You might want to look into a Jenson Lake mower. Then you can cut what you want , no chems and you make up the cost quickly on a lake your size. That is what I would do if our lakes were that big/much weeds. 100 acres is a lot to treat. Plus if you remove the weeds when cut you don't have the DO (O2 depletion) problem.
Jenson Technologies (info@lakemower.com) ask for Annamarie Gervais -- these are great people. Tell them PB sent you.
From the PB web site :
Jenson Lake Mower®
A boat-mounted tool to cut invasive plants such as hydrilla, cattail, milfoil, lily pads, etc., the Lake Mower™ offers the environmentally friendly alternative for aquatic weed control. Affordable yet durable, for both individual and commercial removal projects, the Jenson Lake Mower™ reaches 4 feet wide and up to 7 feet deep. The Lake Mower™ cuts 7 to 10 hours on a 12V deep-cycle marine battery. The Rake™ - 5-feet wide - removes floating weeds like salvinia, hyacinth andcuttings. Satisfaction guaranteed with a 30-day return privilege and a one-year warranty. Request a free video online.
412 Summer Mountain Drive, San Marcos, TX 78666
Contact: Annamarie Mangum
Toll Free: (888) 298-5253FAX: (512) 393-5074
Email: info@lakemower.com
Website:
www.lakemower.com 
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