Despite their microscopic size, planktonic organisms have a big impact on the biosphere: photosynthetic plankton in the oceans produce more oxygen than all the rainforests combined. They bind large quantities of CO2 and provide the basis of life for many marine animals. But the warming of the oceans could reduce the productivity of plankton. It is unclear whether species diversity will also decline. “Up until now, we didn't even know how phytoplankton species are distributed in the oceans,” says Damiano Righetti, a PhD student in the Environmental Physics Group at ETH Zurich.
He has therefore developed, together with WSL researcher Niklaus Zimmermann, a method to map the spatial and temporal distribution of phytoplankton species. He collected more than half a million data measurements for 536 different species from various databases. Using a computer model, he created the first map of their global diversity patterns.
The result: the total number of plankton species decreases from the equator to the poles. This phenomenon is known from terrestrial species, but unlike for them, for plankton species the decrease is not continuous. Between the 35th and 55th latitude, the diversity is significantly lower than would be expected, before increasing again towards the poles. The marked dip in mid-latitudes could be due to the harsh and seasonally changing environmental conditions prevailing there, the researchers suspect.
Thanks to the new model, it has been possible to derive the biodiversity patterns of phytoplankton from very incomplete and unevenly distributed data. “These mostly originate from seawater samples collected along the normal shipping routes. There is hardly any data available from less frequented areas,” says Damiano. The model can be used to correct for uneven sampling density, and also enables predictions to be made about how the biodiversity of the phytoplankton will change if sea temperatures continue to rise. (Claudia Hoffmann, Diagonal 1/19)