Studies have shown that boats have an affect on water clarity and can be a source of nutrients and algal growth in aquatic ecosystems, especially in shallow lakes. Propellers disturb the lake bottom both directly and indirectly through the wash or turbulence they produce. This increases the amount of sediment particles in the water and causes nutrients that are stored in the sediments, such as phosphorus, to become available for algal growth. All of this affects water clarity which impacts the lake ecosystem, the lake aesthetics, and property values.
Shipshewana lake is shallow and the speed limit is 10 mph. The muck bottom of the lake contains phosphorous which is a major nutrient for algae. Speeding on the lake stirs up these nutrients, which makes the algae grow.
There are several studies available on line:
Wawasee Area Conservancy Foundation