We killed nearly all the sperm whales. So their poop is no longer fertilizing the plankton (it was their main source of iron). The end result is there is way less plankton now than 300 years ago.

It is logical to assume that less plankton absorbs less CO2. The health of our oceans is precarious at best, and pretending that we can rely on plankton to clean up our messes is simply wrong.