Turbidity—Through a Water Column, Darkly
There are only two types of ocean water "out there", if you listen to an optical oceanographer. Optical oceanographers refer to these two types of water as cases, so the two cases are Case 1 and Case 2, as you might expect. Case 1 water is the kind of water that is found in the open ocean, far from land, nearly as transparent as glass, a deep azure when viewed over the side of a ship. In Case 1 waters, all of the optical properties are determined by the concentration of phytoplankton and its associated chlorophyll. In Case 1 water, the concentration of phytoplankton and chlorophyll is usually low. This combination of properties means that the optical properties of Case 1 water are relatively easy to analyze, and thus relatively easy to model.
And then there's Case 2 water, which is everything else.
Case 2 water might be the muddy-brown water found at the mouth of the Amazon, Mississippi, or Yangtze rivers, or the coffee-black waters of the Suwanee and Apalachicola rivers in Florida. Case 2 water could be found where hurricane winds have pushed the sediments found in a coral lagoon offshore into the adjacent deep ocean. Or Case 2 waters could be teeming verdant green waters loaded with chlorophyll and mixed with a little mud from the sea bottom, found in a coastal upwelling zone.
The image below shows a SeaWiFS view of the Yellow Sea (Huang Hai) on April 1, 2001, when Case 2 waters extended considerably offshore the coast of China. The cause of the turbidity here is primarily sediments carried by the Yangtze (Changjiang) River (which enters the Yellow Sea at Shanghai) and other rivers during heavy river flow caused by spring rains. Phytoplankton growth offshore also appears to be enhanced due to nutrients carried by the rivers. This SeaWiFS data was acquired by the HRPT station at the Second Institute of Oceanography in Hangzhou, People's Republic of China.
In short, Case 2 waters consist of water that is generally termed turbid. And that means that Case 2 waters are an optical oceanographer's headache, compared to the serene clarity of Case 1 waters.
But scientists love a challenge. So now we will look into the problem of turbidity, even though it might be difficult to see our way clear to a solution.
First of all, why are Case 2 waters important? There are two primary reasons. The first reason is that in many of the situations where Case 2 waters are encountered, these waters are significantly more productive than Case 1 waters. River mouths and coastal upwelling zones are two classic examples of this situation, where primary productivity (the production of carbon by photosynthesis, in this case by phytoplankton) 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.
As an example, let's take a look at the processed (Level 2) chlorophyll concentration image of the scene above.
The large areas of white near the coast of China and extending offshore are areas where the reflection of light from the sediments is so bright that it is interpreted as clouds. The same problem affects the southern coast of Korea. It is obvious that the chlorophyll concentration over a large part of this region can't be analyzed at all.
A second reason that Case 2 waters are important is that the coastal waters where the human influence on the marine environment is most significant are very often classified Case 2. Furthermore, scientists consider Case 2 waters important because they can mislead the algorithms that are used to calculate optical properties and chlorophyll concentration. Case 2 waters usually reflect more light than Case 1 waters, and this increased radiance can exceed the limits where the algorithms are most accurate. Thus, unless Case 2 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. The main condition that is used to assign this flag is high reflectance at 555 nm. Below is the distribution of the turbid water flag for the Yellow Sea scene shown in the above two images (shown in blue).
Because the large, curved area of black area east of Shanghai (corresponding to the curved white area east of Shanghai in the previous image) was covered by the cloud mask, this image demonstrates that there isn't much data in this image that is "above suspicion" in terms of returning reliable chlorophyll concentrations!
Now that we've both established the potential importance of Case 2 waters and seen an example of where they occur, let's look at the main reasons that analysis of Case 2 waters is difficult. Remember that for the optical properties of Case 1 waters, the two contributing factors are the optical properties of clear water and the concentration of phytoplankton. Case 2 waters may have several other factors mixed together to create their optical characteristics. In addition to phytoplankton, these factors might include:
- suspended sediments (sand, clay, or mud particles);
- colored dissolved organic matter (CDOM);
- Gelbstoff (which means yellow substance in German, referring to a polymeric organic material that's slightly too large to be considered dissolved);
- and even the presence of the sea floor, which can reflect some light, a situation called "bottom reflectance".
And there's another problem. Because Case 2 waters are generally brighter (more reflective) than Case 1 waters, the atmospheric correction algorithms that rely on the fact that most ocean water is optically dark at certain wavelengths become less reliable. Furthermore (as if that wasn't enough already), as Case 2 waters are frequently found in coastal areas, the overlying atmosphere is also a bit more muddled by terrestrial input, including smoke, the haze of pollution, and dust. This makes atmospheric correction more difficult as well.
What are we to do?
One thing that can be done is to conduct intensive analyses of how the optical properties of Case 2 water vary, to determine if patterns in their optical properties can be used to help analyze their content. Researchers at the City University of New York have found a unique way to observe changes in water turbidity in the Shinnecock Canal, located on Long Island.
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