Investigations of the influence of physical factors on some marine ecological systems

Sammanfattning: This thesis is about the conditions for life in the sea. The marine life forms may look disparate, but are joined by their common dependence of the fluid medium which they inhabit. The motion and the properties of the water in the sea largely determine the conditions with which all maritime organisms have to comply. By their existence, the organisms also influence and reshape the environment in which they live. Mass occurrences of fast-growing green algae are a common sight in many coast-al areas. These algae are superior competitors for space and nutrients, and may often disrupt the previous functioning of certain shallow water ecosystems. Through a process-based model study, I have tried to explain the relationships of the algae, the nutrient sup-ly, and the water exchange that may lead to such mass occurrences. The macroscopic algae on the seafloor often have a production capacity superior to that of the phytoplankton of the open waters, but this production is frequently overlooked in large-scale estimations of the primary production. In a study including the use of field measurements of macroalgal biomass and coverage, GIS-based analyses of bottom sediment structure and topography, as well as modelled productivity measures, I have tried to compute the total production of macroalgae in the sea of Kattegat, situated between Sweden and Denmark. This objective was, in the present situation, possible to reach only by the use of various methods and a diverse range of data, where all of the above ingredients, however, amalgamated into a useable form and emerged into a plausible result. The joy of swimming in the Baltic Sea on a hot summer day is often disturbed by the presence of blooms of cyanobacteria, or blue-green algae, that turn the tempting waves into an unpleasant mess. These sometimes toxic organisms become competitive in summer, when the nutrient composition is insufficient for other algae. Through analysis of the reports on cyanobacterial blooms in the last three years in the Baltic Sea, I have tried to seek out the antecedents of the blooms, taking the nutrient dynamics throughout the year into consideration, and arrived at the conclusion that the late winter phosphorus conditions have a considerable influence on the presence of cyanobacteria in the following summer. The data also showed that the surface water became depleted of phosphorus in summer, irrespective of whether cyanobacterial blooms occurred or not. The organisms of the seafloor live on the boundary between two worlds, finding shelter in the soft sediment while they rely on food and oxygen to be supplied from the water above. The delicate balance between too little and too much regarding life in the sediment was investigated in the final essay of this thesis. The vital oxygen can be lost to the bottom dwellers by the excessive precipitation of organic matter, either into a stagnant water volume, or directly onto the seafloor. The size of the benthic biomass in the seas around Sweden was found to be directly linked to the supply of food.