Why are there more different ecosystems in the coastal area than in the main body of water?

An important part of any ecosystem is the presence of producers. They create organic substances from inorganic ones under the influence of sunlight. At great depths, sunlight almost does not penetrate and therefore there are much fewer producers there, respectively, there is less food and consumers too. Oxygen at great depths is just as little, and it is necessary for all living things, Therefore, there are more ecosystems in the coastal part, at a shallow depth, than in the main body of water.