Flexi Says: Phytoplankton are the primary producers in the oceans, and they are experiencing several environmental stresses as a result of human activities. The major root causes of these stressors are climate change and pollution. Climate change causes surface waters where phytoplankton live to be warmer, more acidic, and more salty than usual. All of these changes can stress phytoplankton populations. Pollution from plastics that end up in the ocean as well as runoff from both cities and farming areas contain toxins that can harm phytoplankton. Nutrient-rich runoff can also feed blooms of toxic algae that throw phytoplankton populations out of balance. Ozone-depleting chemical pollutants also result in more UV radiation reaching the ocean surface, which can also harm phytoplankton.