PhD ceremony Ms. G. Kulk: Photophysiology of oceanic phytoplankton. Consequences of stratification for primary production and community composition

PhD ceremony: Ms. G. Kulk, 14.30 uur, Academiegebouw, Broerstraat 5, Groningen

Dissertation: Photophysiology of oceanic phytoplankton. Consequences of stratification for primary production and community composition

Promotor(s): prof. A.G.J. Buma

Faculty: Mathematics and Natural Sciences

Phytoplankton are essential for all life in the oceans. By means of photosynthesis (in which phytoplankton use sunlight to convert CO2 into carbon), phytoplankton are responsible for almost half of the carbon production on earth. Gemma Kulk has studied the photophysiology of key oceanic phytoplankton species to gain insight in the ecological consequences of large scale physical processes and global warming.

The success of a specific species depends on the (photophysiological) response to different water column conditions, such as temperature, light and nutrientsMarked differences in photophysiology between several phytoplankton species showed that one of the most important physical processes, stratification, may have considerable ecological consequences. Prokaryotic phytoplankton species may benefit from the high temperatures and low nutrient availability associated with stratification, whereas eukaryotic phytoplankton species may benefit from the high light conditions. This suggests that during stratification, when fewer nutrients are available, small (prokaryotic) phytoplankton species will dominate the phytoplankton community. Ultimately this may lead to a lower productivity of the phytoplankton community and lower export of carbon to the deep ocean. Global warming may have similar consequences when stratification increases in large areas of the open oceans. Specific phytoplankton species will benefit from the changes in the temperature, light, and nutrient conditions. Comparable to stratification, a shift to smaller phytoplankton species is expected during global warming. This may lead to a lower productivity of the phytoplankton community in large areas of the open oceans and simultaneously decrease the export of carbon to the deep ocean.