There are three major factors that govern weed growth. These are (1) the nutrient level in sediments and in the water column, (2) the amount of exposure to sunlight, and (3) water temperature. In essence, the higher each of these factors are the more prolific the weed growth. Since these factors vary from year to year so too does the magnitude of the weed problem. Whereas the nutrient level in sediments does not change that much from season to season, rooted aquatic plant growth is most affected by the absence or presence of cloud cover and water clarity during the period of their most active growth. Water temperature is largely controlled by the amount of sunlight, the source mix of inflowing water (e.g., warm surface vs cold ground water), and water circulation (mixing) patterns.
As water temperatures start to decline and daylight hours are shortened these plants begin to enter a slow growth pattern and eventually die off.
Submersed Weed Control
Desired and/or planned outcome:
Submersed weed control is designed to promote full recreational use of the lake during the summer months in an effort to provide residents and the local community with a safe swimming and boating environment. Timely macrophyte control is utilized to eliminate plant biomass and prohibit seed production in an effort to reduce future years plant densities often a result of unimpeded yearly seed germination.
Weed control is authorized through the Washington State Department of Ecology under the current NPDES permitting system. Treatment timelines are established by the Washington State Department of Fisheries. Lake Steilacoom fisheries support salmonid species. In an effort to avoid treatment during critical timeframes when fish are migrating through the system, herbicide use is permitted after June 15 on a yearly basis. In addition to establishing a treatment timing window the permit also limits treatment to 160 acres of the lakes liitoral zone. Acreages treated on a yearly basis vary from year to year.
Lake Steilacoom supports a diverse aquatic macrophyte community consisting of native and non-native species. Macrophyte communities consist of numerous Potamogetons (pondweeds), Ceratophyllum (coontail), Elodea, Myriophyllum (native milfoil), Callitriche, Nitella, Chara and Fontinalis (moss). There has been only one non-native noxious plant identified within the lake basin, Potamogeton crispus. This particular plant has not increased in density or range over the past ten years.
Contact herbicides have typically been the control technique utilized in controlling submersed weed growth. Contact herbicides afford a quick and economical control alternative throughout the system while maintaining a healthy and productive aquatic environment. Diquat (Reward) and endothol (Aquathol K) are the selected contact herbicides utilized throughout Lake Steilacoom. Typically one treatment performed shortly after June 15 has provided excellent seasonal control in addition to also reducing plant densities the following years. Occasionally a late season application is required to control Ceratophyllum in the deeper areas of the southern basin.
Historical treatments have proven to be cost effective and efficient in maintaining Lake Steilacoom wasters weed free during the summer months. Weed densities remain at acceptable levels supporting a healthy biological environment while also providing a safe recreational environment to the local residents. Past successful applications of contact herbicides to the lake supports product utilization in future years.
Sonar (fluridone), a systemic herbicide has been applied to the lake without success during 1999. This application involved the granular formulation applied to the northern basin. Monitoring of the site four weeks post application revealed fluridone levels below detectable limits of the laboratory equipment. Fluridone use requires aquatic plants to be exposed to lethal concentrations for ten to 12 weeks Failure of fluridone use was attributed to a poor treatment plan associated with not maintaining lethal concentrations during the required exposure timeline. Lake Steilacoom has a swift turnover rate and multiple applications applied at three week intervals would likely have been required. Typically multiple applications are required even in those water bodies that have no outflow. Future sonar use is not being considered at this time because of the high cost associated with the treatment and swift water flow through the system.