Abstract
Phytoplankton community structure was analysed from 2010 to 2017 at C1-LTER, the coastal Long-Term Ecological Research station located in the Gulf of Trieste, which is the northernmost part of the Mediterranean Sea. Phytoplankton abundance and relevant oceanographic parameters were measured monthly in order to describe the seasonal cycle and interannual variability of the main phytoplankton taxa (diatoms, dinoflagellates, coccolithophores and flagellates) and to analyse their relationship with environmental conditions. Overall, phytoplankton abundances showed a marked seasonal cycle characterised by a bloom in spring, with the peak in May. During the summer, phytoplankton abundances gradually decreased until September, then slightly increased again in October and reached their minima in winter. In general, the phytoplankton community was dominated by flagellates (generally <10 µm) and diatoms co-occurring in the spring bloom. In this period, diatoms were also represented by nano-sized species, gradually replaced by larger species in summer and autumn. Phytoplankton assemblages differed significantly between seasons (Pseudo-F= 9.59;p< 0.01) and temperature and salinity were the best predictor variables explaining the distribution of the multivariate data cloud. At the interannual scale, a strong decrease of the late-winter bloom was observed in recent years with the spring bloom being the main phytoplankton increase of the year.
Highlights
Marine ecosystems are experiencing many different changes in response to natural processes, human activities and climate change
This study presents the temporal dynamics, over eight years, of the phytoplankton community at a coastal station located in the north-eastern Adriatic, a highly variable environment
Long-term sampling offers a unique opportunity to analyse multiannual datasets and describe complete seasonal cycles, unveiling possible changes occurring in phytoplankton community structure in highly variable environments such as coastal ecosystems where the distinction between natural variability and temporal trends is more difficult due to local disturbances
Summary
Marine ecosystems are experiencing many different changes in response to natural processes, human activities and climate change. Phytoplankton are a key component of marine ecosystem dynamics, contributing about half the global net primary production (Field et al 1998) They represent the base of the food web and play a pivotal role in global nutrient cycles and particle export to the bottom. Tracking changes in the phytoplankton community structure can help to forecast ecosystem changes and plan sustainable management of the seas and oceans For this reason, for instance, phytoplankton community diversity and temporal modification have been included as useful indicators in the Marine Strategy Framework Directive (MSFD) to determine the achievement of Good Environmental Status for the protection and conservation of the marine ecosystem. This study is based on data collected at the station C1-LTER that, since 2006, has been formally included in the LTER-Italy as part of the northern Adriatic LTER site
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