4.1.13 (HL)—Upwellings

When winds move surface water away, deeper water rises to replace it; this upwelling often brings nutrients into the lighted zone.

Deep water can be nutrient-rich because decomposition regenerates nitrate and phosphate. In oceans, wind-driven displacement can bring this water upward and ENSO can weaken or strengthen the pattern. In stratified lakes, seasonal cooling and wind can erode the density barrier and produce seasonal upwelling or turnover.

Coastal upwelling raises cold nutrient-rich water, supporting phytoplankton and fisheries; El Niño often weakens this delivery in the eastern Pacific.

Upwelling raises productivity only when nutrient-rich water reaches sufficient light and producers are not limited by another factor.

Upwelling is not automatically a fish increase; trace nutrients, light, food-web response and timing.