Decades of data harvested from the seas off Bermuda reveal changes in a drifting and largely invisible plant ecosystem vital for life on Earth as well as in the oceans.

A study published today by the University of Exeter in the journal Nature Climate Change showed two communities of phytoplankton responding to the effects of global warming tracked off Bermuda’s shores.

Research from the Bermuda Atlantic Time-series Study, run by the Bermuda Institute of Ocean Sciences, has yielded insight into changes below as well as on the ocean’s surface, where seas have grown more acidic as well as rising in temperature.

The research, known by the acronym Bats, uses Bermuda’s position in the Sargasso Sea to track physical, chemical and biological data far offshore.

The ongoing study uses regular cruises by the Bios research vessel Atlantic Explorer as well as analysis taken at Hydrostation S, 80km southeast of the island and the most studied site in the world's oceans.

The study, which celebrated its 70th anniversary this year, has clearly shown rising temperatures offshore, while surface waters have also gradually become saltier and 30 per cent more acidic during the past four decades.

Carbon dioxide, a greenhouse gas and one of the principle drivers of climate change, has surged in the atmosphere over centuries of human industrial activity, but is also infiltrating the sea, where the gas leads to ocean acidification.

Phytoplankton, which produce a major portion of the planet’s oxygen through photosynthesis, are key players in the carbon cycle by absorbing carbon dioxide.

They are also the basis of the marine food web for sea life.

The new study examined the tiny marine plants at the ocean’s surface along with a separate layer beneath, known as the subsurface.

The research paper, titled Climate variability shifts the vertical structure of phytoplankton in the Sargasso Sea, shows the surface phytoplankton losing some of its chlorophyll, the green pigment enabling plants and algae to photosynthesise from sunlight.

The biomass of the phytoplankton at the surface, meaning the total weight of living material, has held steady.

However, the study showed that in the subsurface, phytoplankton biomass has increased over the past decade, likely in response to warming.

The Bats research was analysed by Johannes Viljoen, from the Department of Earth and Environmental Science at Exeter’s Penryn Campus in Cornwall, with Bob Brewin and Xuerong Sun from the Centre for Geography and Environmental Science, the coauthors of the article.

The findings drawn from 33 years of Bats data also point to changes in the ocean’s surface mixed layer — the sea’s zone of turbulence, which the article said has shallowed in the area of the Bermuda study during the past decade of rapid oceanic warming.

Dr Viljoen said it was “important to understand these trends because phytoplankton are the foundation of the marine food web and play a key role in removing carbon dioxide from the atmosphere”.

He added: “Our findings reveal that deep-living phytoplankton, which thrive in lowlight conditions, respond differently to ocean warming and climate variability compared with surface phytoplankton.

“We typically rely on satellite observations to monitor phytoplankton but the subsurface is hidden from satellite view.

“Our study highlights the limitations of satellite observations and underscores the urgent need for improved global monitoring of phytoplankton below what satellites can see.”

He said continued monitoring of the phytoplankton would “help scientists better understand ongoing changes in the ocean that might otherwise go unnoticed”.

Dr Brewin said that changes at the base of the food web could lead to “cascading effects on marine life, from tiny zooplankton to large fish and marine mammals”.

He added: “So the future of phytoplankton will have major implications for biodiversity as well as climate change.”