No Project associated with this Finding

Why Measure Chlorophyll?

There are several kinds of chlorophyll pigments, but chlorophyll a is the predominant type found in algae (Wetzel 2001). Chlorophyll a (Chl-a) concentration is used as an estimate of the amount of algal biomass in the water. High concentration of Chl-a is a primary characteristic of nutrient rich water because excess nutrients fuel the growth of algae. Lower algae levels promote better water quality and improved transparency.

What do we know about Chl-a in Lake Tahoe?

While Chl-a concentrations of the plankton in the open-water or pelagic zone of Lake Tahoe has been sampled with some regularity (especially since 1984; TERC 2011), Chl-a data from the nearshore or littoral plankton is much less common and only exists as part of limited, isolated studies.

Recent monitoring using in situ fluorescence to estimate Chl-a also has been done by the Desert Research Institute (DRI), both along the south shore and complete lake perimeter. Remote sensing data was used by Steissberg et al. (2010) in a detailed analysis of spatial and seasonal patterns of distribution of Chl-a in the upper euphotic zone of the nearshore. Recently (August 2011), researchers with the U.S. EPA, TERC and DRI, circumnavigated the lake as part of the PARASOL study (PARticulates And SOLutes in lakes) and took measurements of Chl-a (Kelly pers. comm.).

Where is Chl-a?

Relative Chl-a was measured during different research projects over the years, along with turbidity and transmissivity. These included both full-perimeter surveys and more intensive localized surveys offshore from targeted locations. There were eight full surveys of nearshore Chl-a that met quality control criteria and these data were analyzed in Thiessen polygons (Figure 13-4), as described for turbidity. These relative Chl-a results show greater spatial and temporal variability than observed for turbidity and transmissivity. South shore values were always higher than average, while east shore was generally lower than average. The Chl-a response reflects an integration of multiple highly variable factors including nutrients, temperature and zooplankton. While measurements of Chl-a have improved, the approach is subject to bias inherent in shallow water Chl-a measurements due to the quenching of fluorescence by ambient light, and the time required to complete a whole lake circuit. Improved techniques will be required to overcome some of these unresolved issues.

What’s Next?

No standard currently exists for nearshore Chl-a in Lake Tahoe. Therefore, it is imperative 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 and indicators. Winder and Reuter (2009) developed an extensive monitoring protocol for Chl-a. This should be combined with a routine of full-perimeter nearshore surveys for turbidity, transmissivity, and chlorophyll.

Recommended Monitoring Plan

We recommend a set of depth profiles distributed around the nearshore during perimeter cruises, associated with phytoplankton collections. These depth profiles should include discrete samples taken for absolute Chl-a measurements and AGP, as well as continuous down-cast measurements for relative Chl-a, transmissivity and turbidity. As with turbidity and transmissivity, these surveys are expected to typically require 2 to 3 days for completion and should follow the same path each time. We recommend at least four sampling periods per year, on a seasonal basis, with 3 to 9 depth profiles distributed around the lake perimeter. More frequent sampling may be required initially to establish a robust dataset for calibration. This is an important metric, and is currently an area of active research, but the approach, methods and technology may change substantially as existing issues associated with obtaining reliable high-quality are resolved.

For additional information, view the Nearshore Evaluation below.