Phytoplankton composition and long-term variations in two coastal and offshore marine sites (northern Adriatic sea)

Two long-term (1988–2019) datasets collected at coastal and offshore stations located along the LTER Senigallia–Susak Transect were compared. Multivariate statistical analysis highlighted the role of oceanographic conditions in differentiating the study areas. Environmental parameters at the coastal station were more variable and showed marked seasonality due to direct riverine inputs. Distinct mean annual cycles of physico-chemical parameters and phytoplankton abundances were observed, along with differences in community composition. Higher mean abundance values and lower diversity were found at the coastal station, although the two stations appeared more similar in summer due to the southeastward spreading of riverine waters under stratified conditions. Graph network analysis revealed that species not identified as relevant by IndVal were still able to influence the overall community at both sites. The offshore community exhibited a higher number of strong interactions among species compared to the coastal one.