Why is life so bountiful at the edge of the sea?

Question

Why is life so bountiful at the edge of the sea?

Answer 1

It is so because of the presence of sunlight. Sunlight provides many things for life.  

Answer 2

Life in the oceans is regulated by the amount of sunlight present in a specific column of water. Life Zones are defined by their depth from the surface, so most ocean life zones are layered like a cake according to how much light they receive and how deep they are. Near coasts, intertidal (littoral) and estaury zones are the interfaces between land and water--ecotones.

As water gets deeper, there is less and less light, and at about 150-200 meters twilight turns into eternal darkness. Life in the ocean that lives below the sunlight penetration (the Benthic Zone) lives in a three dimensional blackness. With increasing depth, there are fewer animals, both in numbers and diversity. As depth increases, pressure from the weight of the water column increases enormously.