Optical characterisation of natural waters by PAR measurement under changeable light conditions

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Abstract
Water bodies can be characterized optically by the euphotic depth, zeu (1% of surface light), the diffuse attenuation coefficients, Kd for downwelling light and Ku for upwelling light, and reflectance coefficient, R (ratio of upwelling to downwelling light), all in terms of photosynthetically active radiation (PAR). The estimation of all of these quantities from underwater PAR measurements is simple in principle but very difficult in practice under changing ambient light conditions caused by drifting cloud patches. A method of PAR measurement using a 2-channel instrument which records the ratio of the response of 2 PAR sensors and thus overcomes the problems of changeable ambient light is described. Relative PAR Q(z)/Qr, where Q(z) is PAR (either upwelling or downwelling) at depth z, and Qr is reference (downwelling) PAR, is much less dependent on ambient light when the reference sensor is located in water than when located in air (deck sensor). The profiles of relative PAR can be used to calculate Zeu, Kd, and Ku. R can be measured directly as the ratio of upwelling to downwelling PAR.