Spatial variability of atmospheric fluxes of metals (Al, Fe, Cd, Zn and Pb) and phosphorus over the whole Mediterranean from a one-year monitoring experiment: Biogeochemical implications

Abstract

During the ADIOS EU funded program, the total atmospheric deposition of metals (Al, Cd, Zn and Pb) and elements of biogeochemical interest (Fe, P) were collected monthly during one year (June 01/May 02) at nine coastal sampling sites around the Mediterranean Sea. In order to avoid bias in assessing the spatial and temporal variability, the sampling was performed using the same protocol and the same schedule, with a centralized location for preparation and samples analysis. The highest fluxes were found for Al and Fe (1004 and 665 kg km − 2 yr − 1 respectively), with these fluxes being two times higher in the western basin than in the eastern one, due to the occurrence of strong Saharan dust fall. The comparison of Al fluxes with previous data for the northwest Mediterranean over the past two decades underlines the very high variability, precluding any assessment of spatial trends from our study. The mean phosphorus flux was 33 kg km − 2 yr − 1 , with a possible slight decrease from the western/central basin to the eastern one. Zinc fluxes were slightly higher in the western basin than in the eastern one (12.1 and 7.7 kg km − 2 yr − 1 respectively). Lead fluxes were identical in both basins, at 1.46 kg km − 2 yr − 1 , but gradients within the basins indicated higher fluxes in the south of the western basin and in the north of the eastern one. A decreasing trend was observed over the past two decades for the northwestern basin, linked to reduction in the use of unleaded gasoline in the western European countries. For cadmium, the fluxes were of the same order of magnitude in both basins, with an average value of 0.037 kg km − 2 yr − 1 , and with no clearly-decreasing trend over the past 20 years. Using the element-to-Al ratio in Saharan dust, the non-crustal contribution to the total atmospheric deposition was on average 11, 70, 81, 88, and 96% for Fe, P, Pb, Zn and Cd respectively. Fe and Al were well correlated, with the Fe/Al ratio for western sites being typical of the Saharan end-member already defined in the western basin, but slightly higher in the eastern one, probably due to a variation in the composition of the source of the Saharan dust. Biogeochemical response to atmospheric inputs was considered at the scale of the whole stratification period and at the event scale. During the stratification period dissolved iron concentrations in the surface mixed layer could increase over the whole Mediterranean Sea by 1.8–5.3 nM in response to the accumulation of atmospheric iron from both anthropogenic and natural origins, confirming a strong impact of atmospheric inputs on the iron marine cycle in the Mediterranean Sea. Dissolved phosphate in surface waters was calculated to be significantly augmented by the atmospheric inputs during summer and we estimated that the atmospheric input of phosphate can sustain the new primary production during the stratification period at the scale of the whole open Mediterranean. At the event scale the response of the biota to a strong dust fall, which occurred during the Adios year in the Ligurian Sea, was registered by the water column biogeochemistry and chlorophyll content; this Saharan event did bring a considerable amount of dust and new nutrients, inducing a significant new production and a doubling of the chlorophyll concentration in the surface mixed layer. This event, accompanied by strong winds and heavy rains, also induced profound changes in the physical characteristics of the surface waters, triggering the beginning of the destratification of the surface waters that also induced an input of P from below.