From the observation that the population of phytoplankton in an 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 conclusion is based on the understanding that phytoplankton play a crucial role in carbon cycling. A decreased population means there will be less phytoplankton available to absorb carbon dioxide from the atmosphere (functioning as a carbon sink). Meanwhile, with less phytoplankton, decomposition and respiration processes may dominate, leading to increased carbon release into the ocean and atmosphere (functioning as a carbon source).