A 10 YEAR-MONITORING PROGRAMME (1997-2006) OF A HEAVILY POLLUTED ESTUARY: GEOCHEMICAL AND MICROFAUNAL PROXIES IN THE BILBAO ESTUARY, N. SPAIN

The Bilbao estuary (Northern Spain) as many other estuaries worldwide, has been dramatically modified by urban, industrial and port development. During the last 150 years, the Bilbao estuary has received wastes from many sources that have significantly degraded the environmental quality of this coastal area. Nowadays, estuarine restoration is being undertaken as part of a Revitalization Strategic Plan, which includes preventive measures and remedial action involving dredging of heavily contaminated surficial sediments. Cearreta et al. (Mar. Poll. Bull. 44, 2002, 487-503; Est. Coast. & Shelf Sci. 50, 2000, 571-592) analyzed the record of recent environmental change preserved in the sediment from intertidal mudflats. They found a mantel of sediments with extreme concentrations of metals and barren of indigenous foraminifera, corresponding to the latest industrial period (1950-2000). They showed that heavy metal concentrations were highly dependent on proximity to sources of pollutants while living foraminifera were very scarce or absent from surface sediments in the estuary. This study follows up the superficial evolution of the estuary from 1997 to 2006. Surface sediments consist originally of black sulphidic sandy muds, except in the sampling point located in the open bay area which was brown muddy sand. During the year 2003, a decrease in heavy metal concentration up to 50 % was measured and suggested the possibility of a satisfactory gradual recovery. However, during the initial 7 years of sampling only a reduced number of living individuals has been detected , even thought the species found alive are considered resistant to several polluting agents (i.e.: Alve, 1995). By 2006 the surficial sediments are brownish sandy mud, and present colonization of crabs and oysters in the middle and lower reaches of the estuary. This is probably due to the improvement of the estuarine water quality during this period. However, no significant reduction of heavy metals concentration or increase in microfaunal numbers of species and individuals have been observed discarding any further recovery. Thus, local authorities should consider also the clean up of the pollutants stored in the sediments, not seeking only the achievement of the water quality standards, as these pollutants can act as source of contamination.