1a.Objectives (from AD-416):
Determine efficacy of non-chemical methods for management of invasive aquatic weeds in Lake Tahoe, and provide information on potential movement and dissipation of aquatic herbicides within South Lake Tahoe marinas that currently support nuisance growth of invasive plants.
1b.Approach (from AD-416):
Pre- and post- “application” assessments of target plant distribution and biomass will be determined as part of deployment of “bottom barriers” composed of (1) synthetic material and (2) natural jute mesh fabric. As a surrogate for aquatic herbicides (not yet permitted for use in Lake Tahoe), Rhodamine WT (dye) will be applied in specific locations within the study site and its movement and dissipation will be determined by frequent monitoring and detection using boat-mounted flow-through fluorometers and a fixed station, autologging fluorometer.
Dye dissipation studies will be conducted in spring and fall periods to emulate typical herbicide application timing.
This research specifically addresses Objectives 1 and 2, weed biology and integrated management. Invasive weeds continue to spread and impair ecosystems services at Lake Tahoe, particularly in the economically important South Lake Tahoe (Tahoe Keys Marina). An ARS Scientist from Davis, California, conducted studies to help resolve this problem. Rhodamine WT (RWT) dye was injected in mid-October 2011 at four sites in South Lake Tahoe Keys as a herbicide surrogate to determine movement and dissipation characteristics. Real-time monitoring using ship-board fluorometry showed that RWT injected in “dead end” area had residence times exceeding 30 days and that RWT injected in “open sites” had residence times of one to two weeks. Monitoring also showed that some dye moved from the Tahoe Keys to the lake proper, though at extremely low concentrations. These results, in conjunction with similar studies conducted in early summer 2012 suggest that applications of EPA- approved herbicides to “dead-end” areas (which comprise about 75% of weed-infested areas), would result in efficacious contact times and concentrations either in early summer or early fall. The earlier summer study showed that residence time in open areas was extremely brief (<24 hrs) and that water movement was primarily southward from the lake proper into the Keys.