Effect of atmospheric nutrients on the autotrophic communities in a low nutrient, low chlorophyll system

Abstract

The effect of atmospheric inputs on phytoplanktonic dynamics was investigated in the Mediterranean Sea during the season characterized by a stratified water column, low primary productivity, and low concentrations of nutrients ([nitrate] , 50 nmol L 21 ; [phosphate] 5 20 nmol L 21 ; [silicate] 5 0.7 mmol L 21 ). We report here data obtained during microcosm enrichment experiments performed on the natural assemblage using different combinations of realistic additions (Saharan dust, Fe, Fe 1 phosphate, and anthropogenic particles). Saharan dust and Fe 1 phosphate treatments significantly stimulated primary production. Anthropogenic particles and Fe 1 phosphate treatments increased the chlorophyll a concentrations, enhancing mainly the small cells (pico- and nanophytoplankton). The autotrophic community structure was significantly altered; for example, Fe and Fe 1 phosphate additions benefited prokaryotic populations, indicating possible nitrogen fixation. The colimitation of both phosphate and Fe was removed by these additions. Results emphasized the effect of Fe, although the ambient concentration was close to 1 nmol L 21 . The addition of dust benefited eukaryotic populations, which indicates that the dust was a possible source of nitrogen. An abiotic dissolution experiment of macronutrients attached to dust confirmed this hypothesis. The dissolution of Fe attached to the dust (0.23‐0.61%) and to the anthropogenic particles (0.86‐1.85%) was consistent with previous studies conducted under abiotic conditions. This result suggests that the possible enhancement of the dissolution processes caused by biological activity might have been balanced by Fe consumption by the biota and its adsorption on both mineral and organic particles. Studying the key interactions between the ocean and the atmosphere is essential to understanding the present function of biogeochemical cycles in the ocean and to predict their evolution. Atmospheric deposition is now recognized as a significant source of external iron (Fe) and other nutrients for surface waters. Although the effect of Fe on productivity has been recognized in high-nutrient, low-chlorophyll (HNLC) regions (e.g., Martin et al. 1994), the ecological effects of atmospheric Fe and macronutrients in terms of species response and community structure in oligotrophic environments are poorly understood. Natural and anthropogenic changes in climate and global biogeochemistry can alter the atmospheric input of aerosols to the ocean. It is important to understand how these modifications cause changes in planktonic productivity and food web structure because they could result in altered carbon partitioning and biogenic air‐sea gas fluxes. The Mediterranean Sea is an oligotrophic quasi-enclosed basin receiving the highest rate of aeolian material deposition in the world (Guerzoni et al. 1999) in the form of strong pulses of mineral dust. In addition, it continuously receives 1