Which process brings nutrients from the deep ocean to the surface?

• A. Upwelling brings nutrients to the surface.
• B. Downwelling brings nutrients to the surface.
• C. Tidal mixing brings nutrients to the surface.
• D. Sediment deposition brings nutrients to the surface.

Answer: (A)

Upwelling is the process that brings nutrients from the deep ocean to the surface. Deep waters are rich in nutrients like nitrate, phosphate, and silicate because organic matter has sunk and decomposed there. When winds and currents move surface water away from the coast (or create patterns that lift water from below), deeper, nutrient-packed water rises into the sunlit layer where phytoplankton live. This influx of nutrients fuels blooms and supports the entire marine food web.

Downwelling, by contrast, moves surface water downward into deeper layers, which tends to move nutrients away from the surface rather than toward it. Tidal mixing does mix water layers on a daily cycle but isn’t the main mechanism that continuously brings deep nutrients up to the surface over broad areas. Sediment deposition adds nutrients to the seabed, not to the surface waters.