Phytoplankton communities in the northwestern Adriatic Sea: Interdecadal variability over a 30-years period (1988-2016) and relationships with meteoclimatic drivers

In this study the interannual variability of phytoplankton over a ~30-years period in the coastal site of LTER-Senigallia transect (N Adriatic Sea) was investigated to document patterns potentially related to environmental/climatic drivers. Comparing physical and chemical data of the periods 1988-2002 and 2007-2016 periods, we showed that phytoplankton abundance and biomass and inorganic nutrient concentrations increased in the last decade, indicating that the tendency to oligotrophication due to the drop of the Po River outflow in the years 2002-2007 was reversed in the period 2007-2016. The typical P-limited conditions of the N Adriatic Sea seem to have been attenuated in the study area. P levels were not explained by the P concentrations in the Po River waters, suggesting the possible influence of other local P sources that could be related to the anomalous meteorological events (intense rainfalls) that took place in the 2007-2016 period. In the last decade, the community structure and seasonality of phytoplankton markedly changed, as highlighted by the different indicator species for each season: the blooms of Skeletonema marinoi shifted from winter to spring. A significant decrease of coccolithophores was observed particularly in winter months in the 2007-2016 period: some indicator species among the most relevant in the 1988-2002 period (such as Emiliania huxleyi in winter, and Syracosphaera pulchra in spring) have lost this role in 2007-2016. Dinoflagellate abundances decreased, except in spring when the occasional proliferation of large sized species caused biomass peaks. The phytoplankton annual cycle became irregular with sudden diatom blooms, reflecting the variability of meteorological events in recent years. It is noteworthy that in the last decade, an allochthonous species, i.e. the diatom Pseudo-nitzschia multistriata, became a regular inhabitant of the autumn phytoplankton communities of the NW Adriatic Sea.