From the observation that the population of phytoplankton in the area of the ocean has significantly decreased compared to the previous year, researchers can conclude:
The plankton as a carbon sink will be less than expected, but the plankton as a carbon source will be greater than expected.
This is because a decrease in phytoplankton population means less carbon dioxide is being fixed through photosynthesis (carbon sink), while the decomposition of phytoplankton could release more carbon dioxide back into the water (carbon source) than the amount being absorbed.