Oxygen is a fiercely corrosive gas. As it attacks rock and metal, wood and flesh, the gas itself is consumed. Were it not continually replaced, all the oxygen in the air — over a million-billion-plus tons of it - would disappear within a few thousand years.

Roughly half the oxygen in our atmosphere is belched out, a waste product of photosynthesis, by green plants on land.

The other half, also photosynthetic waste, is disgorged by phytoplankton: Single-celled algae living in lakes and oceans.

Much as land plants have divided up the continents — conifers claiming the colder spots, for instance — so phytoplankton have apportioned the seas (check out the image).

As concern slowly builds over the fate of our forests and grasslands, we ask, “How fare the phytoplankton?”

It takes energy to rearrange the atoms of carbon dioxide and water into food. Plants derive this energy from the red and blue hues of sunlight. The wavelengths they reject, colors other than red and blue, register in our eyes as green.

We can get a handle on the size of the world's phytoplankton population by measuring how much green light the waters reflect — the greener the water, the larger the phytoplankton population. Spacecraft measure ocean color daily.

As important as knowing the phytoplankton population today is knowing how it compares with the population a month, year, decade, or century ago. The mathematics of spaceflight dates back to the early 17th century, but cutting metal to actually leave Earth had to wait until the mid- to late-20th.

Since the early 20th century, however, spectroscopists have been measuring the greenness of the ocean, as a means of measuring phytoplankton. And other means of censusing the seas go back beyond that.

In the mid-1800s, Angelo Secchi tied a bright-white dinner plate to a length of line, and dropped it into the water. The sooner the plate disappeared, the more stuff — including phytoplankton — was suspended in the water.

Scientists have been tossing Secchi disks into the sea ever since (disks today are specially made for the purpose, but they still look like nothing so much as white dinner plates).

A report published in the journal nature by a team of oceanographers and biologists at Dalhousie University (Halifax, Nova Scotia) describes its compilation of a century-plus worth of Secchi disk and spectroscopy data.

On average, in most (though not all) of Earth's ocean basins, the Secchi disk drops farther before disappearing, and the water is less green.

Earth's oceans are becoming more transparent. Phytoplankton populations — base of the oceanic food web, source of half the world's oxygen — are shrinking.

The oceans seem to be in the midst of a vast reorganization. Populations of phytoplankton are shrinking, responding, perhaps, to higher surface temperatures; to increased acidity, as carbon dioxide dissolves in the water to make carbonic acid; to a shortage of essential iron, as acidity allows substances in the water to bind up the metal.

Sooner or later — one hopes sooner, new populations of phytoplankton will grow to replace the old.

Alan Stahler teaches enrichment and remedial classes in science and writing, and talks about science on KVMR 89.5 FM. Contact him at stahler@kvmr.org.