Global warming likely to reduce phytoplankton size, thereby reducing ocean's capacity to remove carbon dioxide from air

Researchers at the Spanish Institute of Oceanography (IEO) Xosé Anxelu G. Morán, Ángel López-Urrutia and Alejandra Calvo-Díaz, with the collaboration of William K. W. Li, from the Bedford Institute of Oceanography in Canada, have evidenced that the contribution to phytoplankton total biomass of its smallest fraction (picophytoplankton, less than 2 µm in diameter) is greater in warmer waters. This study, published last March in the journal Global Change Biology, anticipates future changes in the functioning of marine ecosystems as phytoplankton size largely determines the ultimate fate of organic carbon produced during photosynthesis.

The study was developed with data obtained in different cruises carried out between 1994 and 2007 in the Northwestern Atlantic (between 43 and 60ºN latitude), and with samples taken monthly from 2002-2007 in the Northeastern Atlantic (43° N) under the strategic program of the IEO on time series of oceanographic data called RADIALES.

They analyzed more than 150 plankton samples taken throughout the year within a temperature range between 0 and 22 º C and measured the concentration of the smallest organisms: the picophytoplankton, photosynthetic unicells with a diameter of less than 2 µm, about 40 times smaller than a human hair.

Analyzing the results in terms of biomass (organic carbon content) showed that while total phytoplankton decreased with warmer temperature, as already demonstrated in other recent studies, the biomass of smaller cells increased. This trend, which was practically identical on both sides of the ocean, shows that picophytoplanktonic contribution to total biomass would be greater in a global warming scenario.

The size of phytoplankton is a key variable that determines the structure and functioning of the marine food web and ultimately, the fate of the organic carbon produced during photosynthesis. The impact of the increase in these small primary producers can be summarized as a significant reduction in the potential of the oceans as a sink for atmospheric CO₂. The Spanish Institute of Oceanography (IEO) is a public research organization, from the Spanish Ministry of Science and Innovation, dedicated to research in marine science; especially in relation to scientific knowledge of oceans, sustainability of fisheries resources and marine environment. The IEO represents Spain in most of the international science and technology forums related to the sea and its resources. IEO has nine coastal oceanographic centres, five experimental aquaculture plants, twelve tide gauge stations, one station for receiving satellite images and a fleet of six research vessels.