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Abstract

This dissertation examines the ecological dynamics of phytoplankton communities in the northern Adriatic Sea, focusing on phenology, environmental drivers, and trophodynamics. The complexity of the region, characterized by the richness of phytoplankton communities, environmental variability and intensive human activities, is not seen as an obstacle but as an opportunity to gain new ecological insights. Building on existing knowledge, the research combines established data analysis techniques with innovative ecological modeling methods to unravel the complicated and paradoxical nature of phytoplankton life.
The dissertation comprises three interlinked studies conducted at the Slovenian Long-Term Ecological Research (LTER) site in the Gulf of Trieste. The first study examines the phenology of the phytoplankton community using a time series of monthly taxa abundances collected between 2005 and 2017. By reducing the complexity of the community into constituent assemblages, the study evaluates techniques for species selection, community structure analysis and assemblage definition, highlighting their strengths and limitations.
The second study investigates how atmospheric (winds, temperature, precipitation) and hydrospheric (river inputs, water column stability, salinity) factors shape the ecological niches of the phytoplankton assemblages described in the previous study. Through linear and nonlinear numerical modeling, the study identifies the main niche-forming parameters and their properties. The resulting models are then applied to assess the connectivity of the northern Adriatic Basin at the mesoscale. These results are placed in the context of current ecological theories.
The third study extends phytoplankton research to individual-based biological data collected in the Gulf of Trieste between April 2020 and March 2021. It investigates the differences between taxonomic and trait-based classifications and between abundance and biomass metrics. By scaling individual-level data to community-level dynamics, the study provides insights into the trophic structure of the planktonic food web.
The dissertation concludes by summarizing the main results in a conceptual model that improves the understanding of phytoplankton dynamics in a highly dynamic coastal environment. In summary, the distribution of phytoplankton communities in the Gulf of Trieste exhibits a partially predictable seasonal succession driven by environmental forces and mesoscale connectivity. The baseline community is dominated by nano-sized phytoflagellates, with cell size and biomass distribution following a power law during times of resource scarcity or grazing pressure. Diatoms dominate during biomass peaks in spring and autumn, which is associated with larger cells and multimodal distributions. These shifts reflect the influence of both top-down and bottom-up processes. Phenology is influenced by periodic events such as stratification and river discharge, as well as non-periodic disturbances such as climatic anomalies, which promote the occurrence of short-lived assemblages. At the mesoscale, the Gulf of Trieste is partially connected to the northern Adriatic by cyclonic gyres and wind-driven circulations, especially in winter and autumn, while in spring and summer local phenomena dominate and reduce connectivity.