Investigating environmental changes in the southern North Sea : a combined statistical assessment of climatic and biogeochemical long term time series The work presented in this thesis untangles the effects of changes in various physical and biogeochemical variables on the ecosystem of the southern North Sea with an emphasis on longterm variability. It is still a matter of debate how climate variations may affect the functioning of the ecosystem. Therefore this study provided for the first time a detailed understanding of climate variability of the southeastern region of the North Sea the German Bight during the period 19752004 by combining a wide set of physical biological and chemical data. The first part of this work used Principal Component Analysis to examine the bulk variability of a diverse array of physical and biogeochemical longterm time series in order to understand the main mode of variability of the climate and the ecosystem of the coastal region. The outcomes motivated further investigations on the dynamic of the first trophic levels of the ecosystem. Therefore in the second part of this work the factors that drove the longterm seasonal variability of three most representative species of the diatom community (Guinardia delicatula Thalassionema nitzschioides and Odontella aurita) were determined with multivariate linear regression analysis. Finally in the third part the results on the seasonal variability of the three phytoplankton species were put into the wider context of the German Bight ecosystem by studying the phenology of some important primary and secondary consumers. Hence the longterm abundance dynamics of Beroe gracilis Pleurobrachia pileus and their food calanoid copepods were analysed using Bayesian statistics ...