Changes in vertical stratification and their impact on North Atlantic phytoplankton communities

Summary

Global warming will change physical, chemical and biological processes in the oceans. Ocean-climate models predict that warming of the surface layer may strengthen vertical stratification, starting earlier in spring and lasting longer in autumn. This results in suppressed upward mixing of nutrients from the deep ocean and altered irradiance regimes. Changes in stratification will have major effects on the growth and species composition of phytoplankton. This will subsequently impact grazing, viral lysis and sedimentation rates, with cascading effects on ecosystem functioning and biogeochemical fluxes. Little is known, however, on the exact implications of global warming for these fundamental processes. The highly multidisciplinary research team of Stratiphyt has investigated how changes in vertical stratification affect phytoplankton communities (growth, losses and composition). Two field campaigns were carried out during spring and summer along a North-South gradient in the Atlantic Ocean, from Iceland to the Canary Islands.

The Northeast Atlantic Ocean was chosen, because it is a key area in global ocean circulation, a large sink for atmospheric CO₂ and a major determinant of the climate in Western Europe. Furthermore, the Atlantic Ocean offers a gradient from weak seasonal stratification in the North to strong permanent stratification in the (sub)tropics. Detailed laboratory experiments with representative phytoplankton species (including Synechococcus spp., Prochlorococcus spp.) were carried out. Our integrated approach of physical, chemical, and biological processes will enable a better understanding of the implications of global warming for plankton growth in the Northeast Atlantic Ocean.

(Stratiphyt project: NWO-ZKO)