64PE368 - Day 2

Welcome onboard, this is the last CHARLET cruise in a set of four. What are we up to? If you would walk around the ship, and chat here and there, you would find out that there is one thing that binds us: phytoplankton. Yesterday evening, I was flabbergasted by the spectacular beauty and diversity of the phytoplankton community  Corina Brussaard, the chief scientist, was showing under the microscope at station 11. We were witnessing the remarkable diversity of phytoplankton with respect to colour, shape, and size. Phytoplankton also differ in their requirements for resources as light, nitrogen, and phosphate. Hence, by changes in the relative availability of these resources, the competitive balance between the phytoplankton species is very likely to shift. And this brings us to the purpose of the CHARLET cruises.

In the last decades, there has been a dramatic shift in nutrient availability in the North Sea. The cause of this shift is the reduced input of phosphorus, as our washing powder has become more ‘green’, on top of other measures reducing eutrophication. At the same time, nitrogen and silicate concentrations were less reduced, causing a shift from phosphorus to nitrogen limitation in the North Sea. How is this shift in nutrient limitation affecting the phytoplankton community in the North Sea, and the food-chain as a whole? In the CHARLET project, three teams of researchers are tackling this question, each at a different level. The first team from the UvA (me and Amanda Burson) investigates which nutrients limit phytoplankton growth in the North Sea and how this effects their competition. The next team from the NIOZ-Yerseke (Eric Boschker & Julia Grosse) investigates how changes in these limiting factors affect the cellular composition and species composition of the phytoplankton community. The third team from NIOZ-Texel (Corina Brussaard &Paul O’Conner) brings it to the next level: they study to what extent changes in phytoplankton species composition and their nutritional quality (cellular composition) will affect trophic transfer efficiency of nutrients and energy to zooplankton grazing versus viral lysis.

During this last CHARLET cruises it will be the challenge to bring together the pieces of information collected during the former cruises by the three teams, and to glue them together to get us the essential insight how nutrient limitation in the North-Sea shapes the phytoplankton community and the food-web as a whole.

Chain forming diatoms (typical phytoplankton found during spring when nutrients are still relatively high and there is enough silicate for their skeleton). The ones shown here are single cells connected together; using a specific light their chloroplasts (where there pigments are found) light up bright red. [Photo: Sander Asjes].