evidence: Land Use Changes and Ecosystem Services Supply in Transboundary River Basins. The Case of the Maroni River (French Guiana/Suriname) The Maroni River in the Guiana Shield marks the border between French Guiana and Suriname. In the Maroni River Basin, as in many transboundary river basins of the world, socio-political borders criss-cross through natural ecosystems and hence intersect through ecological boundaries. As neighbouring countries share ecosystems, they also share inherent ecosystem processes, functions and, hence, the ecosystem services (ES) that these areas provide. However, life and land uses are changing in the river basin. A growing need for land, urbanization, a decrease of shifting cultivation patterns towards smallholder agriculture and a run for gold put pressure on natural ecosystems.Our work on ES in this area was twofold: under the umbrella of the ECOSEO Project, we conducted a first ecosystem services assessment for the river basin, extracted from two national ES assessments on the base of Land Use Land Cover data. In participatory expert workshops in Paramaribo, Suriname and Cayenne, French Guiana, stakeholders from the territory assessed the capacity of abundant ecosystem types to supply 21 different ecosystem services using a matrix-based approach. [...] Especially regulating ES in the Upper Maroni River were affected, with consequences on freshwater quality, supply of wild foods and fish and transportation for the entire lower River Basin.As LULC changes, especially gold mining-related activities, on both sides of the river have a severe degrading effect on ecosystem condition and related ES supply, cross- or transboundary conservation efforts are needed to safeguard ecosystems and their services for the population on both sides of the Maroni River.