This report discusses aquatic plant management activities completed by the Horseshoe Lake Public Inland Protection and Rehabilitation District (formerly the Horseshoe Lake Improvement Association) and Lake Education and Planning Services (LEAPS) during the 2016 season and provides a preliminary hybrid Eurasian watermilfoil (HWM) treatment plan for 2017. The 2017 treatment proposal provides the Association with the information needed to contract with a certified aquatic herbicide applicator to complete the necessary WDNR permitting and herbicide treatment. 2015 was the first year of implementation for the new Aquatic Plant Management Plan that was approved in early 2015.

Sediment cores were collected from 3 stations in Horseshoe Lake in Juley, 2016, to examine phosphorus (P) release from sediment under anaerobic conditions and pools of mobile P fractions in the sediment. Overall, rates of P release under anaerobic conditions were moderate, ranging between 2.4 and 3.3 mg/m2 d. Labile organic P (i.e., sediment P that can be broken down to phosphate by bacteria) dominated the mobile P fraction in the upper 5-cm sediment layer at all stations followed by the iron-bound P fraction. Station 10 and 20 exhibited the highest labile organic and iron-bound P (i.e., subject to release and recycling) concentrations in the upper 5-cm layer. Iron-bound P at these stations fell near the median compared to other lakes. Labile organic P was high at all stations compared to the median. Station 10 and 20 sediment cores exhibited a slight surface concentration maximum in mobile P, suggesting the potential for modest internal P loading. Station 30 mobile P concentrations were relatively constant with increasing sediment depth. This information will need to be combined with information on hypolimnetic anoxia and the Fe:P in order to assess the importance of internal P loading to the P budget and algal blooms in Horseshoe Lake.

The Horseshoe Lake Improvement Association is sponsoring a water quality and education project for Horseshoe Lake. Project deliverables include GIS data & maps of areas monitored, water quality and flow data, bed mapping data, shoreline and woody habitat data, sediment P release report, and a hybrid milfoil treatment report. Specific project activities include: 1) Tributary and in-lake monitoring; 2) Precipitation monitoring; 3) Shoreline and woody habitat survey; 4) Lake sediment phosphorus release study; 5) AIS education; 6) Fall EWM bed mapping.