Based on the observation that the population of phytoplankton 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.
Phytoplankton plays a key role in the carbon cycle by absorbing carbon dioxide (acting as a carbon sink). A decrease in phytoplankton population would lead to a reduced capacity for carbon uptake. Moreover, as phytoplankton die and decompose, they can release carbon back into the water and atmosphere, potentially increasing the carbon source. Thus, the overall result would indicate a diminished carbon sink and an increased carbon source.