Developments in German Bight benthic ecology driven by climate change and anthropogenic utilisation

Marine ecosystems are subjected to an unprecedented range of natural and anthropogenic disturbance with an increasing frequency of occurrence over recent decades. Among others, rising sea water temperature, ocean acidification, and coastal water pollution have resulted in alteration of habitats and subsequent changes in the structures of species assemblages. In the face of these challenges, ecological research needs to predict responses of assemblages to global change, a requisite for the adequate prevention of further environmental degradation. However, predicting assemblage responses requires a thorough understanding of ecological processes and of the structure and functioning of assemblages. The present thesis comprises four manuscripts which address in detail a) the temporal variability of benthic macrozoobenthos assemblages and the relative role of different environmental drivers of abundance variations in the North Sea, b) the functional diversity and the dominant functional characteristics of benthic species of the southern North Sea, c) the temporal variations in the functional trait composition, the contribution of different biotic and abiotic predictors to the variation in ecological functioning, the relationship between species diversity and functional diversity and the extent of functional redundancy within benthic communities and d) the degree of functional homogenization and the identification of dynamically-dominant-traits that likely have the greatest effect on biodiversity and ecosystem function. In ‘Manuscript I,’ the model results revealed that temperature and anomalies of the North Atlantic Oscillation Index (NAOI) play a distinct role in controlling the temporal variation of the benthic assemblages. The results also showed substantial variation in the composition of macrozoobenthos assemblages in the North Sea at decadal and sub-decadal scales. Species react differently to environmental disturbances with generalist species being dominant in the region. ‘Manuscript II’ and ‘Manuscript III’ on the functional characteristics of macrozoobenthos assemblages revealed that changes in trait composition were more similar among monitoring sites than changes in the taxonomic composition, emphasizing the role of environmental disturbances in the determination of trait associations in the North Sea system. The relationship between species richness and functional diversity indicates a relatively high functional redundancy within benthic assemblages of the southern North Sea. ‘Manuscripts II - IV’ showed that some trait modalities such as small body size, high dispersal potential, interface- and deposit-feeding were relatively common in the North Sea benthic assemblages. In fact, this suite of traits represents an ‘adaptive strategy’ enabling species to survive and thrive in a stressful environment. The replacement of specialist species by generalist species (Manuscripts I and III), less site-specific temporal variations in functional composition as compared to the taxonomic composition (Manuscript III) and a high degree of functional niche overlap (Manuscript IV), imply functional similarity among species assemblages. An increased spatial similarity of assemblages, in turn, generated ‘functional homogenization’ in the North Sea benthic system. In contrast to the considerable temporal variation in species abundance, the temporal development of functional diversity was relatively stable with only two incidental inconsistencies coinciding with extreme events (i.e. cold winter 1995/96 and extreme negative NAO winter of 2009/10) in the North Sea. Following the temporary changes, the functional diversity rebounded to previous levels after almost one year. The rapid recovery of functionality after disturbance may be attributed to the high functional redundancy in the ecosystem. The results of the present thesis contribute to the understanding of the structural and functional processes of macrozoobenthos in the southern North Sea. The study highlights that a comprehensive understanding of long-term dynamics of benthic ecosystems requires a combined analysis of functionality and taxonomic structure