Phytoplankton Dynamics andWater Quality in the Venice Lagoon

We analyzed the phytoplankton abundance and community structure monthly over a20-year period (1998-2017) at five stations in the Venice lagoon (VL), one of the sites belonging to theLong-Term Ecological Research network of Italy (LTER-Italy). We focused on phytoplankton seasonalpatterns, inter-annual variability and long-term trends in relation to water quality. Diatoms numericallydominated (ca. 60% on average), followed by nanoflagellates (37%), while coccolithophoridsand dinoflagellates contributed less than 2%. We observed distinct seasonal and inter-annual changesin the abundance and floristic composition of the phytoplankton groups, whilst no clear long-termtrend was statistically significant. We also assessed the water quality changes, applying to our datasetthe multimetric phytoplankton index (MPI), recently officially adopted by Italy to accomplish thewater framework directive (WFD) requirements. The index evidenced a temporal improvement ofthe water quality from "moderate" to "good" and allowed us to confirm its reliability to addressthe changes in the water quality, not only spatially--as previously known--but also for followingthe yearly time trends. Overall, our results highlight the importance of long-term observations,for understanding the variability in the phytoplankton communities of the lagoon as well as therelevance of their use to test and apply synthetic descriptors of water quality, in compliance with theenvironmental directives.