Lake Tahoe Particle Characterization 1999-2000 Jennifer Coker and Geoffrey Schladow Department of Civil & Environmental Engineering, University of California, Davis, CA 95616 Lake Tahoe particles were characterized in terms of number concentration, size distribution, and percentage of mineral and organic particles throughout 1999-2000. These results were used to estimate the magnitude of the scattering coefficient and to estimate the roles that individual types of particles had on the clarity of Lake Tahoe water. Particles were analyzed for number concentration and size distribution using an LS-200 (Liquid Sampler) and LiQuilaz-S05-HF sensor manufactured by Particle Measuring Systems Inc (Boulder, Colorado). The system measures particles with diameters between 0.5 mm and 20 mm in diameter, the size range that scatters the greatest amount of light. This size range is also of interest because particles with these small diameters have extremely slow settling velocities, and tend to be retained in the water column. The fraction of organic to inorganic particles was determined using Scanning Electron Microscopy coupled with X-ray energy spectroscopy. In Figure 1, results are shown for 409 particle size distributions measured for both the Index and Mid lake stations throughout 1999-2000. Normalized particle concentrations are presented (normalized by the width of the channel, i.e. the concentration of particles between diameters 1 m m and 2 m m would be divided by 1m m, thus the units are particles/ml/m m channel width). As is consistent with the literature, it Figure 1: Index and Mid lake station Particle Size Distributions 1999-2000   was found that the number of particles increased rapidly with decreasing size, and the majority of particles were smaller than 2 m m in diameter. All particle size distributions were described by a one-parameter hyperbolic distribution with exponents between —2.5 to —3.5, most having r2 values of greater than 0.97. As can be seen in Figure 1, the best-fit trendline has an exponent of —3.1, and the r2 value of 0.95. The highest particle number concentration for the Mid lake station, 130,000 particles/ml, was measured on 9/13/99 at a depth of 230 m. This concentration was confined to thin layer of water, at most stretching from just below 220 m to just above 240 m. Assuming this sample was not contaminated in some fashion, the average depth weighted particle concentration for the Mid lake station is approximately 11,900 particles/ml. However, if we assume that this sample was contaminated, the maximum particle concentration would be 45,000 particles/ml, and the average depth weighted concentration would be closer to 11,600 particles/ml. The minimum concentration measured at the Mid lake station was 1700 particles/ml. The maximum and minimum concentrations measured at the Index station were 25,000 particles/ml and 1,800 particles/ml respectively. The average depth weighted particle concentration at the Index station was 7,600 particles/ml. While both maximum and minimum particle concentrations were measured at the Mid lake station, on average, the Index station has lower particle concentrations. In Figure 2 and Figure 3 results are presented for the fraction of organic and mineral particles determined from X-ray energy spectroscopy for the Index station and Mid lake station respectively. A seasonal pattern is evident from Figure 2. In early October, the Figure 2: Fraction of organic to inorganic particles, Index Station 1999-2000 error bars denote 95% confidence ratio of organic to inorganic particles is about the same. Starting at the end of October through mid December, the ratio shifts, and begins to favor more organic particles. According to Jassby et al. (1999) thermal stratification usually peaks in August, and the thermocline begins to deepen in September. As the mixed layers reaches the 60 — 120 m region around December, it encounters the deep chlorophyll maximum, a common feature of Lake Tahoe and other deep water bodies. It appears that the subsequent upwelling of phytoplankton from this maximum, was the cause of the phytoplankton (organic particle) increase in the surface waters. From early February through early March, the ratio of particles again becomes approximately equal — either due to further deepening of the mixed layer into chlorophyll-depleted water from the hypolimnion which now dilutes the surface water and/or the beginning of the snowmelt. The last measurement, taken in early April shows the ratio has again shifted to be dominated by organic particles — perhaps due to warmer temperatures and increased availability of nutrients. There was considerably more variation in the ratio of organic to inorganic particles observed at the Mid lake station for 1999-2000. This variation in ratios is probably due to nutrient limitations found at the Mid lake station. Figure 3: Fraction of organic to inorganic particles Mid lake station, error bars denote 95% confidence. In Figure 4 the cumulative contribution to the scattering coefficient is plotted versus particle diameter for a typical size distribution and number concentration of particles found in Lake Tahoe Surface water. Two extreme cases are considered —     Figure 4: Fraction of Scattering at 550 nm vs particle size for a typical size distribution of Lake Tahoe surface water, exp=-3.14, C=4500 (Concentration of particles >0.5 um =9500 particles/ml)   all inorganic particles and all organic particles. It is evident from Figure 4 that particles smaller than 10 m m comprise the vast majority of particulate scattering (up to 70% for inorganic particles. Particles of diameters smaller than 3 m m contribute 10% and 45% of the total scattering coefficient for organic particles and inorganic particles respectively. Comparison of estimates of absorption and scattering measured within days of each other have revealed that scattering is typically smaller than measured absorption, though it can range from 30%-200% of the absorption coefficient (Swift, personal communication). Although results are still somewhat preliminary, it appears that absorption may dominate the beam attenuation coefficient during the time of year which does not include the spring snowmelt; this is consistent with the previous findings of Jassby et al. (1999) . Data is currently being collected and analyzed for that important period when clarity is at a seasonal minimum. It is still unclear to what extent nutrients are associated with suspended particles — a line of research that will be undertaken.