No Project associated with this Finding

Why Measure Transmissivity?

Both turbidimeters and transmissometers have been used to measure water clarity in the nearshore of Lake Tahoe. The main advantage of the transmissometer is that it measures light attenuation directly and is more sensitive in low turbidity waters, where its response is linear over the range of typical impurity concentrations. Theoretically, this approach depends only upon inherent optical properties and would be preferred for clarity measurements where water is very clean. The mechanics of making good transmissivity measurements in Lake Tahoe have been explored by various researchers (Taylor et al., 2004; Susfalk et al., 2009; Schladow et al., 2011), and these techniques continue to be improved. Transmissometers are superior to nephelometers  (turbidimeters) in clear waters, where large changes in transmissivity produce disproportionately smaller changes in turbidity. The history of transmissivity measurements at Tahoe is much shorter, however, so there is not as much data available for assessment of reference conditions. It is likely that the best approach for reporting on nearshore clarity will be a combination of both methods, with transmissivity ultimately taking precedence for status and trends monitoring, while turbidity is retained for compliance assessment associated with specific actions (e.g. dredging) or unusual conditions, and for representing a longer historical data set in status and trends evaluation.

Monitoring Data Summary

Recent nearshore clarity monitoring efforts have included measurements of transmissivity, which is better suited for long-term measurements than turbidity at the low background clarity levels typical in Lake Tahoe. As with nearshore turbidity monitoring previously discussed, the transmissivity was measured as part of several different research efforts over the years. These included both full-perimeter transects and more intensive localized surveys offshore from targeted locations, as well as spatial quantification of clarity in extremely shallow water less than two feet deep, and the development of infrastructure and operational protocols for a nearshore buoy monitoring platform to assess changes in clarity at static locations over time (e.g., Susfalk et al., 2009; Fitzgerald et al., 2012).

Transmissivity data from Lake Tahoe nearshore monitoring circuits were assembled from archived sources, and then reviewed for calibration and completeness. Of these data there were four suitable runs that were examined in more detail. These included nearshore circuits from 2008, 2009 and 2012. The sampling months occurred in April, June and August. One was from the anomalous period when the Tahoe Basin filled with smoke from nearby wildfires (August 2008), during which the summer turbidity also was substantially reduced. The same Thiessen polygons developed for turbidity analysis were applied along the nearshore to break it into 1000 m sections (Figure 11-2) for the transmissivity analysis.

Transmissivity Around the Lake

Results overall were similar to turbidity, interpreted as the inverse, with lowest transmissivity values observed in both south and north sections of the nearshore near urban areas (Figure 12-1). But there were some notable differences as well, particularly in the northwest portion of the lakeshore where clarity appeared to be relatively less than would have been expected from the turbidity results. These data only represent four separate runs, however, so any inference beyond overall patterns is probably not warranted.

Emerald Bay shows consistently lower transmissivity values (and higher turbidity) relative to most other areas of the nearshore. This is a consequence in part of its local geomorphological and natural ecological characteristics. It does not necessarily imply that Emerald Bay clarity has diminished. Unfortunately, there is no data prior to development and disturbance in the Tahoe Basin, so there is no basis for estimating a change in clarity for Emerald Bay. The rest of the nearshore may also be much different from pre-disturbance conditions, but it is expedient and probably reasonable to accept the best of existing conditions and evidence from early monitoring data to establish targets for standards and thresholds. In that process, Emerald Bay should be considered as unique from the rest of the lake.re localized scale.

Recommended Monitoring Plan

Although there is no current standard for light transmissivity in Lake Tahoe, it will be important to establish a monitoring program that would collect the data needed to more fully evaluate existing conditions, its variability, and the relationships to other metrics, like turbidity. The most practical monitoring approach is to implement this as a component of the nearshore perimeter surveys described for turbidity. Nearshore transmissivity measurements would be collected at fine spatial and temporal resolutions using a specifically equipped research vessel that samples lake water from a bowmounted sampling probe at a depth of approximately two feet below the water surface. Lake water is continuously pumped into the cabin, where it passes through an array of sensors that include the turbidimeters and a WetLabs C-Star light transmissometer (488 nm). These data are passed to the CR1000 dataloggers for computer processing, storage, and real-time display in conjunction with data from the GPS receiver.

The transmissometer is calibrated prior to each run by filling the reservoir with distilled, deionized water to establish full range response, and then covering the beam with an opaque barrier to set the zero response in mV. As with turbidity, the full-perimeter surveys are expected to typically require 2-3 days for completion and should follow the same path each time, but with excursion loops added at locations of unexpectedly decreased transmissivity or in areas of particular interest and extent (e.g. South Lake Tahoe or at the mouth of the Upper Truckee River).The sampling periods are also equivalent to the recommended turbidity runs. We recommend four sampling periods per year, seasonally, conducted at least 72 hours after significant wind or rain events. More sampling periods may be preferred during initial implementation of the monitoring program to inform assessment of variability. 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. 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.

Additional measurements may be warranted in some cases to describe localized clarity features in response to targeted events or to better understand the dynamics and interactions with other processes. These measurements would typically be conducted, however, as an additional research effort layered onto the established monitoring program.