The microscopic sea creatures form the foundation of marine food systems, and their decline is likely due to global warming.
In appreciating the biodiversity of our oceans, we must not forget some of the smallest members of the food chain: phytoplankton.
“Phytoplankton — the most basic form of plankton — are a massive part of the planet’s overall ecosystems: they account for roughly half the organic matter produced on Earth, produce half the oxygen in the atmosphere, draw carbon dioxide out of the air, and serve as the foundational food source for most of the oceans’ food webs.”
So though you cannot see them as clearly as a dolphin, or a shark, or a jellyfish, they are nonetheless so integral to the food web of the ocean. They also happen to produce 1/2 of the oxygen that we have in our atmosphere, which as humans we certainly take for granted.