Intertidal macrofauna and environmental stress at a riverine-marine boundary

Benthic macrofaunal dynamics and environmental stress across a salt wedge Mediterranean estuary

The spatial distribution of benthic macroinvertebrate community in relation to environmental factors was studied along the Ebro Estuary (NE Iberian Peninsula), a salt wedge Mediterranean estuary. Both ordination methods and generalized additive models were performed to identify the different benthic assemblages and their relationship to abiotic factors. Our results showed a strong relationship between macrofaunal assemblages and the predominant environmental gradients (e.g. salinity); thus revealing spatial differences in their structure and composition. Two different stretches were identified, namely the upper (UE) and the lower Ebro Estuary (LE). UE showed riverine characteristics and hence was colonized by a freshwater community; whereas LE was influenced by marine intrusion and sustained a complex marine-origin community. However, within each stretch, water and sediment characteristics played an important role in explaining species composition differences among sampling stations. Moreover, outcomes suggested a total species replacement pattern, instead of the nestedness pattern usually associated with well-mixed temperate estuaries. The sharp species turnover together with the estuarine stratification point out that the Ebro Estuary is working, in terms of ecological boundaries, under an ecotone model. Finally, despite obvious differences with well mixed estuaries (i.e. lack of tidal influence, stratification and species turnover), the Ebro Estuary shares important ecological attributes with well-mixed temperate estuaries.

Macrobenthic assemblages, sediment characteristics and heavy metal concentrations in soft-bottom Ebre Delta bays (NW Mediterranean)

The spatial distribution of soft-bottom macrofaunal assemblages and their relationship with sediment characteristics were examined in two semi-enclosed shallow-water bays (Alfacs Bay and Fangar Bay) in the Ebre Delta (Catalonia, NW Mediterranean) during 2006 and 2011. Three assemblages were identified: group 1, present in sediments that contain a high percentage of silt-clay, small median particle size and high organic matter content and located at the most external areas of both bays and in the central basin of Alfacs Bay; group 2, present in the shallow northern platform in Alfacs Bay and in both platforms in Fangar Bay, the locations which feature highly variable sediment characteristics; and group 3, present in the shallow areas of both bays, which are characterised by low silt-clay and organic matter content. In Alfacs Bay, the differentiation of these assemblages has persisted since 1987, independently of the increase in organic matter content and heavy metal concentrations and of the shift in macrofaunal composition that have occurred over the past 20 years, most likely the result of increased agricultural activity.