Phosphorus in Rainwater: Partitioning Inputs and Impact on the Surface Coastal Ocean

Phosphate concentrations in rainwater were measured at a Ligurian coastal sampling site (Cap Ferrat, France) from February 1997 to February 1998 to study the impact of wet atmospheric phosphorus (P) input on the surface ocean. Soluble and particulate fractions were differentiated to evaluate the atmospheric supply of bioavailable P. Complexed and reactive phases within the dissolved fraction were also separated. Preliminary results showed a high temporal variability in total concentration (0.05-4.3 μmol liter-1). The factors controlling the partitioning between reactive and complexed components are not clear However, the partitioning between dissolved and particulate fractions is linked to emission sources. Soil-derived dust from the Sahara was identified as an important source of atmospheric P, mainly insoluble. Conversely, anthropogenic emissions are sources of soluble P (i.e., basically bioavailable). A significant part of these emissions could originate from incinerators and/or biomass burning. The different wet fluxes are calculated to total 165 μmol m-2yr-1, and the dissolved and particulate inputs are 95 and 70 μmol m-2yr-1, respectively. Taking into account the respective solubility of such inputs, anthropogenic emissions appear to be responsible for relatively high amounts of bioavailable P. Even if the atmosphere is globally a minor source of nutrients (compared with riverine inputs and marine vertical mixing), it might be the only source of P in oligotrophic conditions. For example, assuming that P is a limiting factor in the Mediterranean Sea, the rain event of 19 June 1997 (17 μmol m-2of bioavailable P) potentially induced a new production of 0.02 g C m-2, which is a significant value in such conditions. Converted into biomass and integrated over a 5-m-thick water layer, such an atmospheric input represents 0.35-0.45 mg chlorophyll a m-2, an appreciable portion of the total biomass during this period. This observation underlines the major role of the atmosphere during oligotrophic periods in the western Mediterranean.