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.