Phytoplankton dynamics and responses to two extreme events in the Gulf of Venice, Northern Adriatic Sea, Italy

This study, based on 12 years of regular monthly observations, aimed to describe the prevalent seasonal pattern and taxonomic composition of the phytoplankton community at the Italian Long-Term Ecological Research (LTER-Italy) site, Acqua Alta Oceanographic Tower (LTER-AAOT), a coastal region of the NWAS (North-Western Adriatic Sea), characterized by significant variability in oceanographic conditions. Although studies on phytoplankton communities in this region dates back to the 1960’, this dataset represent the first systematic and regular observations. Over this period, two years - 2014 and 2022 – stood due to their distinctive climatic conditions: extreme rainfall (2014) and severe drought (2022). According to the Intergovernmental Panel on Climate Change (IPCC), such events are expected to become more frequent and intense in the future. Therefore, we also analysed phytoplankton changes during these two years, to explore the potential impact of extreme weather events on phytoplankton dynamics in this coastal area. The prevalent phytoplankton seasonal cycle is bimodal, with a growing season that starts in late winter and continues until early summer, followed by a decline in summer, and a second growth pulse in early autumn. Diatoms and undetermined nanoflagellates dominate throughout the year. Various taxa of dinoflagellates taxa are present in summer, though in low abundances, while coccolithophores (i.e. calcified Prymnesiophyceae) are present in winter. Despite significant variability in abiotic factors, this seasonal and taxonomic pattern remains a consistent feature of the phytoplankton community highlighting the overall variability of the LTER site as well as its similarities with other areas of the Northern Adriatic Sea. The study also suggests potential shifts in phytoplankton patterns in response to extreme weather events, indicating a trend toward more oligotrophic conditions during droughts and increased brackish water influence under most rainy scenarios. These findings contribute to a better understanding of the future dynamics of coastal phytoplankton communities in the context of climate change.