Question: Because of low nutrient concentrations during the rest of spring and into late summer, the productivity remains low until the ocean surface layer begins the cooling associated with the reduction in incoming sunlight. This cooling and the strengthening of surface winds cause mixing of ocean water to depths where ocean waters are nutrient rich. Why do nutrient levels increase following the fall bloom in October? a. Phytoplankton go into hibernation. b. The sunlight decreases in the region. c. Both a and b are correct.