Plankton refers to the whole assortment of tiny, often microscopic, living things in the ocean.
The more plankton in the water, the cloudier or murkier it is - harder to see through. If there is a lot of phytoplankton in the water, it generally takes on a greenish color. It is the starting point, or bottom of the marine food web, and the health of the whole system ultimately depends on healthy plankton levels. I suspect not. I was told by a marine biologist in Atlantic Canada that plankton levels are as healthy as ever, but I was hardly reassured since I was given no evidence to back up the statement.