Oxygen concentrations in both the open ocean and coastal waters have declined by 2-5% since at least the middle of the 20th century.
This is one of the most important changes occurring in an ocean becoming increasingly modified by human activities, with raised water temperatures, carbon dioxide content and nutrient inputs.
Through this, humans are altering the abundances and distributions of marine species but the decline in oxygen could pose a new set of threats to marine life.
Writing in Philosophical Transactions of the Royal Society B, scientists present support for the theory that marine invertebrates with larger body size are generally more sensitive to reductions in oxygen than smaller animals, and so will be more sensitive to future global climate change.
It is widely believed that the occurrence of gigantic species in polar waters is made possible by the fact that there is more oxygen dissolved in ice cold water than in the warmer waters of temperate and tropic regions.
So as our ocean warms and oxygen decreases, it has been suggested that such oxygen limitation will have a greater effect on larger than smaller marine invertebrates and fish.
The study was conducted by John Spicer, Professor of Marine Zoology at the University of Plymouth, and Dr Simon Morley, an Ecophysiologist with the British Antarctic Survey (BAS).
They investigated how a number of different sized amphipod species – found in abundance in Antarctic waters and relatives of the sandhoppers on temperate beaches) – performed when the oxygen in the water they were in was reduced.
Overall, there was a reduction in performance with body size supporting the theory that larger species may well be more vulnerable because of oxygen limitation.
However, the picture is a little more complex than this with evolutionary innovation – such as the presence of oxygen binding pigments in their body fluids to enhance oxygen transport, and novel gas exchange structures in some, but not all, species – to some extent offsetting any respiratory disadvantages of large body size.