Vital Marine Plants in Steep Decline
Rising sea temperatures can harm the tiny plant life that forms the base of the oceans' food chain as well as affect the diversity of marine life, two new studies have found.
In research published Wednesday by the journal Nature, scientists found a strong link between higher sea-surface temperatures and a major decline over the past century in marine algae, or phytoplankton. These microscopic plants generate roughly half of all organic matter—the building block of life—on the planet.
A second study, also published in Nature, concludes that warmer seas can influence marine diversity, potentially rearranging the global distribution of ocean life.
Over the years, humans have affected the oceans by polluting and over-fishing and through habitat alteration caused by dredging and other activities. Less understood is the role of higher sea temperatures. Scientists, many of whom believe the increase to be linked to global climate change, estimate the oceans have warmed by roughly half a degree Celsius on average over the past 100 years.
Phytoplankton has flourished in many coastal areas because increased runoff from rivers brings nutrients that the algae gorge on. But no one has properly assessed whether the global oceans are losing or gaining phytoplankton, which forms the base of the marine food chain, from crustaceans to fish and ultimately to humans.
Consistent satellite-based measurements of the algae exist only from 1997. Scientists at Dalhousie University in Nova Scotia, Canada, instead used data obtained with a simple oceanography device used since the late 1800s known as a Secchi—a disk lowered into the water to measure phytoplankton abundance.
By collating and analyzing about half a million Secchi observations, plus other direct measurements of algae, the Dalhousie team estimated that phytoplankton levels declined by about 1% of the global average each year from 1899 onward. The data are more reliable for recent decades, translating into a 40% decline since 1950.
The team investigated several factors that could have caused the decline. "We found that temperature had the best power to explain the changes," said Boris Worm, a marine biologist at Dalhousie and co-author of the study.
Marine algae live in the upper layers of the ocean but rely on nutrients that circulate up from lower layers. Rising temperatures mean the different layers mix less with each other, so fewer nutrients reach the algae. However, Dr. Worm noted that algal abundance can be affected by other factors, such as shifts in predator-prey populations.
