What is the overall impact of upwelling on marine ecosystems?

Upwelling is a process that brings nutrient-rich waters from the deep ocean to the surface, which significantly enhances nutrient availability in marine ecosystems. This influx of nutrients stimulates the growth of phytoplankton, the foundational level of the marine food web. As phytoplankton flourish, they provide a vital food source for a variety of marine organisms, including zooplankton, small fish, and even larger predators such as tuna and whales. Consequently, the enhanced nutrient availability resulting from upwelling supports higher productivity and, consequently, increased biodiversity in these ecosystems.

The other options either mischaracterize the effects of upwelling or focus on specific events rather than the broader ecological impacts. While upwelling can lead to population spikes of certain species, such as phytoplankton, it does not inherently decrease biodiversity or reduce fish populations; rather, it typically contributes to a healthier, more diverse ecosystem.