Availability of Nutrients:
The nutrients in the upper layers of water are constantly being taken up by the phytoplankton, who are the producers, and then these pass on to the herbivores, and the carnivores. When these latter organisms die, they are either eaten up by other animals or they get decayed by the decomposers. Some of the decomposed matter sinks to the ocean floor.
It means, the nutrients taken up by the producers from the upper layers of sea water are constantly being drained to the lower layers of sea water. Do you know what would happen if the nutrients in the upper layers of water are not replenished? There would be no phytoplankton and photosynthesis; no production of food to sustain other animal life. Hence all the organisms would die.
Actually, in nature such a situation does not arise. Let us see how a constant supply of nutrients in the upper layers of water is ensured. There are two ways:
(i) As riversend in the seas, they bring along a lot of refuse and nutrients from the land to the water.
(ii) 'Upwelling' takes place, which is a process by which dwp, nutrient rich waters are brought to the surface. What actually happens is that wind blows surface water on to the shores. The watex which now comes to the surface is from below. It is cold and high in nutrient content. Regions where upwelling takes place are very productive. That's why important commercial fishexies in the world are situated in such regions.