Some Earth observing satellites measure the characteristics of light, or radiance, coming from the Earth's surface. To learn about what is in the water using observations from space, we must first know what influences the color of water. Samples of ocean water are taken and their concentrations of phytoplankton and their chlorophyll are analyzed; these concentrations will then be correlated with the measured radiances. As these measurements are made, researchers hope to find consistent relationships between the radiances and the surface variables that are being measured, which will allow them to construct an algorithm. The algorithm will calculate a specific variable, such as chlorophyll concentration, based solely on the radiance data. Satellite data will then be used in these algorithms to calculate the geophysical variables over large areas of the Earth.
The goal of ocean color remote sensing algorithms is to distinguish different types of water, and the constituents that determine a particular color. Ideally, a useful algorithm would calculate the concentration of suspended particulates in the muddy water, and the concentration of chlorophyll in both turbid and clear water.
A part of the ocean with only one kind of phytoplankton will have a fairly uniform color. In these cases, a fairly simple relationship exists between the color that the satellite observes and the density of phytoplankton. Ratios of light intensity detected at various wavelengths of the visible and infrared spectrum have been used in algorithms to calculate vegetation density or chlorophyll concentration. However, if water contains different species of phytoplankton as well as sediments, it becomes much more difficult to find simple relationships between optical properties and geophysical characteristics.
To further complicate the matter, instruments in space tend to change over time, and they usually can't be re-calibrated. Unfortunately, algorithms such as those described above rely on very accurate measurements of radiance. Thus mission operations ensure that calibration of the instrument is known to a very high level of precision. Several different ways of calibrating these sensors while the satellite is in space have been devised.
The primary instruments NASA uses to investigate ocean color are SeaWiFS and MODIS:
- SeaWiFS: The purpose of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Project is to provide useful data on ocean color to the Earth science community. SeaWiFS flies on the OrbView-2 satellite, but beyond the instrument itself, the SeaWiFS Project has developed and operates a research data system that processes, calibrates, validates, archives and distributes data received from an Earth-orbiting ocean color sensor.
- MODIS: MODIS, an instrument so useful it has been placed on two satellites (Terra and Aqua), is ideal for monitoring large-scale changes in the biosphere to yield new insights into the global carbon cycle. MODIS can measure the photosynthetic activity of land and marine plants (phytoplankton) to yield better estimates of how much of the greenhouse gas is being absorbed and used in plant productivity. Coupled with the sensor's surface temperature measurements, MODIS' measurements of the biosphere are helping scientists track the sources and sinks of carbon dioxide in response to climate changes.
As data from SeaWiFS and MODIS are received, they are processed through several "levels". Level 2 products, such as chlorophyll, water clarity, and fluorescence, apply sensor calibration data and atmospheric correction to calculate Earth surface radiances from the radiances measured at the satellite.
With the SeaWiFS instrument, NASA has gathered the first record of photosynthetic productivity in the oceans. The process begins with a measurement of surface chlorophyll concentration. Chlorophyll is the material that allows plant cells to convert sunlight into energy, thus enabling them to grow. It's a green substance, and thus a good indicator of overall plant health: robust forests, lush lawns and vibrant phytoplankton blooms appear green. By measuring chlorophyll concentration, scientists can determine the health and growth of plants in a given area. By extension, healthy color signatures indicate the successful use of carbon, the fundamental building block for life.
Measuring chlorophyll can identify areas rich in nutrients and monitor such processes such as upwelling. Winds blowing southward along the west coast of the United States cause the surface layer of the ocean to move away from the coast. As the surface water moves offshore, cold, nutrient-rich water upwells from below to replace it. This upwelling provides nutrients needed for the growth of marine phytoplankton which, along with larger seaweeds, in turn nourish the incredible diversity of creatures found along the northern and central California coast.
Sensors such as SeaWiFS can "see" the effects of this upwelling-related productivity because the chlorophyll-bearing phytoplankton reflect predominantly green light back into space as opposed to the water itself which reflects predominantly blue wavelengths back to space.
Water in the open ocean, far from land, is nearly as transparent as glass, a deep navy blue when viewed over the side of a ship. Closer to shore, water becomes turbid for a variety of reasons.
Turbid waters are often significantly more productive than clear waters. River mouths and coastal upwelling zones are two classic examples, where primary productivity is enhanced by the nutrients delivered by the river water or contained in the upwelling deep water. The increased productivity in these coastal regions makes them important to the global carbon cycle, and it is therefore vital to accurately quantify the productivity in these regions. Turbidity makes accurate quantification of the amount of chlorophyll in these waters difficult.
Turbid waters usually reflect more light than clear waters, and this increased radiance can exceed the limits where the algorithms are most accurate. Thus, unless turbid waters and the conditions that cause them are recognized, the algorithms may return erroneous overestimates of the chlorophyll concentration and primary productivity in these regions.
SeaWiFS data processing recognizes that the chlorophyll concentration algorithms might be incorrect in areas where turbid water is present. For that reason, such areas are assigned the "turbid water" flag.
Phytoplankton vary significantly in their ability to convert light to carbon (lengthy exposure to light can reduce their efficiency, for example) and this variability is directly related to their physiological state.
Complex organic molecules are often fluorescent, emitting light of one wavelength when struck by light of another wavelength. Chlorophyll is weakly fluorescent, and this property can be used to determine the physiological state of phytoplankton. The amount of fluorescent radiation that is released by phytoplankton exposed to light is related to the amount of light that is utilized by the phytoplankton for photosynthesis. MODIS is equipped to observe fluorescence.
Productivity, production of carbon by photosynthesis, can't be directly measured, but rather is derived from a combination of other measurements. Net primary production is the amount of carbon created, less the amount reused in cellular respiration. To estimate net primary production, chlorophyll data is analyzed in context of sea surface temperature, incident solar irradiance and mixed layer depths. Current estimates place net primary production by the oceans at between 45 and 50 gigatons of carbon per year.