Context : Long-term quality checked and integrated datasets for the Belgian Part of the North Sea (BPNS) are essential to detect changes in this complex ecosystem and support policy related decisions.Objectives: - Compile and safeguard quality checked, intercalibrated and integrated datasets and make them publicly accessible for further research and policy purposes.- Improve or develop methods and protocols to assess and interpret environmental change in the BPNS and compare trends with neighbouring areas.- Provide support and advice for policy related decisions and legal measures, like MSFD and OSPAR.Conclusions: A scheme with data management tools has been worked out for efficient data flow throughout the project. The scheme, including the mandatory metadata fields and standardization, can be used as a guideline for future projects. Inventories of datasources, projects and data-originators were compiled and the final datasets are available via the central dataportal. Even after compiling and quality checking the long-term datasets, the scientists had to consider some limitations, like changing methodologies and low data resolution, and incorporate these into their trend analyses procedures. Some remarkable environmental changes over time were observed. Model results of the contaminants showed decreasing trends for heavy metals and PCBs. Zn concentrations, however, were found increasing in marine sediments while As concentrations were found increasing in groyne mussels. Nearby the port of Zeebrugge and the mouth of the Scheldt, PCB concentrations were found slightly increasing again over the last decade. The study on eutrophication showed that nutrient (N, P) riverine concentrations and loads have decreased continuously from the end of the 1980‟s to now. However, this did not result in a comparable decrease of marine nutrient concentrations. No clear long-term trend effect was observed on in situ chlorophyll a concentrations following the nutrient decrease. However, a clear change in chlorophyll a phenology followed the increase in sea surface temperature in the period. Furthermore, biomass, seasonality and structure of diatom and dinoflagellate communities were compared between the 1970s and 2000s for the Belgian Part of the North Sea (BPNS), derived from the newly established Belgian Phytoplankton Database. Distinct changes were observed: changes in diatom and dinoflagellate biomass and shift of fulcrum; changes in community structure, with a trend towards seasonal homogenization in the diatom community; increased occurrence of harmful diatom (Pseudo-nitzschia) and dinoflagellate (e.g. Prorocentrum) genera. The observed changes correlate well with overall increases in temperature and changes in nutrient loads and ratios.And finally, for acidification, pH data reveal an increasing trend from the mid-70‟s to the mid-80‟s and a decrease of pH from the mid-80‟s onwards that seems consistent with changes in primary production patterns. The comparison of CH4 concentrations obtained in 1990 and 2016, showed a decreasing trend consistent with alleviation of eutrophication in the area.Based on the outcome of this successful project, the partners defined various recommendations regarding future monitoring strategies for policy makers.