Abstract 2017-410 Mangroves are among the most sensitive marine ecosystems to oil pollution due both to the sensitivity of mangroves species and to the high persistence of hydrocarbons in these environments. Despite their ecological and socio-economic value, the potential effects of an oil spill on French Guiana mangroves remain so far unknown. Yet, there is an increasing transboundary risk of oil spill due to Brazilian offshore oil exploitation (in mid-April 2013, there were 122 offshore drilling rigs in Brazil, including 29 under construction – Brazilian Amapá region located in the French Guiana border area is thought to become an important world oil production area in the coming years). The aim of the PRISME project was to assess the natural degradation of oil in mangrove sediment as well as its impact on benthic communities (micro, meio and macrobenthos): a one-month in situ experiment was conducted in the young French Guianese mangrove (around 3 years old) at the mouth of the Sinnamary estuary. The experimental units consisted in eight plastic cores (Ø : 10 cm ; height: 30 cm) manually introduced within sediments. A thin layer of oiled sediment (2 cm, 20 000 ppm) was applied on four cores while the remaining four cores were considered as control (no oil addition). Three cores were additionally sampled at the beginning of the experiment as initial control sediments. After one month in situ, the eight cores were collected and sliced on site into different sedimentary layers aliquots for later analyses (hydrocarbons, bacterial, meio, macrofauna fauna diversity, bioturbation, biogeochemical parameters). Samples were sent to the different laboratories involved in this multidisciplinary project. Results and knowledge gained from this experimental work were used to develop an approach for assessing coastal vulnerability for oil spills preparedness in mangroves.