Lake Tahoe Optical Model Theodore J. Swift * , John E. Reuter * , Jennifer Coker § , Geoffrey Schladow§ and Charles R. Goldman * Tahoe Research Group and § Dept. Civil & Environmental Engineering, University of California, Davis, CA 95616 Introduction One of the earliest signs of ecological change in Lake Tahoe, and one of the most highly valued measures of its aesthetics, is its world-famous clarity and color. In the early 1960s, a 20-cm (8-inch) diameter white Secchi disk could be discerned at a depth of 32 m (105 feet), on average. By the late 1990s the Secchi depth had reduced to approximately 20 m (65 feet). Secchi depth is among the target thresholds, along with nutrient and sediment loading, etc., driving the strategies for environmental restoration and management of the Tahoe basin. The optical model under development by the TRG is part of the effort to quantitatively couple the effects of nutrient and sediment transport processes to the present and future clarity of the lake. The model will provide a tool for evaluating management and restoration strategies in the watershed and studying ecological processes in the lake. Secchi depth is a sensitive measure of light attenuation in clear water. Secchi depth, and light absorption and scattering As a beam of light travels, it is attenuated by scattering and absorption. Scattering and ab-sorption are called inherent optical properties (IOPs) because they do not depend on the amount or direction of sunlight. Suspended particles, dissolved material, and pure water itself contribute to attenuation. Pure water absorbs light in the red portion of the spectrum, and scatters very little light overall, causing very clear water to have a bright blue or azure appearance. Inorganic sediments strongly scatter light, and scatter more short wavelength (blue) light than long wavelength (red) light, lending sediment-laden water its reddish-brown color. Algal par-ticles scatter somewhat, but primarily are strong absorbers in the blue and red portions of the light spectrum. In a sense, their primary function is to absorb light and thereby convert it into chemical energy stored in organic matter. By absorbing red and blue light, the chlorophyll and related pigments in algae make the water appear green. Colored dissolved organic matter (CDOM) consists of humic substances and tannins from decayed plant matter, including algae and terrestrial plants. It strongly absorbs blue and ultraviolet light and in sufficiently high concentrations, looks like tea. All these sub-stances are present in Tahoe water. The chal-lenge has been to separate and quantify the contributions of these sub-stances to the loss of clarity. A Secchi observation provides a sensitive and integrated measurement of attenuation. By understanding the scattering and absorbing characteristics of the dissolved and particulate matter in Tahoe, developing a mathematical model relating material concentration to clarity and Secchi depth, and verifying the model by comparing the predicted to the observed values, we can begin to develop a management tool. Light scattering and absorption by particles and water itself. Optical Model Components The formal relationship between the IOPs and Secchi depth are well developed (e.g., Tyler, 1968; Preisendorfer, 1986): The Secchi depth is the eye's contrast threshold (a quasi-constant) divided by the sum of the diffuse attenuation for down-welling light and the beam attenuation of light returning from the disk to the eye. These attenuations are functions of the absorption and scattering coefficients. The optical model takes CDOM and organic and inorganic particle concentrations, calculates the spectral scattering and absorption coefficients, weights these by the eye's photopic response, and uses Tyler's equation to arrive at a predicted Secchi depth. Scattering The wave nature of light causes it to scatter more strongly from very small particles of sizes on the order of a few wavelengths. The optical model takes the organic and inorganic particle size distributions (PSDs) into account in the scattering calculations. As is reported elsewhere in these summaries (Coker and Schladow), the typical PSDs in Tahoe water follows a hyperbolic distribution found in most natural systems, in which the number of particles is proportional to an inverse power of their diameter. A majority of the particles in the lake are 2 µm or less in diameter, in the size range that scatters light most strongly. Materials in Tahoe water absorb light at distinctive wavelengths. Absorption CDOM, though detectable in Tahoe water, contributes only ~4.5% of visible-light ab-sorption. This proportion drops to ~1.5% when weighted by the eye's photopic response to color. Attention has therefore focused on particulate material in the lake and tributary streams. We concentrate particles from measured volumes of lake water (often 1 to 2 liters) onto glass fiber filters and measure their spectral absorption in the laboratory. Particulate absorp-tion varies seasonally and with depth, but typ-ically about 55% of photopic absorption is due to soluble pigments; the remainder is due to other, insoluble organic and inorganic matter. Principle parts of the Tahoe clarity model. Findings and comparison between the model and observation Field measurements and preliminary model calculations agree well over a wide range of Secchi depths (see below). Very fine particulates, both organic and inorganic, cause the reduced clarity of Lake Tahoe. Dissolved organic matter, while found in higher concentrations in streamwater, is not the cause of the seasonal or long-term loss of clarity. Inorganic (sediment) particles contribute disproportionately to clarity loss due to their higher scattering efficiency and slow settling rate. Continued study of the physical and biological processes affecting these particles is needed. Comparison between observations and model. For further information, contact: Ted Swift Dept. Environmental Science & Policy University of California, Davis, CA 95616 tjswift@ucdavis.edu (530) 752-2913