With the advent of DNA-based molecular technologies, microbial ecologists now have the tools to test whether general ecological patterns apply to microorganisms. In this study, we selected 11 high-mountain lakes from Sierra Nevada (Spain) to test the predictions of island-biogeography theory in relation to ecosystem size and isolation, and to assess the influence of other factors (i.e., ecosystem productivity, resource richness, and biotic interactions) on bacterial community structure. Bacterial operational taxonomic units (OTUs), generated by denaturing-gradient gel electrophoresis of polymerase chain-reactionamplified 16S rRNA genes, were used as a surrogate of predominant ''biodiversity units.'' OTU composition among lakes was heterogeneous, and the number of site-specific OTUs was near 50%. Lake remoteness did not affect the number of bacterial OTUs although the spatial distribution of the lakes significantly influenced bacterial composition. Lakes that were closer together had more similar bacterial fingerprints. We found a consistent positive association between bacterial OTU richness and lake area. The slope of this relationship (0.161 Ϯ 0.026, including literature data) was similar to slopes obtained for organisms with high dispersion rates.

We quantified dry and wet deposition of dust, nitrogen, and phosphorus over the southwest Mediterranean region (Sierra Nevada, Spain) and assessed its effects on the nutrient status and the chlorophyll a (Chl a) concentration in two high mountain lakes. Atmospheric deposition of particulate matter (PM) and total phosphorus (TP) were mainly associated with dryfall and showed a seasonal pattern similar to that reported for Saharan dust export toward the Mediterranean region, with maxima during spring and summer. In contrast, total nitrogen (TN) deposition was related to rainfall and did not follow the pattern observed for PM and TP. The molar TN : TP ratio was significantly lower (i.e., phosphorus-enriched) in dry deposition (TN vs. TP slope ϭ 11.2) than in wet deposition (TN vs. TP slope ϭ 95.5). In the study lakes, the molar TN : TP ratios and the Chl a concentrations were significantly influenced by the molar TN : TP ratio and the TP content of atmospheric deposition, respectively. Lake responses were more pronounced in the more phosphorous-limited system. These results establish a direct connection between atmospheric deposition and lake nutrient status and Chl a, making evident that in the Mediterranean region these inputs are an important source of phosphorous affecting biogeochemistry of oligotrophic systems.

Remote lakes are usually unaffected by direct human influence, yet they receive inputs of atmospheric pollutants, dust, and other aerosols, both inorganic and organic. In remote, alpine lakes, these atmospheric inputs may influence the pool of dissolved organic matter, a critical constituent for the biogeochemical functioning of aquatic ecosystems. Here, to assess this influence, we evaluate factors related to aerosol deposition, climate, catchment properties, and microbial constituents in a global dataset of 86 alpine and polar lakes. We show significant latitudinal trends in dissolved organic matter quantity and quality, and uncover new evidence that this geographic pattern is influenced by dust deposition, flux of incident ultraviolet radiation, and bacterial processing. Our results suggest that changes in land use and climate that result in increasing dust flux, ultraviolet radiation, and air temperature may act to shift the optical quality of dissolved organic matter in clear, alpine lakes.

We assessed the effects of Saharan dust inputs of particulate matter (PM), total phosphorus (TP), total nitrogen, and water soluble organic carbon (WSOC) on bacterial abundance (BA) in two alpine lakes and two reservoirs in the Mediterranean region. We also experimentally assessed the effects of dust inputs on bacterial activity and community composition and explored the presence of airborne bacteria. We found synchronous BA dynamics at least in one of the study years for each corresponding pair of ecosystems, suggesting an external control. The link between BA dynamics and inputs of PM, WSOC, or TP occurred only in those ecosystems with severe P-limitation and low dissolved organic carbon. The response was most intense in the most P-limited ecosystem. Dust addition had a significant positive effect on bacterial growth and abundance, but not on richness, diversity, or composition of the indigenous bacterial assemblages. We also obtained experimental evidence that some airborne bacteria could develop in oligotrophic waters by observing the growth of gamma-proteobacteria, a group poorly represented in natural aquatic environments.

Abstract. Previous studies have provided some insight into the Saharan dust deposition at a few specific locations from observations over long time periods or intensive field campaigns. However, no assessment of the dust deposition temporal variability in connection with its regional spatial distribution has been achieved so far from network observations over more than 1 year. To investigate dust deposition dynamics at the regional scale, five automatic deposition collectors named CARAGA (Collecteur Automatique de Retombées Atmosphériques insolubles à Grande Autonomie in French) have been deployed in the western Mediterranean region during 1 to 3 years depending on the station. The sites include, from south to north, Lampedusa, Majorca, Corsica, by satellites, more than 80 % of the major dust deposition events are recorded at only one station, suggesting that the dust provenance, transport and deposition processes (i.e. wet vs. dry) of dust are different and specific for the different deposition sites in the Mediterranean studied area. The results tend to indicate that wet deposition is the main form of deposition for mineral dust in the western Mediterranean basin, but the contribution of dry deposition (in the sense that no precipitation was detected at the surface) is far from being negligible, and contributes 10 to 46 % to the major dust deposition events, depending on the sampling site.