Microphytobenthos in cold-water sublitoral systems : their ecological role and response to changing environmental conditions

In my thesis, the main aim was to study and compare subtidal microphytobenthos communities and their activity from sandy sediments of an Arctic (Spitsbergen, Svalbard) and a temperate (Helgoland, North Sea) study site. Helgoland was chosen as a comparative site because of the tight link between the North Sea and the western coast of Spitsbergen due to ocean currents. The research objective was realized in three consecutive studies. The specific questions in this study were: (i) How do MPB net photosynthesis and respiration (in terms of oxygen production and consumption) compare in Helgoland and Svalbard? Who are the main players of the MPB community, and what is the daily net oxygen budget under natural (in situ) conditions? Will the communities and their photosynthetic performances (light adaptation, maximum photosynthesis) be site‐specific, or will they be similar? (ii) Will the photosynthesis and respiration response of the temperate and sub-Arctic MPB community to short-term temperature increases be different? Will the sub-Arctic site be more adapted as it experiences higher temperature fluctuations in situ on a short term scale (daily/ hourly) than the temperate site? Will the MPB communities develop towards a net-heterotrophy as previously shown for benthic communities? (iii) Will an upward shift of the temperature baseline and increase of nutrients have consequences for the growth of MPB communities from Helgoland and Svalbard? Will the MPB community grow best under lowest, intermediate or highest temperatures from within the in situ temperature range? Will their growth be dependent on the addition of nutrients? Will they grow with the same speed and reach similar biomasses, or will there be differences because of differences in their natural light and temperature regimes?