April 21, 2020
Author: Emmanuel Boss
Taxonomies of water types
Classiﬁcation of natural waters into optical water types, as many other taxonomies, is done in order to generalized and systematize the science of ocean color. Arnone et al. (2004), in a review paper on water mass classiﬁcation, credits the Secchi disk (invented around 1865) as the ﬁrst quantitative optical instrument that has been used to measure water transparency and hence provided the ability to diﬀerentiate water mass based on their optical properties. Classiﬁcation schemes that are based on water color are primarily based on absorption (as scattering contributes to intensity but less to color).
Classiﬁcation Schemes of water types are based on
- Human eye: compare color to a ﬂuid sample
- Analysis of AOP spectra
- Analysis of inherent optical properties (ternery diagrams)
The Forel-Ule Scale is a method to approximately determine the color of bodies of water. Diﬀerent inorganic compounds with standard formulas are used to produce a color palette with twenty one ﬂuid vials covering blue to green to brown. The analysis consists of ﬁnding the vial whose color best matches the color of the water body; the result is an index from 1 to 21. The method is often used in conjunction with the Secchi disk and is included in data bases such as NOAA’s NODC. The method was developed in the 1890s by Francois-Alphonse Forel and William Ule. Although now replaced by instruments for quantitative work, the Forel-Ule color measurements are still of interest because of the ease of making measurements and the large database of historical observations, which has value for studies of long-term changes in ocean ecosystems (Wernand (2011). There is now even a smart-phone app for making Forel-Ule measurements.
Another comparative scale is the Hazen Platin-Cobalt-Scale adopted as the American Public Health Association (APHA) color scale (described in ASTM D1209, ”Standard Test Method for Color of Clear Liquids (Platinum Cobalt Scale)”). It is based on diluting a standard stock solution (500ppm of PtCo) and ﬁnding the dilution whose color best matches the ﬂuid compared.
Jerlov (1968) introduces a classiﬁcation of water bodies based on their spectral optical attenuation depth . Since ocean color is proportional to this spectra is linked to the color observed. Jerlov discritizes his observations into a set of ﬁve typical oceanic spectra and nine typical coastal spectra (e.g. Table XX in Jerlov (1968) which is linked here).
Morel and Prieur (1977) have described the spectral shape of and it changes as separating two types of waters.
Case 1 corresponds to situations where the chlorophyll concentration is high relative to the scattering coeﬃcient. As the pigment concentration increases several changes occur:
- The R values in the blue-violet region decrease progressively and a minimum is formed around 440 nm, which corresponds to the maximum absorption of chlorophyll. The maximum shifts toward 565-570 nm, which is the wavelength where simultaneously the absorption due to pigments is at its minimum and the absorption due to the water itself rapidly increases.
- The second maximum of absorption by chlorophyll a in vivo creates an R minimum near 665 nm. This minimum is only slightly marked because the increase in absorption due to the presence of chlorophyll remains weak compared with the absorption due to water itself.
- At 685nm a second reﬂectance maximum appears due to ﬂuorescence by chlorophyll superimposed on absorption/scattering interaction near the 676nm absorption band.
- A hinge point in R values in the 560-640-nm band is observed with relatively little change as chlorophyll varies.
In case 2 inorganic particles are relatively more dominant than phytoplankton. As the turbidity increases the following modiﬁcations appear:
- The reﬂectance values are generally higher than for case 1 throughout the spectrum and of a diﬀerent shape. There is no longer, as in case 1, a minimum at 440 nm. On the contrary, the curves become convex between 400 and 560 nm (inverted ’U’ shape rather than inverted ’V’). The maximum is ﬂatter than in case 1, but located at the same wavelength, 560 nm.
- R values become higher as turbidity increases. This is opposite of what was observed in case 1, particularly for wavelengths above 550 nm.
- As a result of the ﬂat shape of the R curves, the dominant wavelength does not shift beyond 510 nm. These waters are blue-green or green with a bright and milky appearance due to the combined eﬀects of high E values and of low purity values.
In recent year Case I and Case II water-types have changed in meaning. They are often used to deﬁned waters where all component vary with chlorophyll (Case I) to those where it is not the case (Case II). This distinction has been also used to delineate the waters where reﬂectance ratio algorithms work well to obtain chlorophyll concentration (Case I) vs. those where it doesn’t. Mobley et al. (2004) however, suggest that it is time to drop this type of classiﬁcation as it sometimes obfuscates more than it helps. For example, water in the same area may be case I during some time of the year and case II in other times or the fact that waters with similar chlorophyll may have big variations in ocean color in open ocean environment (e.g. Eastern Mediterranean vs. South Paciﬁc gyre).
Ternary diagrams are used to diﬀerentiate water masses optically by designating a point in a triangle based on the relative contribution of absorption by phytoplankton, non-algal particles, and CDOM to the total absorption (minus that of salt water) at a speciﬁc wavelength. A point at the center of the triangle designate equal contribution by all, while each points at the apex designate water where all the contribution is from a single component. An example of a ternary diagram applied to absorption is given in Figure 16 of Babin et al. (2003b) at various wavelengths for European coastal waters. The diagram shows well how at diﬀerent wavelengths diﬀerent constituents (dissolved material, algae and non-algal particles) dominate absorption.