Over one year study (12 campaigns), particulate arsenic and dissolved inorganic arsenic (total inorganic Asi and arsenite As(III)) levels were determined along the whole salinity gradient of the Penzé estuary (12 stations) in Brittany (France). Concentrations, in the range 2-26 nM and 3-56 μg g− 1 for dissolved inorganic and particulate arsenic, respectively, were relatively high in comparison to pristine systems and were attributed to pig and poultry farming activities taking place in the catchment area. Our results show that variations of dissolved inorganic and particulate arsenic up to mid-estuary were significantly linked to fluvial inputs, these latter being controlled by water discharge and precipitations. Arsenite variations over a large salinity range (S > 4) were controlled by those of the marine end-member mainly through bacterial processes. Removal of dissolved species within the water column by adsorption on oxide material was found to be minimal while arsenic fluxes from the sediment control arsenate and arsenite levels in the upper part of the estuary.