The correct option is It increases the temperature of water by 10°C and reduces plankton..
Explanation
El-Nino is a complex weather pattern resulting from variations in ocean temperatures in the Equatorial Pacific. It is characterized by the appearance of warm ocean currents off the coast of Peru, which replace the normally cold Peruvian (Humboldt) Current.
Detailed Analysis:
- Temperature Change: Under normal conditions, the Peruvian coast is influenced by the cold Humboldt Current. During an El-Nino event, warm water from the central Pacific moves eastward towards the South American coast. This results in a significant rise in sea surface temperatures, sometimes by as much as 10°C.
- Impact on Marine Life: The warm water layer suppresses the natural upwelling process that usually brings cold, nutrient-rich water from the ocean depths to the surface.
- Reduction in Plankton: These nutrients (nitrates and phosphates) are essential for the growth of phytoplankton. When upwelling is blocked, plankton levels drop drastically.
- Effect on Fish: Since plankton forms the base of the marine food web, its scarcity leads to a sharp decline in fish populations (particularly anchovies), adversely affecting the local fishing industry.
Key Takeaway:
El-Nino causes the warming of waters off the Peruvian coast, which halts nutrient upwelling, thereby reducing plankton density and causing a decline in fish stocks.