Contrary to what you might believe, a clear, blue ocean isn't necessarily a good thing.
Phytoplankton -- which form the base of ocean food chains -- have declined 40 percent since 1950.
The trend is linked to warming of the surface of the oceans.
The die-off could affect climate, fisheries and ocean health.
In oceans around the world, there has been a surprisingly large and extensive decline in phytoplankton -- the tiny algae that keep marine food webs afloat.
The drifting green flecks have been dying off for at least a century, with a staggering 40 percent decline since 1950, according to a new study.
Phytoplankton make up half of all plant matter around the globe, said marine ecologist Daniel Boyce, whose study appears this week in the journal Nature. Its disappearance threatens the stability of climate, the well-being of fisheries and the overall health of the oceans.
"It's hard to really imagine phytoplankton could be so important because most people don't see them in their daily lives. They're microscopic and they live out at sea," said Boyce, of Dalhousie University in Halifax, Canada. "But everything that happens to them affects the entire marine food chain, including us."
Some recent satellite images have shown the ocean turning from green to blue as a result of phytoplankton declines, but those data stretch back only 13 years. Other studies have offered mixed results.
To get a more accurate picture and to look further into the past, Boyce and colleagues collected a half-million measurements of ocean clarity from a public data set that dated back to 1899.
Over the last century-plus, analyses showed, phytoplankton levels have dropped by one percent each year in eight out of 10 large ocean regions. The greatest decline occurred in areas around the poles, near the equator and in the open oceans. The rate of disappearance picked up after 1950, totaling a 40 percent drop-off since then.
"It's really big," said David Siegel, a marine scientist at the University of California, Santa Barbara. "I'm a little leery about how big that number is."
The scientists can't yet say what's causing the mass die-off of phytoplankton, but temperature data offer a clue. The declines were worst in places where the surface of the sea has warmed the most. Warmer ocean water limits the amount of nutrients that can get from the depths to the surface. Phytoplankton need those nutrients to live.
With less phytoplankton around, fish have less to eat. As the decline works its way up the food chain, fishermen will have less to catch and fish-eaters less to eat. Phytoplankton even affect climate by taking up carbon dioxide and absorbing heat.
"Everyone looks at blue oceans and goes: 'Isn't that beautiful?'" Siegel said. "But a blue ocean is full of nothing. You really want something, and we're only making more of the blue ocean."