"The higher quality data available since 1950 has allowed the team to calculate that since that time, the world has seen a phytoplankton decline of about 40%."
"Phyto-" means plant. Phytoplankton, like all green plants, take in more carbon dioxide than they produce, and release more oxygen than they use.
Phytoplankton produce about half the oxygen available to us.
We have plenty of oxygen, so we're not in Spaceballs territory yet (remember Megamaid?), but if this trend plays out, we are screwed.
So how about this for an essential question? Which organisms depend on phytoplankton to survive?
Yep, we're on that list, too.
"Megamaid" picture is obviously from Spaceballs, and taken from here.