An Adaptive Approach to Urban Lake Management

Published on by in Case Studies

An Adaptive Approach to Urban Lake Management

CRWD-WorkingonSonde-600x300-aaa8355.jpg

 

Overview

Continuous monitoring data helped Capitol Region Watershed District make informed adjustments to internal lake management strategies for Como Lake in Saint Paul, Minnesota.

Challenge

Como Lake inspired the founding of Saint Paul’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 in 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.”

Como Lake faces several water quality challenges. Urban runoff carries large amounts of phosphorous into the lake each year, feeding algal blooms that can choke out native plants and deplete dissolved oxygen. In winter and spring, rain and snowmelt take deicing salts off roads and into the lake, increasing chloride concentrations. And prior to management actions, the invasive aquatic plant curly-leaf pondweed had established stubborn residence there, 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.”

Solution

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 if 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, it 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.

CRWD-CollectingSedimentCores.webp

“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.”

Starting in 2018, CRWD installed three In-Situ Aqua TROLL 600 Multiparameter Sondes at three locations of varying depths in the 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. Telemetry was not installed to prevent potential tampering and equipment damage on this public lake. 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.

Results

In 2020, CRWD carried out an alum treatment to inactivate sediment phosphorous and...

READ MORE

Taxonomy