Natural waters differ optically from one another in color, transparency, and composition. Oceanographers and limnologists have developed different types of optical classification schemes to account for one or more of these attributes, and these are reviewed by Mobley  and Kirk . Jerlov  was first to establish the concept of optical classifications for regions of the ocean, and described classes of open-ocean and coastal water based on transparency. Morel and Prieur  divided ocean waters according to optically dominant constituents: in Case 1 waters, phytoplankton and their products dominate; in Case 2 waters, the dominant constituents are either mineral particles or dissolved organic matter not associated with phytoplankton. Kirk  classified inland waters according to optical constituents (W, G, A, and T are used alone or in combinations to indicate the role of Water, Gilvin = CDOM, Algae = Phytoplankton, and Tripton = inorganic particles). Prieur and Sathyendranath  proposed a similar optical classification system for seawater. Kirk's scheme is used below in the description of different optical constituents of natural waters.
The terms DOM, DOC, and CDOM can be confusing and are sometimes used interchangeably. DOM describes the uncharacterized dissolved organic matter in natural waters. While DOM concentration could be quantified on a dry-mass basis (g m-3) and values are found in the older literature, current analytical techniques (e.g., high temperature oxidation ), are calibrated in terms of the concentration of carbon atoms. The term DOC is now used when specific concentrations are reported. A molar or mass-based carbon concentration (e.g., g C m~3) is thus the preferred unit of measure for either DOM or DOC. In contrast to these measures, CDOM is a generic description of the "optical concentration" of DOM, or the concentration of colored substances such as humic and fulvic acids. CDOM is measured as a spectral absorption coefficient (acdom,/i) with units ofm-1.
Table 1 summarizes data on UV optical properties of natural waters from different regions. Section A describes marine sites while Section B describes freshwater sites. Each section presents UV-A and UV-B values, and entries are nominally sorted by value of attenuation or absorption coefficients. Attenuation, absorption and scattering coefficients have been converted, where feasible, to either 380 or 320 nm for easier comparison. The values of Kd 380 in the marine environment range from 0.03-0.8 m-1; in freshwater the range is 0.02-32 m_1. The values for Kd32oin the marine environment range from 0.07-37 m-1; in freshwater the range is 0.05-165 m~l. The lowest values are found in open ocean (Sargasso Sea and eastern Mediterranean) and deep lakes (Crater Lake and L. Vanda). These low values occur in environments where the "hydraulic residence time" is long and where the water is isolated from terrestrial sources of DOM and nutrients either by distance from land, high elevation, or high latitude. In most cases this UV-transparent water is also exposed to high levels of UVR. The values for Kdm in the region of the Baltic Sea and North Sea  and other coastal areas with large rivers  tend to vary with salinity in response to mixing of high-CDOM waters discharged by major rivers with low-CDOM waters from the open ocean. UV attenuation has been reported rarely in turbid systems (but see ). High values in Table 1 reflect either high DOM loading or evapoconcentration. UVR in lakes and estuaries where erosion or bottom resuspension contribute to extremely high turbidity can be assumed to exhibit rapid attenuation with depth.
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