new brightness conditions.

MODIS data is freely available and can used by anyone who so desires, but SeaWiFS data is proprietary, and you must be an authorized user t o access the data. AOOS offers data from all of these sensors, but as this point, to access SeaWiFS data, you must contact Rachel Potter ( rpotter at ims.uaf.edu) to verify your authorization and set up a data transfer.

All products available through AOOS use the OC4 algorithm.