Algae

Wetlands have come under natural and human threats (from subsiding or sinking land to draining or filling for new development). Scientists estimate that the lower 48 United States have lost more than half of their wetlands since colonial times.
First of all, plants naturally grow in and around lakes. Maybe you're asking about a lake that is being choked off by too much algae. In many cases, humans are responsible. Actually, these lakes are being fed too much food for plants!
Eutrophication is the process by which a body of water acquires a high concentration of nutrients, especially phosphorus and nitrogen. These typically promote excessive growth of algae.