What can lead to a significant drop in nutrient supply in ocean waters?

A significant drop in nutrient supply in ocean waters can occur due to abnormal wind patterns during an El Niño event. During El Niño, the typical trade winds weaken, or even reverse, which affects ocean currents and ultimately disrupts the upwelling of nutrient-rich waters from the deep ocean to the surface. This upwelling brings essential nutrients such as nitrogen and phosphorous to the photic zone, where photosynthetic organisms like phytoplankton thrive. When these nutrients are diminished, it leads to lower productivity in marine ecosystems and can have cascading effects throughout the food web.

In contrast, a rise in atmospheric pressure does not directly influence nutrient supply in ocean waters. Increased salinity levels, while affecting water density and stratification, do not inherently lead to a drop in nutrient supply. Regular tidal changes are a natural process that typically helps to maintain nutrient distribution and movement in coastal areas rather than cause a drop in nutrients. Thus, the disruptions caused by abnormal wind patterns during an El Niño are primarily responsible for significant changes in nutrient availability in the ocean.