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.

Intense Saharan dust deposition occurs over large oligotrophic areas in the Mediterranean Sea and in the Tropical Atlantic, and its impact on the biogeochemical functioning of such oligotrophic ecosystems needs to be understood. However, due to the logistical difficulties of investigating in situ natural dust events, and due to the inherent limitations of microcosm laboratory experiments, new experimental approaches need to be developed. In this paper, we present a new experimental setup based on large, clean mesocoms deployed in the frame of the DUNE (a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem) project. We demonstrate that these tools are highly relevant and provide a powerful new strategy to in situ studies of the response of an oligotrophic ecosystem to chemical forcing by atmospheric deposition of African dust. First, we describe how to cope with the large amount of dust aerosol needed to conduct the seeding experiments by producing an analogue from soil collected in a source area and by performing subsequent appropriate physico-chemical treatments in the laboratory, including an eventual processing by simulated cloud water. The comparison of the physico-chemical characteristics of produced dust analogues with the literature confirms that our experimental simulations are representative of dust, aging during atmospheric transport, and subsequent deposition to the Mediterranean. Second, we demonstrate the feasibility in coastal areas to installing, in situ, a series of large (6 × 52 m³) mesocosms without perturbing the local ecosystem. The setup, containing no metallic parts and with the least possible induced perturbation during the sampling sequence, provides an approach for working with the required conditions for biogeochemical studies in oligotrophic environments, where nutrient and micronutrients are at nano- or subnano-molar levels. Two, distinct "seeding experiments" were conducted by deploying three mesocosms serving as controls (CONTROLS-Meso = no addition) and three mesocosms seeded with the same amount of Saharan dust (DUST-Meso = 10 g m⁻² of sprayed dust). A large panel of biogeochemical parameters was measured at 0.1 m, at 5 m and 10 m in all of the mesocosms and at a selected site outside the mesocosms before seeding and at regular intervals afterward. Statistical analyses of the results show that data from three mesocosms that received the same treatment are highly reproducible (variability < 30%) and that there is no significant difference between data obtained from CONTROLS-Meso and data obtained outside the mesocosms. <br><br> This paper demonstrates that the methodology developed in the DUNE project is suitable to quantifying and parameterizing the impact of atmospheric chemical forcing in a low-nutrient, low-chlorophyll (LNLC) ecosystem. Such large mesocosms can be considered as 1-D ecosystems so that the parameterization obtained from these experiments can be integrated into ecosystem models