El papel de la deposición atmosférica en la biogeoquímica de laguna de alta montaña (Sierra Nevada, España)

Abstract

Atmosphere is an important route through which many elements are transported long distances from their sources. Atmospheric particles, known as aerosols, have multiple origins, both natural and anthropogenic. Natural sources, such as sea salt and dust, are responsible for almost 90% of global aerosol emissions (SCHLESINGER, 1997). Arid and semiarid regions are the main sources of soil dust to the atmosphere and, among them, the Sahara desert is the world’s major source of soil dust, with an annual production of 400-700·106 tons per year, almost 50% of the global dust production (SCHÜTZ ET AL. 1981; D’ALMEIDA, 1986; SWAP ET AL. 1996). Every year, large amounts of desert dust are exported from the Sahara to the North Atlantic Ocean and the Mediterranean region. The transport towards Atlantic Ocean is linked to the general atmospheric circulation that has predominant wind directions from west and northwest. By contrast, northward transport of dust over the Mediterranean is influenced by the presence of cyclones (MOULIN ET AL. 1997). The seasonal pattern of African dust transport over the Mediterranean shows a maximum dust load in the central and eastern basins during spring, and in the central and western basins during summer (MOULIN ET AL. 1997). The Iberian Peninsula, located in the western basin, receives African dust mainly during late spring and summer under particular meteorological conditions (RODRIGUEZ ET AL. 2001). Atmospheric deposition can play a major role in biogeochemical cycling in both terrestrial and aquatic ecosystems. Inputs of elements from the atmosphere have sustained the productivity of Hawaiian rainforests during millions of years (CHADWICK ET AL. 1999), and atmospheric deposition can account for 34% of the calcium required for Quercus ilex forest growth in northeastern Spain (AVILA ET AL. 1997). On the other hand, there are growing evidences that atmospheric inputs have an impact on marine chemical cycling and on biological processes in the sea (DUCE ET AL. 1991). Atmospheric deposition is the major input of iron to the open ocean (DUCE & TINDALE, 1991) and atmospheric input of phosphorus can represent up to 14% of total primary production demands in the open western Mediterranean Sea (RIDAME & GUIEU, 2002). The role of atmospheric deposition in the biogeochemistry of freshwater ecosystems has been less explored relative to marine ecosystems and most of the effort has been focused on the effects of acid rain (SULLIVAN ET AL. 1990; KOPAČEK ET AL. 1995; TAIT & THALER, 2000). The contribution of atmospheric inputs of nutrients and Elvira Pulido Manuel de Villena. TESIS DOCTORAL 4 oligoelements to biological processes and chemical properties of lakes has been scantly assessed (PETERS, 1977; GIBSON ET AL. 1995). There are around 50 lakes in the Sierra Nevada mountains (SE Spain) located at elevations of 2800-3100 m above sea level. They are located above tree line in a remote area far from human activity. This location along with their oligotrophic nature make these lakes ideal sites to assess the impact of dust deposition on freshwater ecosystems. In this study, two lakes with contrasting catchments located in the Sierra Nevada mountains were selected. La Caldera Lake is located on rocky terrain, has no inlets and, hence, does not receive any inputs from stream runoff. In contrast, Río Seco Lake is located in a catchment covered by alpine meadows and has temporary inlets that drain water from the catchment. The main goal of this PhD thesis is to assess the influence of atmospheric deposition on the biogeochemistry of these two high mountain lakes. This question is addressed in four different chapters that represent the main topics of this study.