Answer: a. dead organisms from the marine food web.
c. liberation through ATP hydrolysis in living organisms.
Upwelling is a wind driven motion of lower bottom nutrient rich and warmer water on the surface of the water body. This wind driven motion facilitates the movement of nutrients available for growth of primary producers like phytoplanktons growing on the surface of water body. The dead organisms from the marine food web get decomposed and the organic matter obtain after decomposition is a rich source of phosphorous. This phosphorous gets transferred to the upper layers of the water body by upwelling. In aquatic organisms ATP hydrolysis occurs which is a catabolic process that uses water to split the bonds present in the ATP molecule and hence, releases energy for functions performed by them along with a release in phosphate atom. This phosphate gets mixed with the water. Therefore, PO32 come from dead organisms from the marine food web and liberation through ATP hydrolysis in living organisms that circulates due to upwelling.
Most of the photosynthetic organisms are located in the epipelagic zone of the oceans where sunlight can penetrate before diffusing and becoming too weak to support photosynthesis. The epipelagic zone is filled with plankton. When this plantkon die, they begin to sink slowly to the bottom of the ocean. These plankton are food for small fish. These small fish are food for bigger fishes. This is how an ecosystem is sustained in the deep dark oceans.
