Based on the observation that the population of phytoplankton has significantly decreased compared to the previous year, researchers can conclude that:
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 implies that there is less biomass to sequester carbon (acting as a carbon sink), while a reduced population could also mean that more carbon is being released back into the environment due to the breakdown of organic materials (acting as a carbon source).