The aquatic systems of Doñana (SW Spain): watersheds and frontiers

Doñana includes an extraordinary variety of aquatic systems. They are broadly classified according to their location (on either aeolian sands or marshland) as their hydrology largely depends on the geomorphology of their basins. Their chemical composition is mainly influenced by rainfall, evaporative concentration, groundwater discharge, biogeochemical interac- tions at the sediment-water interface, and the quality composition of their watersheds. The influence of the watershed can be studied at different scales of observation. Rainfall infiltration in sandy soils is usually high so surface runoff becomes a rare event of very short duration during floods which, nonetheless, exerts a huge influence on the limnology of temporary ponds on aeolian sands. The water quality of the Doñana marshland, on the contrary, is influenced by long-term processes taking place on large-scale areas: sediment deposition, eutrophication and heavy metal pollution. The review of the main literature on the limnology of the Doñana aquatic systems during the past two decades, enable us to make a comparison in time focusing on the interactions at the frontier between terrestrial and aquatic systems within watersheds. Presently, the eastern area of the Doñana marshland is particularly affected by the low quality of the incoming flowing water compared with the more isolated southern marshes within the National Park. Water from the lower strech of the Guadiamar River ("Entremuros"), that floods the marshes of "Lucio El Cangrejo Grande", showed a significant correlation between inorga- nic suspended solids and total P (r=0.807, p<0.05) during 2003-04, indicating an important contribution of inorganic parti- culates to the eutrophication of this area. The northern streams of the "Arroyo del Partido" watershed have not significantly improved their water quality in the last two decades despite the construction of two waste-water treatment plants, being total P correlated to dissolved phosphate concentration (r=0.995, p<0.01) during 2003-05. A general increase in NO3- concentra- tions have been detected in all studied aquatic systems of the Doñana marshland, including those with the highest water quality ("Arroyo de la Rocina") during the last two decades. Despite wetland management requires a watershed approach, successive hydrologic projects in Doñana have failed to address the great spatio-temporal variability of processes affecting water quality in this area.