Toward a standard method of measuring color in freshwater

Researchers use various wavelengths between 270 and 450 nm to measure color. Data from 20 Quebec lakes indicate that spectrophotometric estimates of water color at different wavelengths are significantly different. We propose the measurement of light absorbance at 440 nm as a standard method for measuring color in freshwater and provide an equation to convert absorption coefficients at 440 nm (gaaO) to standard Pt units. An equation for standardizing reported color values estimated at different wavelengths is also provided. We show that PtCo standard solutions do not adequately mimic the spectral properties of aquatic humic compounds and that conventional Pt units are inappropriate for measuring these compounds in water. Tannic acid equivalents may provide a suitable alternate unit for quantifying color in freshwater. An equation is provided to estimate tannic acid equivalents from g,,, in lake water. The humic fraction of dissolved organic carbon (DOC) plays an important role in the functioning of aquatic ecosystems. Humic compounds are products of plant decomposition and affect aquatic organisms by reducing light energy transmission through water (Kirk 1976) and by affecting the solubility, transport, and bioavailability of nutrients and contaminants (Mierle and Ingram 199 1; Peterson 199 1). Humic substances may enhance plankton metabolism by providing chemical energy (Benner et al. 1986) or may inhibit secondary production in aquatic communities (Rasmussen and Kalff 1987). The importance of humic compounds warrants their quantitative assessment. However, precise fractionation and measurement of the humic compounds in water is a costly, exacting, and laborious Acknowledgments We thank W. Everett for field and laboratory technical assistance. Thanks also to J. B. Rasmussen for suggestions and to R. Peters for criticism of the manuscript. We thank J. F. Koprivnjak and Environment Canada’s Centre St. Laurent for dissolved organic carbon analyses. Contribution 298 ofthe McGill Limnology Research Center. undertaking (Malcom 199 1). Because of the difficulty involved in the direct measurement of aquatic humus, limnologists and oceanographers have instead used assays that measure properties of the humic substances, such as color, phenol content, or tannins and lignin (Thurman 1983). The most common of these, color, provides a simple index for assessing the concentration of humic substances in natural waters. For over a century, color has been measured by visual comparison of water samples with various standard color solutions. Europeans used the methyl orange (Ohle 1934) and the Forel-Ule methods (Thienemann 1925), whereas most North Americans used the Hazen method (use of Pt-Co standard solutions and corresponding Pt units) (see Am. Public Health Assoc. 1989), which is now the standard in limnology (Wetzel 1983). Although visual color measurement techniques are still widely used (e.g. Tranvik 1990), in the past two decades many researchers have switched to the spectrophotometric determination of color. For this method, the absorbance of light energy is measured at one or more wavelengths from within the near ultraviolet (UV) to visible range of the spectrum. The most frequently used wavelength is 440 nm (e.g. Kirk 1976, 1983; Bowling et al. 1986), although several others have also been used (e.g. 270 nm, Davies-Colley and Close 1990; 340 nm, Davies-Colley and Vant 1987; 400 nm, Koenings and Edmundson 199 1; 420 nm, Heikkinen 1990; 430 nm, Tranvik 1990; 405450 nm, Mierle and Ingram 199 1). Researchers convert sample absorbance values to color units (Pt, liter-l) with standard curves derived from spectrophotometric readings of Pt-Co solutions (e.g. Heikkinen 1990), or they report spectrophotometric measurements simply as absorbance values, or as base e absorption coefficients (Kirk 1983).