Lake Tahoe Nearshore Evaluation and Monitoring Framework- Turbidity

No Project associated with this Finding

Finding Details

Nearshore clarity has been measured with grab samples of turbidity and/or snapshot boat-based surveys of turbidity and light transmissivity (Taylor et al., 2004; Susfalk et al., 2009). Short-term monitoring buoys in the nearshore in 2009 and 2011 have also measured turbidity and light transmission at specific locations over extended periods of time. From a terrestrial perspective, where the nearshore is treated as an extension of on-shore activities, the use of turbidity as a nearshore-zone metric is consistent with current urban runoff and stream monitoring uses. However, as discussed previously, light transmissometers are generally better suited for measurements that track changing conditions at the high clarity (low turbidity) levels typical for this lake.

Monitoring Data Summary

Historical data from Lake Tahoe nearshore monitoring circuits were assembled from archived sources, then reviewed for calibration and completeness. Of these data there were nine suitable runs that were examined in more detail. These included nearshore circuits from 2001 through 2003, as well as more recent runs from 2008, 2009 and 2012. The sampling months were typically from March through September, which were separated into what are considered both winter-spring and summer-fall periods. One anomalous period was included, representing a year when the Tahoe Basin was filled with smoke from nearby wildfires, during which the summer background nearshore clarity was reduced.

Turbidity Around the Lake

Waters within the nearshore zone reflect both on-shore influences and lake environmental factors within the immediate vicinity, where it has not yet undergone mixing with cleaner midlake waters. Whole lakeshore surveys presented here, in Taylor (2002) and in Taylor et al. (2004) found that areas of decreased water quality were associated with zones of greater onshore urbanization. This is clearly evident in the compilation of plots for historic and recent nearshore circuits (Figure 11-3). South Lake Tahoe shows consistently higher turbidity values than observed at most other parts of the lake, while the zones around Tahoe City, Kings Beach and Incline Village show somewhat elevated readings compared to the lower readings typical of northeast and southwest shorelines.

How to Measure Turbidity

Monitoring approaches can include measurements taken from both buoy- and boat-based platforms. A buoy-based platform excels at collecting continuous longer-term temporal data at select locations, whereas boat-based measurements can collect extensive spatial data during select time periods. Although a robust monitoring plan should include both buoy- and boat-based measurements, the initial monitoring plan suggested here includes only a boat-based approach for two reasons. First, the proposed boat-based measurements include the manual collection of data that provides a better-integrated dataset of nearshore ecology by directly supporting fishery and periphyton thresholds. These types of measurements are either not suitable for routine remote measurement or would be prohibitively expensive to implement on a buoy. Second, previous studies (Susfalk et al., 2009, Fitzgerald et al., 2012) have suggested that at least four buoys would be needed for a cost-effective program, but an even larger number would be necessary to adequately assess the highly variable nature of the nearshore. A potentially significant amount of maintenance (with associated costs) would be needed for ongoing calibration activities and for responding to quality assurance issues in a timely manner due to unplanned events that affect sensor performance. This is of less concern for boat-based measurements, where sensors will be calibrated at least daily and the boat crew can immediately address unpredictable anomalies and recollect data, if necessary.

In-lake turbidity measurements are collected at fine spatial and temporal resolutions using a specifically equipped research vessel built for year-around use in Lake Tahoe’s shallow nearshore zone. Lake water is continuously sampled from a bow-mounted sampling probe at a depth of approximately two feet below the water surface, depending on boat speed, depth to bottom, and ambient wave conditions. Surveys typically consist of full-perimeter lakeshore runs over the course of 2–3consecutive days.

Recommended Monitoring Plan

We recommend four sampling periods per year, seasonally, conducted at least 72 hours after significant wind or rain events. More sampling periods would be preferred during initial implementation of the monitoring program to inform assessment of variability, but four is considered a reasonable number if funding is limited. The objective is to define both low and high periods of clarity, which occur seasonally associated with lake mixing, snowmelt, recreational boating, and other factors. Four sampling circuits around the lake also will provide enough flexibility so that at least two sampling runs could be available for each of the high and low clarity periods if needed to assess measurement variability. Each whole lakeshore survey should be conducted at the 3 meter water-depth contour, relative to existing lake level. In shallow areas where this 3-m water-depth contour exceeds a lateral distance of twice the minimum nearshore width (700 feet) from shoreline, additional survey data will be collected at a lateral distance of 350 feet from the shoreline or as close as safely possible. In these shallow areas, water will be collected from a depth of approximately 1.5 to 2 feet from the surface. The primary areas where this will occur are offshore of Tahoe City and the City of South Lake Tahoe.