Behaviour of arsenic on the continental shelf off the Gironde estuary: Role of phytoplankton in vertical fluxes during spring bloom conditions

Abstract

The arsenic (As) cycle in the marine environment is known to be sensitive to biological activity. Within the scope of the National Coastal Oceanography Program, we undertook a specific study of the behaviour of this element in a water column on the continental shelf of the Bay of Biscay off the Gironde estuary during two oceanographic cruises conducted in May 1994 and May 1995. Various chemical forms were measured: arsenate (As3), arsenate (As5), monomethylarsenic (MMA), dimethylarsenic (DMA) and dissolved total As after ultraviolet mineralization. The net flux of total As from the Gironde estuary was evaluated as well as its effect on the concentration in surface marine waters. The vertical profiles in the coastal zone during a period of active primary production confirmed a certain analogy between arsenates and phosphates as well as the formation of As3 and DMA. The importance of organoarsenic species not directly accessible to the formation of volatile hydrides is demonstrated. Their presence modifies assessments, reducing the deficit of dissolved total As in the euphotic layer to just 5%. For particles, the As content in phytoplankton was estimated at 6 μg·g −1 compared to 20 to 30 μg·g −1 in iron- and aluminium-rich terrigenous particles. These results do not invalidate the role of phytoplankton in the speciation of dissolved As but indicate that their involvement in vertical transfers was apparently not predominant in the zone under study.