Investigatin environmental changes in the southern North Sea: a combined statistical assessment of climatic and biogeochemical long-term time series

Abstract

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 long–term 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 1975–2004, 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 long–term 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 long–term 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 long–term abundance dynamics of Beroe gracilis, Pleurobrachia pileus and their food calanoid copepods were analysed using Bayesian statistics. Taken together, the results of this work revealed a more coherent picture of how and on what time scales biology responds to physical changes and the inherent consequences for the ecosystem. It was documented for the first time that the climate and the ecology of the German Bight experienced patterns of variability similar to the ones of the entire North Sea. The German Bight, however, is exposed to different natural and anthropogenic perturbations when compared to the whole of the North Sea, detailed investigations on some key organisms have thus revealed the differential impacts of major driving factors on different species. Although temperature variation was the major driver of changes in the abundance and seasonality of primary and secondary consumers (top down effect), evidence was provided suggesting the importance of bottom up processes. This work highlights the complex responses of a coastal ecosystem to climate change and the high nonlinearities in the atmosphere–ocean–biosphere system.