06-09-2025 - Agenda Packet City Council - regular meeting

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Issue Analysis Phosphorus Reduction Stubbs Bay has elevated phosphorus concentrations, leading to algae blooms and impairment of aquatic recreation. In previous years, external phosphorus loading to the bay has been reduced through the following actions: • Protection of upland wetlands; • Enforcement of stormwater management regulations; • Reduction of agricultural land in the upstream watershed; and • Projects carried out by the City of Orono and the Minnehaha Creek Watershed District in the early 2000s (e.g., Stubbs Bay Ravine Stabilization (https://www.legacy.mn.gov/projects/stubbs-bay-ravine- stabilization)). Because these projects have reduced the external phosphorus loading to the lake, it has been suggested that the elevated phosphorus concentrations are at least partly due to internal loading. However, it appears that this conclusion has not been verified by phosphorus budget models. About 21% of the catchment basin remains in agricultural use, 10% is in developed -open space use, and an additional 24% is in developed low -high density use; together, these represent a bit over half of the land use types in the catchment, which could contribute to runoff and external loading of nutrients. The semi -ditched wetland complex is a particularly likely source of external phosphorus loading, and should be investigated, in collaboration with the MPCA or MCWD. The LID proposal does not include any specific program or project to reduce phosphorus concentrations in the bay, nor does it include a firm commitment to study this problem and identify a feasible course of action to mitigate phosphorus concentrations. The LID's first concern is to reduce AIS abundance in the bay, and this is appropriate: the seasonal dynamics of CLP, which grows early in the year and dies back in late summer or fall, releasing nutrients back into the water column as the plants decay, could have a significant impact on the water quality patterns in the bay, which would be a factor in determining any feasible phosphorus mitigation strategy. According to the MPCA Lakes and Streams Water Quality Dashboard, the Minnehaha Creek Watershed District (MCWD) has been monitoring phosphorus concentrations on a monthly basis since 2007. A sampling frequency of at least monthly is necessary to identify possible sources or causes of phosphorus loading to the bay, and therefore DNR strongly recommends that the LID collaborate with the MCWD to maintain or enhance this water quality monitoring program to determine seasonal patterns in phosphorus concentration, possible remaining external sources of phosphorus loading (e.g., stormwater runoff, discharge from the semi -ditched wetland complex), and possible impacts from CLP reduction. DNR also recommends conducting a sediment sampling program to assess the extent to which internal loading is driving the phosphorus concentrations in the bay. This data would be necessary to develop a phosphorus loading model for the bay and determine appropriate mitigation approaches (e.g., alum treatment if internal loading is determined to be a problem, or nonpoint or stormwater capture and mitigation projects if external loading remains a problem). If the continued water quality monitoring program identifies continued external sources of phosphorus, then the LID would need to collaborate with the MCWD, Lake Minnetonka Conservation District (LMCD), or the MPCA to implement projects to reduce these loads, since the areas on the landscape where such projects would need to be implemented would likely be outside the proposed LID boundaries. 280