Int. J. Environ. Res. Public Health 2010, 7 2441 location, three kinds of radiances were measured: radiances of water surface (Lsw), a standard grey board (Lp), and skylight (Lsky). As shown in Figure 2, the instrument was positioned at an angle ϕ v of 90–135° with the plane of the incident radiation away from the sun. The view of the water surface θ v was controlled between 30 and 45° with the aplomb direction. Immediately after measuring water radiance, the spectroradiometer was rotated upwards by 90–120° to measure Lsky. The view zenith angle in this measurement was kept the same as that in measuring water radiance. Contrary to the other two radiances, Lsw was measured ten times at each sampling location. Reflectance spectra were checked and the dataset with obvious errors were abandoned. Then the average of the remaining dataset was used as the value of field measurements. Hyperspectral reflectance is defined as: Rrs = Lw  Ed (0+ ) Lw = Lsw − rLsky (1)  Ed (0+ ) = π Lp /ρ p   where Lw is the water-leaving radiance; Ed (0+ ) is the total incident radiance flux of water surface; Lsw stands for total radiance received from the water surface; Lsky refers to diffused radiation of the sky; r represents the reflectance of skylight on the air water interface, depending upon solar azimuth, measurement geometry, wind speed, and surface roughness; Lp is the radiance of the gray board; and ρp stands for the reflectance of the gray board [26]. Figure 2. Viewing geometry of spectra sampling. Surface water samples (2.5 L volume) were collected at a depth of 0.5 m below the surface immediately after the reflectance measurements. These samples were stored in a cooler with ice in the dark, and taken back to the laboratory for Chl-a concentration analysis within 6 hours. The pigment samples were extracted in hot (80 °C) 90% ethanol, and Chla concentration was quantified fluorimetrically.