Longo, R.S., Picado, A., Vaz, N. and Dias, J.M., 2016. Biological response of a coastal plain estuary to torrential episodes: a modelling study. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 78–82. Coconut Creek (Florida), ISSN 0749-0208.Estuaries are highly dynamic systems with an important impact on biogeochemical cycles and primary production, which may be affected and modified in a climate change context, namely due to extreme rainfall events. This study aims to research chlorophyll-a (Chl-a) and nutrients dynamics in the Tagus estuary under extreme freshwater discharge in a climate change context, using a 2D biophysical model. Three scenarios were set changing the inputs from the main tributaries – Tagus and Sorraia rivers. First, a scenario with one day of extreme discharge for both rivers was considered. Next, and in order to understand the importance of each river, two more scenarios were set considering the extreme discharges separately. Results show that Chl-a concentrations follow the same trend as the imposed discharges, however with a delay of one day. The results also reveal that the biogeochemical characteristics of the Tagus estuary are mainly influenced by the Tagus River inflow. Moreover, in the scenario where the extreme discharges are imposed for both rivers, Chl-a levels increase in the entire estuary and consequently a decrease in nitrate concentrations is observed. Otherwise, phosphate concentrations slightly increase. This suggests primary producers inside the estuary preferentially consumes nitrate, at a higher rate than it is being loaded.