A global citizen science project has handed scientists fresh insights into how climate change might affect ocean species and their ecosystems.
A new study, involving New Zealand co-authors and divers, suggested warming oceans could push fish species away from the equator, while driving a decline in the diversity of invertebrates like crabs, lobsters, sea urchins and octopus.
Just published in the journal Science Advances, the research offered the first evidence of how ecological interactions affect marine species' abundance at global scales.
It was built off data collated by the Reef Life Survey (RLS), a citizen science project involving hundreds of scuba divers around the world, including staff and students from the University of Auckland's Leigh Marine Laboratory.
It now included information on 4000 species in 50 countries, allowing a better understanding of how and why species are distributed, while also providing an early-warning mechanism for climate-driven changes.
The new study found the total number of fishes and large invertebrates seen underwater changed little from the tropics to polar latitudes.
However, while fishes were more numerous in the tropics, they became progressively rarer towards colder latitudes, while large invertebrates showed the opposite pattern.
The data took in surveys at more than 2400 sites, including records of more than 2500 bony fish, 66 sharks and rays, and assorted other species such as mammals, reptiles, sea cucumbers, sea urchins, sea stars, octopus, cuttlefish and crustaceans like crabs, lobsters and mantis shrimps.
In all, the fauna within the study's scope represented some 10 per cent of all marine species, but more than half of species on regional checklists and field guides of shallow reef taxa.
Study lead author and RLS founder Professor Graham Edgar, of the University of Tasmania, said the distribution was changing as waters became warmer, with implications for food webs, ecosystems and fisheries across the planet.
But the effects of climate change on marine life varied greatly between geographic regions, and while the "tropicalisation" of marine life was already underway in Southeastern Australia and Tasmania, similar effects were yet to be seen in New Zealand.
Previous research had suggested climate change would directly affect the range of species due to rising temperatures, but the RLS data showed that wasn't the end of the story, Edgar said.
"We found that the local presence and abundance of a species reflect not only its need for particular temperatures and environmental conditions but also its ecological interactions.
"As fish extend their range further from the equator with warming water, their advantage as predators will affect the abundance and diversity of large mobile invertebrates."
Broad changes would likely spread across the ecosystem, affecting human activities, such as fishing.
Edgar said ongoing monitoring of marine life at both local and regional levels was needed to allow the early detection of such changes.
This would allow adaptive fisheries and conservation management, and help to minimise the social and economic impacts.
"Species monitoring of shallow reef communities at national scales is only possible with the support of citizen scientists, such as the RLS divers who contributed data to our study."
The new study followed a 10-year science stocktake by Niwa which suggested climate change was slowly shifting the chemistry of New Zealand's oceans.
Between 1909 and 2009, New Zealand's sea-surface temperatures had warmed by a statistically significant 0.71C, while pH levels of subantarctic waters had dropped by 0.0015 units per year since 1998.
Globally, the oceans' average pH is currently 8.1, which is 0.1 lower than it was 250 years ago - a decrease of just one pH unit represented a 10-fold increase in the acidity.
The decline in pH was projected to continue in line with the increase in atmospheric CO2, leading to the most rapid decrease in ocean pH in the past 50 million years.
The effect was associated with decreases in nutrients such as nitrate and phosphate in the surface ocean, where most marine organisms live.
Even small shifts had big consequences: mussels and paua might struggle to build their carbonate shells, while some fish species could experience changes in behaviour, physiology and even habitat distribution.
Niwa scientists estimated that perhaps 25 per cent or less of the existing cold water coral locations around New Zealand will be able to sustain their growth by 2100 due to ocean acidification.
Another five-year, $800,000 study has also just been launched to investigate why some New Zealand species may be able to cope more easily with ocean acidification.
Because of their highly soluble calcium carbonate skeletons, reef-building algae were widely considered to be among the species most at risk.
But marine botanist Dr Christopher Cornwall has challenged this idea, suggesting certain species of calcifying algae might pack the physiological machinery needed to tolerate change.
He aimed to find out whether the resilience seen in some populations of local coralline algae was due to them having evolved in more variable pH environments.