daniel boyce and boris worm hypothesized that phytoplankton was declining globally, while data from the northern atlantic ocean during the continuous plankton recorder (cpr) survey suggest that phytoplankton biomass has increased in the last 20-50 years. boyce and worm supported their hypothesis by using a combination of historical secchi disk readings and laboratory-determined levels of chlorophyll in the water. the cpr survey directly measured biomass in the sampled ocean water. which of the following might be the most plausible explanation for the difference between the cpr survey results and boyce and worm's conclusions?