During an educational trip, a botany student saw a beautiful lake in a village. She collected many plants from that area. She noticed that villagers were washing clothes around the lake and at some places waste material from houses was destroying its beauty. After few years, she visited the same lake again. She was surprised to find that the lake was covered with algae, a stinking smell was coming out and its water had become unusable. Can you explain the reason for this condition of the lake?