Remote Measurement of Algal Chlorophyll in Surface Waters: The Case for the First Derivative of Reflectance Near 690 nm Donald C. Rundquist, Luoheng Han, John F. Schalles, and Jeffrey S. Peake Abstract Remote sensing is an important technology for measuring al- gal-chlorophyll concentrations in su$xe waters. Our paper provides hyperspectral signatures, in fhe visible and near-in- frored, associated with two experiments conducted outdoors in large water tanks; one involving relatively low amounts of chlorophyll over a narrow range and a second involving rela- tively high amounts over a wide range. The principal finding was that the commonly used near-infraredlred ratio is best for estimating pigment amounts when the concentration of chlorophyll is relatively low, and the first derivative of reflec- tance around 690 nm is best when the concentration is rela- tively high. Introduction Algal chlorophyll is measurable and quanaable in a labora- tory by extracting the plant pigments from the cells and ana- lyzing the pigment concentration in the extract (Nusch, 1980). However, because the amount of algal chlorophyll is indicative of the productivity and bophic status of surface waters, it is desirable to monitor pigment densities over ex- panses of geographic space and at numerous points in time, both within one "growing season" and from one year to the next. Therefore, remote sensing is viewed as an important technology for assessing chlorophyll concentration (e.g., Johnson, 1978). Purpose of Study The general goal for our research project was to investigate basic relationships between spectral reflectance and widely varying densities of algal chlorophyll, while the specific ob- jective was to compare techniques for analyzing amounts of algal chlorophyll in surface waters. The work summarized in this paper differs from previous studies in two ways. First, while most researchers study only a narrow range of relatively low densities of algal chlorophyll in water, we chose to analyze spectra for two experiments, one compris- ing a narrow range of relatively low chlorophyll amounts and one based on a wide range including very high pigment D.C. Rundquist is with the Center for Advanced Land Man- agement Information Technologies, Conservation and Survey Division, University of Nebraska-Lincoln, Lincoln, NE 68588- 05 17. L. Han is with the Department of Geography, University of Alabama, Tuscaloosa, AL 35487-0322 J.F. Schalles is with the Department of Biology, Creighton University, Omaha, NE 68178-0103. J.S. Peake is with the Department of Geography-Geology, University of Nebraska at Omaha, Omaha, NE 68182-0199. densities. Second, while we tested established procedures for measuring amounts of algal chlorophyll in surface wa- ters, we also introduced the use of the first derivative of re- flectance. A fundamental contention guiding our research is that empirical relationships between spectral radiance and algal chlorophyll in water can best be elucidated by collecting data at close range in manipulated tank mesocosms using a hyperspectral sensor. This approach allows one to eliminate entirely, or at least minimize, certain problems associated with data acquired at aircraft and satellite altitudes, not the least of which is the atmosphere. Literature Review Several investigators have analyzed the relationship between algal chlorophyll and remotely acquired measures of reflec- tance using hyperspectral instruments at close range. Gitel- son (1992) pointed out that the magnitude and the position of the maximum reflectance in the near-infrared (near 705 nm) can be used in predicting chlorophyll concentration. Mittenzwey et al. (1992) found a high correlation (0.98) be- tween chlorophyll and a near-infrared (~IR)/red reflectance ratio. Quibell (1992) examined the links between reflectance measured over different freshwater algal genera and chloro- phyll concentrations and found that, for inland waters, the volume reflectance in the NIK was best for estimating algal- chlorophyll concentrations. Rundquist et al. (in press) found that there was a stronger relationship between chlorophyll concentration and N1~1red for data acquired over a black sur- rogate bottom in shaIlow water than for white, and that the NIRlred ratio was more robust for predicting algal-chlorophyll concentration than was a simple red difference. The reader is referred to other related works, including Gower et al. (1984), Carder and Steward (1985), Vos et al. (1986), Hoge and Swift (1987), Vertucci and Likens (1989), Gower and Borstad (1990), Dekker et al. (1991), Gitelson and Kondratyev (1991a), Gitelson and Kondratyev (lgglb), Gower and Kon- dratyev (1991), and Quibell (1991). Experimental Design Study Site and Sky Conditions During Data Collection Two chlorophyll-dilution experiments were conducted at the Mead Research and Development Center [96"25'51" W and 41°10'34" N), an agricultural research station of the University of Nebraska-Lincoln, on 8 June and 4 August 1993, both un- Photogrammetric Engineering & Remote Sensing, Vol. 62, No. 2, February 1996, pp. 195-200. 0099-1112/96/6202-195$3.00/0 0 1996 American Society for Photogranmetry and Remote Sensing PE&RS February 1996