las comunidades de peces del río guadiamar y el accidente minero ...

Las comunidades de peces del río Guadiamar
y el accidente minero de Aznalcóllar
Discussion
Guadiamar River fish assemblages at the different sampling sites evolved in
different ways throughout the 13 years following the spill. Several barriers hampered
recolonization from tributaries; however, this process was carried out and is still
underway.
The fish assemblage after the spill event almost disappeared, and was therefore,
highly dissimilar to that found in the previously undisturbed Guadiamar River main
stem (Doadrio, 1996 and 2001). Different trajectories at each sampling site reduced or
increased this similarity through time, but no clear patterns were found. Available prespill
information was limited, so only species presence could be used as reference data.
Species richness commonly ranged from 1 to 3 every year for both native and exotic
species. Therefore, species presence was a weak changing variable for pre- and postspill
similarity comparisons. Although fish diversity allowed us to draw a general
outline of assemblage dynamics, this approach did not help us identify the dynamics of
fish assemblage structure through time. The PRC and NMDS analyses, based on
abundance, offered both overall and specific approaches to explain the observed
patterns. First, the early spill effect and subsequent cleaning works, especially the
withdrawal of vast amounts of soil in the summer of 1999 that cut and dried several
main stem reaches (Arenas et al., 2008), impeded fish establishment in the affected
reach (E2-E5) until 2000 - Fernández and Drake (2008) caught some individuals before
but were scarce and probably reduced during these soil movements (personal
observation) -. From that year, fish assemblage structure in affected sites tended towards
that of the reference site (E1), where native species were dominant and exotics were
scarce (Table 1, Fig. 6 and 7). The increase in assemblage similarity was especially
relevant in E2, which was the nearest sampling site to the reference. Thus, between
2002 and 2004 (four-six years after the spill), fish assemblage structure in affected sites
stabilized, with slight increases or decreases in similarity, depending on the sampling
year, to that of the reference site (Fig. 5). These first signs of recovery were
considerably belated compared with those of other studies where a defaunated river
stretch, experimentally or by accident, was considered. In Illinois (USA), a fish
assemblage took between 3 and 10 days to reach background levels in a river without
barriers and drained by many tributaries (Peterson and Bayley, 1993). In another case,
in South Carolina (USA), several downstream pools recovered in 1 month, while
upstream sites took longer to recover (Sheldon and Meffe, 1995). Several studies both
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