One cause is something - runoff of phosphorus into the lake.

Phosphorus, or phosphate, is a fertilizer component that is critical to weed growth in lakes. It is present in many lawn and garden fertilizers. It's the second number in the three number label which most fertilizer bags have. A typical lawn fertilizer might be 28-4-6; the 4 means 4 percent phosphorus. The good news is that many lawn fertilizers now on the market in NH have zero percent phosphorus - the middle number is 0. Please use those whenever you can. In NH there is enough phosphorus in the soil and in rainfall to meet the need for most lawns.

If on a septic system, another significant source of phosphate is from dishwasher detergent. Not dish soap used when washing dishes by hand and not laundry soap; most of that is phosphate-free. But detergent used in automatic dishwashers, whether solid, powder or gel, usually contains phosphates. But there are some dishwasher detergents available in local stores which are phosphate-free. Examples are: Planet Free and Clear, Cascade Complete, Seventh Generation Ultra Power Plus Gel, Ecover Automatic Dishwasher Soap Tablets, and more. Check the label to be sure.

For the Top Ten Ways to keep lakes healthy, check out this short summary: https://nhlakes.org/wp-content/uploads/1-Lake-Friendly-Actions-Top-10-Ways-to-Keep-Lakes-Healthy-NH-LAKES.pdf