Atmospheric inputs of phosphorus and nitrogen to the southwest Mediterranean region: Biogeochemical responses of high mountain lakes

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 5 11.2) than in wet deposition (TN vs. TP slope 5 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. Atmospheric deposition is a significant input of gaseous (e.g., nitrogen) and lithosphere-derived (e.g., phosphorus) elements to aquatic and terrestrial ecosystems (Chadwick et al. 1999; Guerzoni et al. 1999). Nitrogen deposition in the Northern hemisphere has increased as the result of anthropogenic activity, and its effects on lake chemistry have been intensely studied (Sullivan et al. 1990; Stoddard et al. 1999). In contrast, atmospheric inputs of phosphorus derived from Saharan dust deposition and their effects on ecosystems have received attention only recently (Ridame and Guieu 2002; Okin et al. 2004). Every year, massive airborne plumes of dust from the Sahara Desert are exported to the Atlantic Ocean by the predominant westerly winds. In the Mediterranean region, maximum loads occur during spring and sum1 Corresponding author ([email protected]).