The main factor contributing to the nutrient enrichment in sea is:

Correct option is D

Correct Answer: (d) Upwelling
Explanation:
→ Upwelling brings cold, nutrient-rich deep waters to the surface, supplying nitrates and phosphates that stimulate primary productivity.
→ This process is a principal natural driver of high marine productivity in coastal zones.
Information Booster:
→ Upwelling regions support major global fisheries (e.g., Peruvian, Arabian).
Additional Information (Other Options):
(b) Decomposition recycles nutrients locally but is secondary to large-scale upwelling.
(c) Eutrophication is the consequence of nutrient enrichment (often anthropogenic).
(a) Spring tide affects mixing but not major nutrient supply compared to upwelling.