Abstract
SUMMARY: The community composition and spatial distribution of benthic macroinvertebrates were studied along the Ebro estuary, a highly stratified estuary located in the NE Iberian Peninsula. During the last decade the oligotrophication process occurring in the lower Ebro River and its estuary has allowed a complex benthic macroinvertebrate community to become established; these results contrast with the poor community found there in the early nineties. A total of 214 taxa were identified, and polychaetes dominated the community both in abundance and species richness. The results showed spatial differences in the structure and composition of macroinvertebrates, which suggests that there are two distinct communities along the estuary. Each community was found in a specific stretch (upper and lower estuary) in function of the presence of the salt wedge. The macrobenthos of the upper estuary was dominated by freshwater taxa, but some euryhaline species were also found. The lower estuary showed a marine community typical of shallow Mediterranean environments. The transition between these two communities fits an ecotone model. The highest abundances, richness and diversities were recorded at the lower estuarine stations, especially those closer to the river mouth, whereas the lowest values corresponded to the stations adjacent to the tip of the salt wedge.
Highlights
The Ebro estuary (NE, Iberian Peninsula) is a salt wedge or highly stratified estuary (Hansen and Rattray, 1966; Ibáñez et al, 1997)
Fluctuating estuarine systems produce strong environmental gradients, which leads to a patchy distribution of organisms that must cope with a wide variety of stresses (Morrisey et al, 1992; Gray and Elliott, 2009) due to both natural and anthropogenic factors (McLusky, 1999; Dauer et al, 2000; Dauvin, 2007; Elliott and Quintino, 2007)
During the study period the salt wedge was only found in the lower estuary stations
Summary
The Ebro estuary (NE, Iberian Peninsula) is a salt wedge or highly stratified estuary (Hansen and Rattray, 1966; Ibáñez et al, 1997). The benthic invertebrate communities, often used as indicators of the health of an ecosystem, can be very similar in both impacted and non-disturbed estuarine systems This increases the difficulty of distinguishing natural from anthropogenic stresses. In Mediterranean regions and in the Iberian Peninsula, besides the spatial fluctuation there is strong temporal environmental variability in the aquatic systems due to limited water availability during part of the year (Caiola et al, 2001; Ferreira et al, 2007a) This variability is exacerbated by a long history of human-induced pressures that have led to serious changes in the natural ecological cycles of estuarine systems from this region (Ferreira et al, 2007b). This study establishes a robust basis so that macroinvertebrates can be used as indicators of the ecological status of the Ebro estuary
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