Phytoplankton diversity and bloom dynamics in coastal ecosystems under the influence of terrestrial runoff

Abstract

In order to investigate the various issues raised in the present study, a number of objectives were defined and approached in different chapters. The first three Chapters (1, 2, 3), include an introduction to phytoplankton ecology and diversity which is followed by a short presentation of the study sites from where information was compiled, and a detailed presentation of the Kalloni Gulf. The physicochemical regime and phytoplankton community of the Kalloni ecosystem have never been analyzed for a full annual cycle; therefore, Chapter 4 contains a thorough description of its physical, chemical, and biotic environment. In Chapter 5, the response of the marine ecosystem of Kalloni Gulf to terrestrial nutrient loading was analysed and threshold values of nutrient enrichment for the development of eutrophication episodes were established. This investigation was based on whole community parameters - such as biomass - and not at the organism level. In the next Chapter (Chapter 6), a more thorough research was conducted on the development of blooms after nutrient enrichments of terrestrial origin. Emphasis was placed on the species composition of blooms and on the temporal variability and spatial distribution of blooming species. This work is essential in order to obtain insights into the mechanisms of initiation and expansion of phytoplankton blooms and particularly harmful algal blooms (HABs) in relation to anthropogenic inputs from the coastal zone. In Chapter 7, an attempt was made to associate the structure, diversity, and species succession patterns of phytoplankton assemblages during a full annual cycle with environmental parameters. This involved the examination of the diversity and composition of all assemblages during the studied year, placing special focus on the differences between natural and anthropogenic causes of blooms. In combination with the study of Chapter 6, this work provided insights into the mechanisms regulating the response of a phytoplankton community to external stressing factors related to land-ocean interactions in the coastal zone and particularly to nutrient inputs of anthropogenic origin. A more detailed research into the way phytoplankton communities are structured and assembled involved the application of various statistical and niche-based models. Statistical models are mainly used to describe the shapes of phytoplankton species abundance distributions and also to explore possible shifts in those shapes with respect to different 6 productivity levels. On the other hand, niche-based models were applied to challenge the hypothesis that phytoplankton communities are niche-assembled. These aspects of community structure were analyzed and discussed in Chapters 8 and 9. Phytoplankton diversity shifts along productivity gradients were analyzed by investigating changes in the shape of the diversity-productivity relationship (Chapter 9). The inclusion of bloom events into this work provided the opportunity to explore the relationship, considering extreme cases of biomass production and therefore explore patterns along a wide productivity spectrum. Moreover, shifts in the diversity-productivity relationship were associated with the variability of physicochemical factors and the level of disturbance in order to explore a new hypothesis that abiotic stress is affecting both sides of the observed shapes. Finally, the main points addressed in the study and concluding remarks were presented in Chapter 10. Comments on the applied methodologies were made and the potential for future development, elaboration and expansion of research in the field of phytoplankton ecology was discussed.