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

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 dinoflagellates. However, the sedimentation of these groups is different: diatoms tend to sink to the sea floor at the end of the bloom, while dinoflagellates to a large degree are been remineralized in the euphotic zone. Understanding phytoplankton competition and species specific 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 dinoflagellates in the Baltic Sea. This was achieved by specific studies of: (1) seasonal vertical positioning, (2) dinoflagellate life cycle, (3) mixotrophy, (4) primary production, respiration and growth and (5) diatom silicate uptake, using cultures of common cold-water diatoms: Chaetoceros wighamii, C. gracilis, Pauliella taeniata, Thalassiosira baltica, T. levanderi, Melosira arctica, Diatoma tenuis, Nitzschia frigida, and dinoflagellates: Peridiniella catenata, Woloszynskia halophila and Scrippsiella hangoei. The diatoms had higher primary production capacity and lower respiration rate compared with the dinoflagellates. This difference was reflected 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 dinoflagellates. Among diatoms there were species specific 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 dinoflagellates 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 dinoflagellates did not have higher growth rates or carbon assimilation rates and would thus probably not perform better than S. hangoei in the model. The dinoflagellates do, however, have competitive advantages that were not included in the model: motility and mixotrophy. Previous investigations has revealed that the chain-forming P. catenata performs diurnal vertical migration (DVM), and the results presented here suggest that active positioning in the water column, in addition to DVM, is a key element in this species' life strategy. There was indication of mixotrophy in S. hangoei, as it produced and excreted the enzyme leucine aminopeptidase (LAP). Moreover, there was indirect evidence that W. halophila obtains carbon from other sources than photosynthesis when comparing increase in cell numbers with in situ carbon assimilation rates. The results indicate that mixotrophy is a part of the strategy of vernal dinoflagellates in the Baltic Sea. There were also indications that the seeding of the spring bloom is very important for the dinoflagellates to succeed. In mesocosm experiments dinoflagellates could not compete with diatoms when their initial numbers were low. In conclusion, this thesis has provided new information about the basic physiological and ecological properties of the main cold-water phytoplankton in the Baltic Sea. The main phytoplankton groups, diatoms and dinoflagellates, have different physiological properties, which clearly separate their life strategies. The information presented here could serve as further steps towards better prognostic models of the effects of eutrophication in the Baltic Sea.

Hur mycket IKT behöver en studerande? Virtuelle kurskonzepte und ihre auswirkungen auf leistung, zufriedenheit und arbeitsstrategien

In dem vorliegenden Aufsatz wird eine vergleichende Untersuchung eines Deutschkurses (Tyska V: Interkulturelle Themen) an der Schwedischen Wirtschaftsuniversität Helsinki (Hanken) präsentiert, der im Jahr 2002 vollständig virtuell und im Jahr 2003 mit virtuellen Lernphasen und Kontaktunterricht im Wechsel realisiert wurde, wobei sich das virtuelle Kursmaterial jeweils in der Lernumgebung WebCT befand. In der Untersuchung werden die Leistungen, die Kursevaluationen und die Arbeitsstrategien der Studierenden in den beiden Kurskonzepten analysiert und miteinander verglichen. In Bezug auf die Leistungen der Studierenden hat sich gezeigt, dass die guten Studierenden in beiden Kurskonzepten gleich gute Ergebnisse erzielen. Die Untersuchung zeigt jedoch, dass der Kontaktunterricht des teils-virtuellen Kurskonzeptes gerade für die schwächeren Studierenden eine wichtige Funktion erfüllt – die schwächeren Studierenden erzielen im Kurskonzept mit Kontaktunterricht deutlich bessere Ergebnisse als die schwächeren Studierenden im ganz virtuellen Kurskonzept. Ein Vergleich der Arbeitsstrategien zeigt, dass die schwächeren Studierenden im Unterschied zu den guten Studierenden deutliche Schwierigkeiten mit dem Zeit- und Materialmanagement haben. Für die Weiterentwicklung des Kurskonzeptes sind zum einen die technischen Rahmenbedingungen zu verbessern, und zum anderen muss der Kontaktunterricht neu überdacht werden, da die Einführung des Themas Arbeitsstrategien in der virtuellen Lernumgebung als eigenständiger Themenbereich dringend notwendig erscheint.