19/01/2021 · 2. Algal blooms and oxygen depletion. As algae begin to form, it blocks sunlight from entering the bottom of ponds, lakes, and rivers. As more nutrients drain into the water, eutrophiion repeats in a vicious algal bloom cycle and releases more nutrients in the water.. When algae receive enough sunlight, they produce oxygen through photosynthesis and release it in the water.