The living space of marine organisms is shrinking

On the one hand human activities expand, on the other hand the living space for marine ecosystems shrinks. The climate crisis affects the oceans, warms them up, acidifies them, reduces the presence of oxygen in the surface layers, and now it turns out that its colour is also changing. Large portions of the planet's oceans have become darker over the past two decades, according to a study published last week in Global Change Biology, and researchers fear that this trend will have a serious impact on marine ecosystems worldwide.

Satellite data and numerical modelling have revealed that more than one fifth of the oceans have darkened between 2003 and 2022, reducing the thickness of the section where 90 per cent of marine species, which depend on sun and moonlight, live. The effect is evident over 75 million square kilometres of water, equivalent to the land area of Europe, Africa, China and North America combined. Thomas Davies, a marine biologist at the University of Plymouth and first signatory of the study, says the findings have potentially serious implications for marine ecosystems, global fisheries and the crucial exchange of carbon with the atmosphere. In fact, 95 per cent of photosynthesis in the ocean (and thus carbon dioxide uptake and subsequent oxygen release) occurs in the first belt.

Davies and his colleagues relied on the colour measurements of the Moderate Resolution Imaging Spectroradiometer (Modis) aboard the Aqua satellite, which has been monitoring ocean colour for 23 years, and a dedicated algorithm to calculate the depth of this first band, called the euphotic zone, normally about 200 metres deep, where there is an optimal level of incoming sunlight, sufficient to allow photosynthesis by the algae and bacteria that make up phytoplankton, the basis of marine life. Darkening has affected 21% of the global oceans in the 20 years to 2022. In 9% of the surface this has led to euphotic zones shrinking by 50 metres and in 2.6% by as much as 100 metres.

The oceans become darker as light has a harder time penetrating the water and this indicates that ecosystems within the ocean surface are also changing, as the colour of the ocean is a reflection of the organisms and materials in its waters, which rise with cold currents. "Areas where there are major changes in ocean circulation, as a result of ocean warming caused by climate change, are darkening, such as in the Antarctic Ocean and along the Gulf Stream to Greenland," Davies explains. 'Marine organisms use light for many functions,' Davies points out. - If it's missing, they have to move up the water column and cluster towards the surface'.

The reduction in living space for marine organisms is also occurring for other reasons. As the oceans warm, oxygen levels drop and entire species migrate to colder (and more oxygenated) waters or to the surface. A study of coral reef fish conducted in California between 1995 and 2009 found that 23 species moved an average of 8.7 metres to the surface every decade as oxygen levels dropped. In the north-east Atlantic, tuna have been pushed into a smaller water layer, losing a total of 15 per cent of their available habitat from 1960 to 2010.