In this research study, NANO members from five Southeast Asia countries (Thailand, Vietnam, Philippines, Indonesia and Malaysia) investigated eutrophication enhanced by nutrient load and modulated by residence time. As a result of unsustainable human activities and coastal development, water quality is degrading which results in hypoxia, algal blooms and pollution of the coastal environments. Long-term effects of eutrophication are a grave concern, and collecting data on water quality is essential to improve the sustainability of the coastal environment and provide information for decision makers. The study included fieldwork, lab measurements and application of the Land-Ocean Interactions in the Coastal Zone (LOICZ) approach and/or models such as Delft 3D to estimate the eutrophication status using the Unscaled Trophic Index (UNTRIX) and residence time. In addition, remote sensing data were applied to quantify chlorophyll-a from satellite information where relevant. This provided useful insights that can form the basis to inform stakeholders and give suggestions on how to manage water quality on a local and regional level. Our project also enhanced networking between NANO SEA members and other collaborators.