Aquatic Plant Monitoring and Evaluation Plan and 2018 Lake Tahoe Nearshore Aquatic Plant Status Report

SIG teamed up with Marine Taxonomic Services, Oregon State University and Quantum Spatial to develop the Lake Tahoe Aquatic Plant Monitoring and Evaluation Plan (Plan) and 2018 Aquatic Plant Status Report. The Plan will be used by Tahoe agencies to guide future nearshore aquatic plant monitoring efforts. The Aquatic Plant Status Report provided the first ever comprehensive survey and map of aquatic plants in Lake Tahoe nearshore habitats.

Maintaining the biological integrity of Lake Tahoe’s nearshore environment is a longtime goal of resource managers in the region and has been set forth in a variety of planning documents. Submerged aquatic vegetation is an important biological component within Lake Tahoe’s nearshore context. Knowing the status of submerged aquatic plant populations at Lake Tahoe has become increasingly important to nearshore managers due to the introduction and spread of invasive species such as Eurasian watermilfoil (Myriophyllum spicatum) and curly-leaf pondweed (Potamogeton crispus). Aquatic invasive plants when left unchecked can affect fishing experiences, water quality, fish and wildlife habitat conditions, human and animal health, vessel navigation, and the recreation values of water bodies. For these reasons, this survey was conducted to assess the extent and distribution of submerged aquatic plants within Lake Tahoe’s nearshore zone.

Both in-situ (field) diver survey data and the analysis of remote sensing data were used to characterize the status of aquatic plants. Quadrat and transect survey methods were employed for in-situ field surveys. These methods were effective for determining aquatic plant species presence, composition, relative abundance and distribution. Field survey data were used in combination with high-resolution topobathymetric LiDAR data and 4-band imagery to map the distribution of submerged aquatic plants.

A total of 107 transects and 521 quadrats were sampled to evaluate submerged aquatic plant communities within the study area. Overall, aquatic plant cover was greatest in the marsh and lowest in the open-water nearshore transects. Results indicated that diver survey data was especially valuable for identifying different species of submerged aquatic plants, including aquatic invasive species, while the analysis of remotely sensed data could only resolve the extent and distribution of submerged aquatic plants. Remotely sensed data could not be used discriminate between individual species, including aquatic invasive plants. Nonetheless, estimates of aquatic plant extent and distribution were possible when mapped aquatic plant features were combined with diver data.

Mapping results showed that aquatic plants occurred throughout Lake Tahoe nearshore with a clear majority occurring in the southern portion of the lake. Results estimated the Eurasion watermilfoil was the most abundant of aquatic plant species, occupying approximately 29.7 acres of nearshore and associated habitats. A total of 8.8 acres of curly-leaf was estimated to occur within the survey area. Combined, aquatic invasive plants comprised nearly 20% of the aquatic plants detected across the nearshore survey area.

Although automated mapping efforts showed promise for mapping the extent and distribution of submerged aquatic vegetation, additional improvements in remote sensing data and technologies are needed to distinguish and map individual plant species, including discriminating native aquatic plants from aquatic invasive species.