It’s Not Easy Being Green: Battling Egeria Densa in Green Lake

Over the past 30 years, Herrera has assisted Seattle Parks with the development and implementation of water quality and aquatic plant management plans for Green Lake in Seattle, WA. Additionally, Herrera staff members provide their expertise to Friends of Green Lake (FOGL), a grassroots organization of citizens, devoted to securing a healthy lake for recreation and wildlife.

Herrera’s team has been tracking the spread of highly invasive Eurasian watermilfoil (milfoil) in Green Lake, as a response to the die-off of grass carp planted 20 years prior. While assisting with FOGL’s milfoil cleanup last fall, Herrera aquatic scientist and Friends of Green Lake President Rob Zisette, discovered pieces of Egeria densa (Brazilian Elodea) on the northeast shoreline. If left uncontrolled, this Class B Noxious Weed poses a great threat to the lake, as scientists predict Egeria to reach the water surface, severely impacting lake recreation and habitat as it has in nearby Lake Union and Lake Washington.

During a survey in May of 2021, Rob along with a team of local experts discovered more Egeria than expected, particularly concentrated near the center of the lake. With too much Egeria for divers to remove by hand, Seattle Parks must look for alternative plans of action. When developing a plan for removal, scientists must consider several factors. For one, controlling large amounts of aquatic plants at one time can result in an algae bloom, further disrupting the lake’s ecosystem. In addition, the team must carefully weigh the detrimental impacts of Egeria on the lake with the pros and cons of various chemical and non-chemical control methods.

This August, the team hopes to survey Green Lake once more to determine how much of the water column Egeria occupies. These findings will give scientists more insight into the current impacts of this invasive species on Green Lake, helping them to develop a comprehensive solution. More information on these efforts can be found here.