There are four major factors that govern toxic blue-green algae growth. These are (1) the nutrient level in the water column, (2) the intensity of solar irradiance, (3) water column stability, and (4) water temperature. High soluble phosphorus concentration in the water column favors the growth of toxic blue-green algae. A characteristic of blue-green algae is that they can control their buoyancy. This allows them to rise or fall in the water column to optimize their exposure to sunlight and nutrients which are found in greater concentration at the interface of the warm upper layer of water and the underlying cold lower layer of water. Since blue-green algae grow slowly prolonged periods of exposure to warm water temperatures, nutrients and a stable water column fosters their growth.
As the slow growing blue-green algae multiply they begin to block out the sunlight that the bottom dwelling rooted aquatic plants require for continuation of their growth. The aquatic plants begin to die off. By late summer and early fall blue-green algae populations have reached bloom proportions and the lake becomes a lethal soup of dead and decaying aquatic plant fragments and toxin laden blue-green algae cells.
Chemical Alternatives Utilized
Steilacoom Lake is plagued with toxic cyanobacteria blooms on a seasonal basis. In general, blooms have historically created conditions that have warranted Pierce County Health to post the lake advising lake users that toxic conditions, according to World Health Organization standards, are present. Circumstances that require lake closure usually occur after September 1. Formation of surface scum is typically not a proficient indicator of water clarity since Steilacoom Lake supports healthy secchi disc readings in excess of three meters throughout a majority of the summer and fall months.
Over the past twenty years the homeowners association has been very aggressive in trying to find a solution to the cyanobacteria blooms. The association expects healthy water quality to prevail during the high use summer months. A number of residents feel that bloom conditions that may arise during the fall months, when lake use is at its lowest level, possess a reduced risk and is acceptable. Control philosophies currently center on the need to provide lake residents with water quality conditions that do not produce toxic blooms during those high use summer and early fall timelines. Until the early 1990’s copper sulfate was applied two to three times per year in an effort to manage the blue-green algae problem. During this time frame no toxic blooms were reported. Copper sulfate use was banned in Washington State during the mid 1990’s and the search by the homeowners association to discover new tools to combat the cyanobacteria blooms began.
Hydrothol 191 is a contact algaecide that attacks the cell wall. Effective use of Hydrothol requires application rates being applied within the targeted water column between .3 and 1.5 ppm. Higher rates are necessary in those situations where cells counts are elevated and greater longevity of control is desired. Repeat applications may be necessary when alga reappears. Elevated algae densities require higher concentrations of material to be applied in order to impact the targeted species.
Three applications of Hydrothol 191 were made to the lake during the 1999 season in an effort to control blue-green algae species. One application was administered to the northern basin and two applications were applied to the southern basin. All three applications failed to prevent toxic algae blooms. Application failure was determined to be a result of inappropriate application rates, not treating a large enough volume of the water column containing the targeted species and treating when algae concentrations were elevated above threshold limits. As a result of the poor performance during 1999 further use of Hydrothol 191 was not considered until 2007.
Hydrothol use was revisited during the late summer of 2007. After designing a treatment scenario supported by the product manufacturer, 200 acres of the lake were treated and evaluated. Monitoring pre and post application acknowledged that blue-green algae concentrations had declined from 440,000 cells/ml to 120,000 cells/ml. within the immediate treatment site. Monitoring at the control site revealed no change in the pre and post blue-green cell counts at 340,000 cells/ml. Sampling at the treatment and control sites two weeks post revealed algae concentrations within the treatment area at 160,000 cells/ml while concentrations at the control site remained elevated at 320,000 cells/ml.
As a result of the successful 2007 Hydrothol application, the homeowners association has approved future use of Hydrothol when warranted to avoid blue-green bloom conditions.
Green Clean is 100% soluble in water and when applied it rapidly dissociates to sodium carbonate and hydrogen peroxide (H2O2) molecules. Green Clean’s active ingredient is a form of concentrated Sodium Carbonate Peroxyhydrate. Once active in the water column Green Clean exhibits both external and internal modes of action. Hydrogen peroxide reacts quickly with algae cells and requires quick delivery of the active ingredient to the targeted site due to the short residual time the product remains in its active state.
In an effort to evaluate the potential use of Green Clean for lake wide blue-green algae control, application to a ten acre protected cove during an occurring bloom was undertaken. The cove treated was shallow in comparison to the main water body having an average depth of only five feet. Application was completed by 1:00 pm on a bright warm sunny summer day. At the conclusion of the treatment, visual inspection of the site determined that the application was already producing algae cells that were bleached and floating on the waters surface. Algae counts and secchi disk readings were taken pre and post application and evaluated.
Sampling and visual observation of the site, forty eight hours post application, identified no observable differences between the cove treated and the conditions throughout the rest of the main water body. Cell counts and secchi disk data supports the visual observations made at the time of inspection.
Green Clean is a rapid acting material with a relatively short active life span in the water column. As applied in the granular formulation Green Clean slowly sinks through the upper part of the water column dissolving within four to five feet of the waters surface.
The short life span of the active ingredient requires the use of quick and efficient application protocol in order for the material to be effective. Following the application windy conditions created surface movement of untreated algae back into the treatment zone after the active ingredient was no longer at concentrations high enough to effectively control algae re-entering the site.
Steilacoom Lake is a 320 acre water body that generally requires treatment on a whole lake basis. Re-infestation from areas of the lake waiting to be treated during an application cycle would likely be re-colonized from wind blown untreated portions of the lake. In addition to the problem created by the short life span of the material in the water Steilacoom lake blooms typically occur throughout all of the upper waters to about the twelve foot depth. Green Clean as a granular formulation would not have the ability to impact those algae cells residing below five feet of the waters surface.
Green Clean appears to be an effective tool at addressing algae problems on a site specific and under specific lake conditions. Successful product use appears to be dependent upon the size of the area treated in relation to the entire impacted water body. Future use of Green Clean as a whole lake treatment method is no longer under consideration.