So it’s no surprise that ocean life, from the smallest plankton to the largest whale, is showing signs of damage. Only this week the US National Oceanographic and Atmospheric Administration designated the eastern North Pacific basking shark a “species of concern” because of the dramatic drop in its numbers despite years of protection from fishing.

And earlier this summer researchers in Canada found that the amount of phytoplankton in the ocean has decreased by 40% since 1950 in 8 out of 10 large ocean regions. They ascribed this decline to rising sea-surface temperatures, but added that there may be other factors that they haven’t yet discovered.

“As the surface of the ocean continues to warm, the water column appears to be getting increasingly stable,” Dan Boyce of Dalhousie University told environmentalresearchweb. “This increased stability is likely limiting the amount of nutrients which are delivered to phytoplankton at the surface from deeper waters.”

Despite their small size, phytoplankton play a key role in the health of the ocean. “They are the foundation of the vast majority of the ocean food web, so changes in their abundance will affect everything higher in the ecosystem, including humans,” said Boyce. “Phytoplankton are also necessary to maintaining the stability of our global climate system and the sustainability of fisheries, and to ensuring that our oceans remain healthy and productive.”

Most studies of large-scale phytoplankton trends have used satellite data. But there’s a snag – the measurements are only continuously available back to 1997, which isn’t long enough to separate out short-term fluctuations and long-term trends. “By revisiting historical records of phytoplankton abundance collected by scientists while onboard marine research vessels throughout the 20th century, we were able to extend the phytoplankton record back to the year 1899,” said Boyce, who worked with colleagues at Dalhousie.

The team believes that it is essential to carry on monitoring global phytoplankton levels and to understand the drivers and consequences of the declines. “Prerequisite to addressing these pressing issues will be an enhanced in situ and space-borne observational basis so that scientists have access to good quality data,” said Boyce.

The heat’s on for hotspots

It’s not just phytoplankton that are affected by sea surface temperature. It’s also number-one contender as the explanation for the occurrence of marine biodiversity hotspots – areas that are rich in species for many types of organism.

That’s according to another study conducted by researchers from Dalhousie along with scientists from Yale and Rutgers Universities in the US. The collaboration used data from the Census of Marine Life and elsewhere to synthesise the first global map of marine biodiversity across 13 species groups, from plankton to whales.

“We found that sea surface temperature was strongly linked with diversity across all groups,” Derek Tittensor of Dalhousie told environmentalresearchweb. “The consistent link with marine biodiversity and temperature for so many organisms also suggests that the diversity patterns will likely re-arrange in response to future climate change. There is some evidence from other studies that this may already be happening; our study can be used as a baseline to help quantify future changes.”

Coastal species such as corals and some fish tended to peak in diversity around Southeast Asia. Open-ocean dwellers, by contrast, like tuna and whale, showed broader hotspots across the mid-latitude oceans. The diversity of coastal species also showed some relationship to factors such as habitat availability and history, as well as temperature.

The researchers believe that the higher metabolic rates of organisms associated with warmer temperatures may promote higher rates of speciation, or that more species may be able to tolerate a warm environment.

Worryingly, the hotspots tended to overlap with regions where humankind has had an impact through fishing, habitat alteration and pollution.

“Marine biodiversity faces multiple threats and indeed is declining in many areas,” said Tittensor. “In order to protect it, we need to know how it is distributed – where the hotspots are. Yet our knowledge of the spatial distribution of marine biodiversity lags behind that on land.”

Now Tittensor would like to delve deeper into the relationship between marine and terrestrial diversity. “What is responsible for the different patterns we observe?” he pondered. “Can we synthesise across these two realms to try and further untangle the forces that shape planetary-wide biodiversity?” He’d also like to explore diversity patterns in the deep sea, which he says is a very different environment to the surface oceans.

The resource at the core of this study – the 10-year Census of Marine Life – has brought together researchers from more than 80 countries and added more than 5600 new creatures to the roughly 230,000 known marine species. To date the results from 25 ocean areas have been published; inventories for areas such as Indonesia, Madagascar and the Arabian Sea are still underway and will be revealed on 4 October at a briefing in London.

Let’s hope that the census reaches its full potential as a tool for conservation and oceans management and does not become merely a historical record of species that used to exist.