Since plants need sunlight in order to carry out the process of photosynthesis which produces Oxygen and Glucose
The main problem for photosynthesis in water is that the plants underwater might not receive enough sunlight or none at all ... which effects the food web
When fertilizers and other chemicals flow into the water they result in growth of algae which forms on the top of the water surface which completely blocks of the sunlight which might lead to great disturbance in the aquatic life
