5.8.3 Long Lake Internal Load Management Project

ProjectLong Lake Internal Load Management
DescriptionDesign and implementation of strategies to reduce internal phosphorus loading, including: feasibility study; aquatic vegetation survey update and management plan; fishery survey update and management plan; biomanipulation strategies that may include aquatic vegetation management, zooplankton community and fishery manipulation, and chemical treatment.

The modeled watershed phosphorus wash-off load to Long Lake is not sufficient to explain the in-lake concentration of total phosphorus that exceeds the lake’s total phosphorus goal. An “unknown load” to Long Lake is attributed to internal loading, and is likely a combination of bottom sediments, aquatic vegetation, and model accuracy. An alum treatment was previously applied to Long Lake, which responded with a few years of improved water clarity but which soon returned to its pretreatment condition.

This project would include a subwatershed and lake diagnostic / feasibility study to refine the water quantity and quality models used to prepare a phosphorus reduction plan and to investigate strategies for improving water quality and clarity. Strategies may include aquatic vegetation management, biomanipulation and fishery management, chemical treatment, or improvements to existing Best Management Practices such as the Park Ponds in the City of Long Lake. The District may consider cooperative projects with the City of Long Lake such as improvements to Park Ponds based on the results of the diagnostic and feasibility study. The specific share of construction cost as well as ongoing maintenance costs associated with this pond would be determined by cooperative agreement prior to implementation of this project.

The project includes an ongoing vegetation and fishery management plan. As improvements are implemented and water quality improves, the increased clarity can result in extensive aquatic vegetation blooms that if not managed can again increase internal phosphorus loads. At least 3-5 years of aquatic vegetation treatment would be required to manage the plant community.

OutcomeThe goal is to achieve a 70 percent reduction in internal/unknown load, estimated as 288 pounds of phosphorus annually at ultimate development. Reduction in phosphorus load from internal sources estimated at 201 pounds annually; improved water clarity; more diverse aquatic vegetation community; improved aesthetics.
Estimated Cost
Investigation, permitting, cooperative agreement, fish,
vegetation, and zooplankton surveys, construction.
Funding source is the District capital levy.

$16,900 Design

$271,100 Construction

$288,000 Total


2011 Feasibility study, design, cooperative agreement

2012 Implementation of strategies