Recovery of an Estuarine Ecosystem after the Stopping of Wastewater Discharge: Intertidal Macrobenthic Community Characterization in the Estuary of Oued Souss (Southwestern Morocco)

The communities of benthic macroinvertebrates, living in the estuary of Oued Souss (Agadir Bay, Morocco), were studied in parallel with the changes that this ecosystem had underwent after the stopping of pollution, caused by untreated wastewater discharge which occurred in November 2002. Three campaigns were undertaken during the summer season. Two of them were carried out while the estuary was receiving wastewater discharge in 2001 and 2002. The 2003 campaign was carried out after the cessation of the pollution. The intertidal macrobenthic fauna shows a similar faunal composition for both seasons during the period when the ecosystem was receiving wastewater discharge. During this period, Hydrobia ulvae was the dominant species, followed by Hediste diversicolor and Scrobicularia plana in decreasing order. In 2003, the number of individuals harvested was significantly larger compared to the period when the ecosystem received wastewater. For the dominance of species, H. diversicolor was dominanted, followed by H. ulvae, Cerastoderma edule, and S. plana in decreasing order. However, in both periods: before and after the end of wastewater discharge, the phylum of mollusks is dominant, followed by that of Annelids and Crustaceans. The study of the coenotic affinity between settlements of different radials allowed us to separate 2–3 groups of radials in 2001 and 2002, respectively, and 3 groups after stopping discharges in the estuary. The specific richness was greater in the year following the end of discharges (22 species in 2003 instead of 14 found during the pollution period: 2001–2002). The longitudinal distribution of the species living in this site in 2001 and 2002 became wider in 2003 and average biomass, determined by the study of the ash-free dry weight, became clearly greater (20.46 g/m² in 2003; and 15.54 g/ m² or 15.84 g/ m² in 2001 or 2002 respectively). This investigation allowed us to acquire a qualitative (species list, species richness) and quantitative (abundance, biomass, species density) database of intertidal benthic macrofauna of the Oued Souss estuary. It represents a reference state. It responds to the requests for information about the quality of the environment immediately after the cessation of wastewater discharge and could serve as a basis for conducting impact studies later.