On the ecology of cold-water phytoplankton in the Baltic Sea

Temporary cysts enables long-term dark survival of Scrippsiella hangoei (Dinophyceae). – Submitted manuscript. and pH during a sub-ice bloom of dinofl agellates in the Baltic Sea. – Submitted manuscript. The printed research article included in this thesis (paper I) is reproduced with the kind permission of Oxford University Press. Increased anthropogenic loading of nitrogen (N) and phosphorus (P) has led to an eutrophication problem in the Baltic Sea, and the spring bloom is a key component in the biological uptake of increased nutrient concentrations. The spring bloom in the Baltic Sea is dominated by both diatoms and dinofl agellates. However, the sedimentation of these groups is different: diatoms tend to sink to the sea fl oor at the end of the bloom, while dinofl agellates to a large degree are been remineralized in the euphotic zone. Understanding phyto-plankton competition and species specifi c ecological strategies is thus of importance for assessing indirect effects of phytoplankton community composition on eutrophication problems. The main objective of this thesis was to describe some basic physiological and ecological characteristics of the main cold-water diatoms and dinofl agellates in the Baltic Sea. This was achieved by specifi c studies of: (1) seasonal vertical positioning, (2) dinofl agellate life cycle, (3) mixotrophy, (4) primary production , respiration and growth and (5) diatom silicate uptake, using cultures of common cold-water diatoms: Chae toceros wighamii, C. gracilis, Pauliella taeniata, Thalassiosira baltica, T. levanderi, Melosira arctica, Diatoma tenuis, Nitzschia frigida, and dinofl agellates: Peridiniella catenata, Woloszynskia halophila and Scrippsiella hangoei. The diatoms had higher primary production capacity and lower respiration rate compared with the dino-fl agellates. This difference was refl ected in the maximum growth rate, which for the examined diatoms range from 0.6 to 1.2 divisions d-1 , compared with 0.2 to 0.3 divisions d-1 for the dinofl agellates. Among diatoms there were species specifi c differences in light utilization and uptake of silicate, and C. wighamii had the highest carbon assimilation capacity and maximum silicate uptake. The physiological properties of diatoms and dinofl agellates were used in a model of the onset of the spring bloom: for the diatoms the model could predict the initiation of the spring bloom; S. hangoei, on the other hand, could not compete successfully and did not obtain positive growth in the model. The other dinofl agellates did not have higher growth rates or carbon assimilation rates and would thus probably not …