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The distribution of phytoplankton abundance and species composition is described for coastal waters of the NW Adriatic Sea, for the period 1990–1999. Hydrochemical and biological properties were investigated every month, at 48 stations. This coastal system is quite unstable and heterogeneous, due to the influence of several rivers, intense water exchange with the Lagoon of Venice, and complex advective processes. In the vicinity of river plumes, dissolved nitrogen and dissolved silica appear to be in excess, rarely limiting phytoplankton growth. Examining the general trend of phytoplankton over the years, the main limiting factors for phytoplankton growth in the Adriatic coastal system are light, temperature, and the strong influence of meteorological events. The NW Adriatic coastal area was subdivided into three zones, each characterised by different trophic conditions. In each zone, phytoplankton was similar in terms of community structure and was different only in abundance. Three main periods of growth were observed: February, April and July. As expected in a nutrient-enriched system, the community structure was dominated by diatoms (both micro- and nanoplankton fractions), over most of the year. The importance of dinoflagellates in the communities was generally low, with significant abundances present only in June–July, once the spring bloom of diatoms had left ‘nutrient-poor’ conditions in the water. These low concentrations of nutrients favoured the growth of dinoflagellates, which have lower nutritional requirements. Over the 10-year study, a decrease in orthophosphate concentrations was observed. This pattern could be explained as a consequence of reduced quantities of phosphorus used in detergents, in the late 1980s. No significant change in any hydrochemical or biological property was observed.

A first synoptic and trans-domain overview of plankton dynamics was conducted across the aquatic sites belonging to the Italian Long-Term Ecological Research Network (LTER-Italy). Based on published studies, checked and complemented with unpublished information, we investigated phytoplankton and zooplankton annual dynamics and long-term changes across domains: from the large subalpine lakes to mountain lakes and artificial lakes, from lagoons to marine coastal ecosystems. This study permitted identifying common and unique environmental drivers and ecological functional processes controlling seasonal and long-term temporal course. The most relevant patterns of plankton seasonal succession were revealed, showing that the driving factors were nutrient availability, stratification regime, and freshwater inflow. Phytoplankton and mesozooplankton displayed a wide interannual variability at most sites. Unidirectional or linear long-term trends were rarely detected but all sites were impacted across the years by at least one, but in many case several major stressor(s): nutrient inputs, meteo-climatic variability at the local and regional scale, and direct human activities at specific sites. Different climatic and anthropic forcings frequently co-occurred, whereby the responses of plankton communities were the result of this environmental complexity. Overall, the LTER investigations are providing an unparalleled framework of knowledge to evaluate changes in the aquatic pelagic systems and management options.