Lake Mohegan

Westchester

County

Town of Yorktown

Presentation by Ken Belfer

July 24, 2015

We were # 1 in NYS in 2014

• For the highest BGA counts!!!!

Description of Lake

• Natural Lake

• 103 Acres

• About a mile long

• Much of lake is 12’ deep or less (maximum of 22’)

• Dense residential development around lake

• Municipal storm water systems dump directly into

lake

Lake Conditions

• Low turnover rate

• Eutrophic (rich in nutrients, thus significant growth of rooted aquatic plants and phytoplankton

• High levels of phosphorus in water in summer

• Anoxic conditions exist in deep areas in heat of summer

Background

• Long-time participant in CSLAP

• Also participated in NYS HAB Study (Harmful Algae Blooms)

• Copper sulfate used to treat algae for decades

• Used Alum in 2002 – significant improvement

Watershed

• Relatively small watershed

• 2011 County law banning lawn fertilizer containing phosphorus

• Most of the area sewered in past 25 years

• Expected to see improvement based on these watershed changes

What happened?

• Deterioration in water quality has increased

• In 2013, one test showed phosphorus content of 172 ug/l – NYS guidance value is 20 ug/l

• Stormwater testing in 2013 and 2014 also showed high concentrations of phosphorus in stormwater

Mohegan Lake Improvement District (MLID)

• District created in 1975 after residents petitioned Town of Yorktown

• Special tax district to address Mohegan Lake environment

• Approximately 1500 tax parcels in Yorktown and Cortlandt

• Volunteer board, 3 seasonal staff

• Current annual budget approximately $95,000

Challenges Faced by MLID

• Invasive Species

• Blue Green Algae (cyanobacteria)

• Maintaining suitability of lake for recreational uses (swimming, fishing, boating, picnicking)

Invasive Aquatic Plants

• Eurasian Watermilfoil (myriophyllum) • Was once a dominant plant

• Would make swimming difficult

• Harvesting with weed harvester

leaves fragments, propagating

further spread

• Curly-Leaved Pondweed (potamogeton crispus)

• Has become dominant spring plant in past few years

• Hard to harvest with weed harvester

• Dies off in late June/early July and decays at bottom

Invasive Aquatic Plants

• Water Chestnut (trapa natans)

– Not to be confused with the Chinese vegetable

Invasive Aquatic Plants

• Water Chestnut (trapa natans)

– Can literally cover the lake surface in water less than 6 feet deep, choking out other plant life

– Eradication has been a significant focus for past 6 years

• Initial volunteer effort could not keep up

• Subsequently addressed through weed harvester + hand pulling

Serious Setback

• 2011 – 2012 saw increased public awareness of health risks associated with BGA toxins

• 2012 County Health Dept closed down beaches for season in mid-July due to presence of blue green algae

• 2013 -2014 County mandated beach closures for summer season starting mid-July

• County testing became more sophisticated

– Included identification of algae, and of toxins

Current Activities

• Hand harvesting

water chestnut

• Weed harvesting with

mechanical harvester

– to clear beach areas

– to help clear invasives

– To remove some mass

of nutrients from lake

ecosystem

Current Activities

• Maintaining and expanding aeration system

– 5 sites, multiple compressors and lines

– About 2/3 of lake is covered currently

Current Activities

• Alum

– Submitted permit application to NYS DEC in 2014

– Considering a lobbying strategy

• Copper Sulfate

– Applied for permit in 2015

– County Health Dept closed beaches June 19th

– Allied Biological commenced applications June 23rd

– Applying every two weeks in July and August

– After 2 applications, predominant form of algae in lake was green algae, not blue-green algae

– Beaches reopened June 24th and so far, still open . .

Lake Day July 12, 2015