by Annmarie Delfino, In-Situ, Inc.
Como Lake inspired the founding of Saint Paul (MN)’s Capitol Region Watershed District (CRWD). In 1998, community members petitioned the Minnesota Board of Water and Soil Resources to create the district after expressing concerns about poor water quality in the lake. CRWD’s work encompasses a variety of projects, but its origin story points to Como Lake’s importance to the community.
“It’s one of the most recognizable and well-loved water resources in our area,” says Sarah Wein, Water Resource Project Manager for CRWD. “People really care about it and want to see it improved.”
Cascading effects demand holistic lake management
CRWD installed instruments at three locations to monitor DO, pH, temperature and conductivity. Photo courtesy of CRWD.
Como Lake faces several water quality challenges. Urban runoff carries large amounts of phosphorous into the lake each year, feeding algae blooms that can choke out native plants and deplete dissolved oxygen. In winter and spring, rain and snowmelt take de-icing salts off roads and into the lake, increasing chloride concentrations. And before management actions, the invasive aquatic plant curly-leaf pondweed had established stubborn residence in Como Lake, dominating the plant community and producing a pungent smell that hung over the lake in summer as the plants decomposed.
To address these challenges, CRWD developed a multi-faceted plan in 2019 featuring internal and external lake management strategies. Every three years, they reevaluate plan implementation and revise their approach based on monitoring data and outcomes observed.
“Monitoring data drives all of the work we do,” says Wein. “We don’t make a lot of decisions based solely on hypotheticals or modeled data.”
Data-driven adjustments improve management plans
CRWD collects sediment cores to determine phosphorous release rates from sediment. Photo courtesy of CRWD.
CRWD knew that the 2019 plan required more internal management strategies to reach water quality goals. External management tools are limited for urban watersheds, Wein explains, because “there aren’t endless opportunities in a developed area to reduce external sources of pollutants.” But developing a plan with increased emphasis on internal lake management measures required more data on current conditions. In particular, CRWD wanted to determine whether an alum treatment would reduce phosphorous coming from sediment buildup.
“We wanted to understand if an alum treatment would be an effective tool for phosphorous reduction in the lake. To do that, we needed more robust data on dissolved oxygen levels at the bottom of the lake,” says Wein. This would help CRWD get a better picture of when and where the lake was experiencing low oxygen levels, or anoxia. When low oxygen conditions exist above the sediment at lake bottom, they can cause phosphorous bound to sediment to release back into the water column–a process called diffusive flux–thereby contributing to internal loading of phosphorus in the lake. Comparing data on diffusive flux before and after the alum treatment would allow CRWD to evaluate its success as a management strategy.
“We did GIS mapping to get the total sediment area,” said Wein. “We collected sediment cores to measure the phosphorous release rates from the sediment before any alum treatment occurred, and knew we could collect cores after to determine the release rates post-treatment. But we needed information on the duration and frequency of anoxic periods at different bottom depths. That was the missing piece.”
Anoxia data key to management actions
CRWD visits the installations weekly to maintain the sondes and collect profiling data. Photo courtesy of CRWD.
Starting in 2018, CRWD installed three In-Situ Aqua TROLL 600 Multiparameter Sondes at three locations of varying depths in Como Lake. Each year, CRWD deploys the sondes after ice melt in spring and leaves them as long as possible before the lake freezes over.
Sitting a half meter above the bottom, the instruments take dissolved oxygen, temperature, conductivity and pH readings every 15 minutes. To avoid potential tampering and equipment damage on this public lake, telemetry was not installed. So, staff visit weekly to download data and take profile readings at half-meter intervals at all three locations using the VuSitu mobile app.
With continuous DO data, CRWD could identify the frequency and duration of anoxic periods in the hypolimnion and show that, while shallow (the lake has a maximum depth of only 15 feet), there was a large area of Como Lake that experienced anoxia for the majority of the open-water season. This indicated that an alum treatment could be an effective tool for managing phosphorus in the lake.
Assessing effectiveness of alum treatment with monitoring data
In 2020, CRWD carried out an alum treatment to inactivate sediment phosphorous and thus reduce the contribution of diffusive flux to the overall phosphorous level in the lake. CRWD continued to collect dissolved oxygen data with the Aqua TROLLs after the treatment. When combined with sediment areas and phosphorus release rates calculated from post-treatment sediment cores, measurements from the sondes allowed CRWD to calculate the total pounds of phosphorous coming from diffusive flux before and after alum treatment to evaluate how well the treatment worked to reduce phosphorous loading.
“The alum treatment significantly reduced the phosphorus load from diffusive flux,” reports Wein. “Using our monitoring data, we were really able to understand how well the alum treatment worked as a management tool.”
Enhancing lake health through community engagement
CRWD works with the county and many other partners to evaluate and revise Como Lake management plans. It’s a collaborative project that requires communication and consensus.
“No one entity is doing total management or total monitoring at Como Lake,” Wein explains. “It’s all a partnered effort, which is why we do so much work to develop and outline the plan and ask, ‘Are we all on board with these strategies?’ All of the agencies need to have buy-in for this to work.”
The Como Lake management plan includes water quality initiatives related to all aspects of lake health. Herbicide treatments have reduced curly-leaf pondweed by 96 percent. Lake vegetation management practices have increased biodiversity. Fish surveys and removal of invasive common carp have also improved water quality, clarity and native plant longevity. Shoreline assessment and management engage city organizations, volunteers, and community members. And CRWD educates the community on de-icing best practices and works with partners to promote innovative de-icing methods, in addition to monitoring for chloride and phosphorous pollution in storm sewer outlets and other points within the watershed.
“Data collection has allowed us to be adaptive in how we address and continue managing the health of this lake,” says Wein. “We created a robust monitoring program multiple years in advance of the plan, so we had a lot of background data to inform new goals and actions.”
You can find more information on Como Lake and the Capitol Region Watershed District at https://www.capitolregionwd.org/projects/.
