by Rob Zisette, Timothy Clark, Katie Sweeney, and Eliza Spear, Herrera Environmental Consultants, Inc.
Herrera has been busy evaluating algae and aquatic plants in Washington lakes to develop plans aimed at improving public use and wildlife health. Over just the past two years, our team has been fortunate to work on seven lake cyanobacteria management plans (LCMPs) and four integrated aquatic vegetation management plans (IAVMPs) for lakes, primarily funded by Washington State Department of Ecology grants or legislature provisos. Here we provide examples of how we objectively evaluate management alternatives to best meet the needs of the lake and its community.
Lake Cyanobacteria Management Plans
Sediment coring at Pattison Lake
In accordance with Ecology’s guidelines and our detailed quality assurance project plans, we typically collect one year of water quality and hydrological data in a lake and watershed to understand the drivers of cyanobacteria and how best to prevent harmful algal blooms (HABs). We develop water and phosphorus (P) budgets in monthly time steps to quantify where algae get their most critical nutrient. A key step is estimating internal P loading from sediment release using multiple methods based on sediment P fraction analysis of sediment cores and the accumulation of P in the hypolimnion (bottom layer) during the summer period (May through October).
We then evaluate the effectiveness of many methods for phosphorus and cyanobacteria control. Stormwater and septic system management are often recommended to reduce long-term watershed inputs. A wide range of in-lake controls are evaluated, including physical methods (e.g., mixing, oxygenation, or dredging), chemical methods (e.g., P inactivation with alum or lanthanum), and biological methods (e.g., carp removal or goose deterrence). We work with the lake community to determine which methods best meet their needs, and provide a comprehensive list of possible funding sources for implementing the planned actions.
We recommended using oxygen saturation technology (OST) for HAB control for three of the seven LCMPs. OST is a relatively new and efficient way to prevent sediment P release caused by anoxia (no oxygen) in the hypolimnion while improving fish habitat without adding P inactivation chemicals. As described in our presentations at the 2024 OLA-WALPA and NALMS conferences, we recommended OST for those three lakes with anoxic hypolimnia because sediment P release was identified as the primary source (~65 %) of phosphorus for summer algae growth in each lake, and OST was found to be of similar or lower long-term cost than phosphorus inactivation by alum or lanthanum in a 20-year period.
Oxygen Saturation Technology A) Onshore oxygen supply B) Umbilical – O2 and power lines C) Pump & O2 contact chambers D) Automation with DO sensor E) Suction & discharge headers
We are currently designing a large OST system for the 251-acre Spanaway Lake in Pierce County, which has been plagued by HAB closures nearly every year in recent history. In addition, the Spanaway Lake Management District (LMD) funded a lanthanum treatment in 2024 to provide short-term relief while OST permits and funding are obtained. We are planning an alum treatment of the 61-acre Blackmans Lake this spring because the City of Snohomish preferred this proven method of HAB control for now. The City of Shoreline is focusing its efforts on stormwater management for long-term HAB control in 13-acre Echo Lake while they seek funding for OST or P inactivation.
Alum treated core in Lake Campbell Sediment Incubation Study
OST was not recommended for three lakes that don’t stratify. We recommended low-dose, short-term stripping of phosphorus from the water, in addition to watershed source control, for the 33-acre Lake Marcel in King County because its bottom waters drain from the lake and prevent sediment release. Alum treatment was recommended for the 367-acre Lake Campbell because this technique was very effective in the past, and our recent sediment incubation experiments showed high P release from both elevated pH and low dissolved oxygen.
Modification of an existing flushing channel was recommended for the 2,300-acre Vancouver Lake to increase by a factor of 10 the flushing of algae and P from the lake with adjacent Columbia River water. We also experimented in 2024 with an algaecide treatment of only the public swimming beach for short-term control while we further explore and model flushing options. Finally, we evaluated oxygenating the 921-acre, 130-foot-deep Curlew Lake in Ferry County, which has three separate basins, each with anoxic hypolimnia, but determined OST was too expensive. We instead recommended focusing on watershed controls and further exploration of seasonal lake level management. Lowering lake levels with a new flow control structure at the outlet during fall mixing would increase P export from the lake, and low levels through the winter would suppress undesirable aquatic weed growth by freezing exposed plant roots.
Integrated Aquatic Vegetation Management Plans
In accordance with Ecology’s guidelines, we work with lake stakeholders to prepare IAVMPs that map the density and distribution of noxious weed and native aquatic plant species, identify and prioritize plant species and areas for control, evaluate cost-effectiveness of plant management alternatives, and recommend a preferred management scenario to meet the community’s objectives and budget.
Egeria densa sample from Green Lake
Our IAVMPs incorporate adaptive management, recognizing the importance of using monitoring data and stakeholder feedback to modify plant management methods in the future. A good example of this is the multiple updates we have prepared since publishing the City of Kenmore IAVMP in 2017 covering 3.4 miles of the Lake Washington shoreline and 1.8 miles of the lower Sammamish River. Management methods initially included herbicides for milfoil, Egeria, and fragrant waterlily, then changed to diver-assisted suction harvesting (DASH) in response to City Council preferences for physical methods, and recently changed back to harvesting and then herbicides to save costs.
We are finalizing the Green Lake IAVMP in time for Seattle Parks to apply for an implementation grant from Ecology to fund the initial ProcellaCOR treatment of milfoil in 2025, focusing on clearing the proposed racecourse for the national rowing championships regatta. A unique and exciting aspect of this IAVMP is the native species plantings proposed after milfoil and Egeria treatments are complete, because native plants have been absent in the lake since it was stocked with sterile grass carp in 2002.
Aquatic plant sampling at Pattison Lake
Draft IAVMPs are being prepared for Pattison Lake and Offut Lake LMDs in Thurston County, both of which focus on dense growth of native plant species and fragrant water lily, which is the only invasive plant in the lakes. We are working with the LMD steering committees and membership to find a balance between practical physical methods and lower-cost herbicides, and addressing concerns about contaminating groundwater supplies with the applied herbicides and nitrates from plant decomposition.
For more information about Herrera’s work protecting lakes and water resources in the Pacific Northwest, visit our website at https://www.herrerainc.com/ or contact Timothy Clark at tclark@herrerainc.com or 971-361-2238.
