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The polychaete assemblages inhabiting the coastal waters of the Oualidia lagoon were studied during winter 2013 and summer 2013. Taxonomic composition and diversity were determined at 43 sample sites. Among the 13 species of polychaetes recorded, 10 were newly reported for Oualidia lagoon. Hediste diversicolor and Capitella sp. were the most abundant taxa in both seasons. Temperature and salinity were higher, and chl-a and OM were lower, in summer than in winter. The structure of the polychaete assemblages was characterized by forming three main clusters, either based on sampling stations or on polychaete species. These clusters were organized according to a downstream gradient, with the stations having fine sediments and a H. diversicolor assemblage in the inner lagoon being replaced by stations with medium grain-sized sediment and a Capitella sp. assemblage in the mid-lagoon, which were in turn replaced by stations having sandy sediments and assemblages dominated by Glycera alba (winter) and P. africana (summer) in the areas closer to the lagoon inlets. The shift was, in fact, from a classical, brackish, lacunar assemblage to two different, temporal aspects of a marine assemblage (close to the inlets), with a transition assemblage in between. This corresponded with a typically paralic spatial structure whose main descriptors responded to a confinement gradient. Despite the absence of a river, the organization of the polychaete assemblages closely resembled that of an estuarine system, with the tidal regime playing a major driving role.

The study of the Oualidia lagoon aims to establish the state of knowledge of the environment of this ecosystem after the construction of an inner dam in 2011 and the ecological functioning of the lagoon. For that, several campaigns were carried out between February 2011 and October 2012 and were the subject of a seasonal monitoring for the collection of the main biotic and abiotic parameters of the environment of this ecosystem. During the construction of the inner dam on 2011, a hydrological variability was marked in summer by low oxygen levels and maximum temperature variation between the two tides. Since the completion of the construction of the inner dam, there has been a significant increase in nutrients, mainly nitrogenous elements, and an increase in turbidity caused by an improvement in hydrodynamics, which has particularly increased of the sandpit modified the morphology and moved on south, creating a new main channel structure in 2011. This hydrodynamic was manifested on the upstream area by a decrease of the sedimentary organic European Scientific Journal June 2018 edition Vol.14, No.18 ISSN: 1857 -7881 (Print) e -ISSN 1857 94 matter due to the trapping of fine sediments in the inner dam. At the immediate level of the upstream area (Slikkes and Schorres), the hydrodynamic is relatively low, which has reduced the sedimentation rate on the upstream of the lagoon. The Oualidia lagoon is situated in a littoral zone influenced by the enrichment in deep cold waters loaded with nutritive salts, which invade it during the high tide, and would be, consequently after construction of the inner dam, protected from any confinement of the ecosystem.

The marine circulation of Dakhla bay (South of Morocco 23°30'N-16°W) have been studied from January to February 2010 by 2D hydrodynamic model. This study aims to understand the marine circulation of this bay and its hydrodynamic functioning. Mainly governed by M2 tide and during spring tides, tidal currents are very strong in intensity near the inlet of the bay and its surrounding (between 1 and 2 m.s -1 ). In its central part and the nothern part of the bay, their intensity are very less (< 1 m.s -1 ). During neap tides, tidal currents are less in intensity (< 0.6 m.s -1 ) in all the areas of the bay. Keywords: Dakhla -bay -South of Morocco-marine circulation-2D hydrodynamic model RésuméLa circulation marine de la baie de Dakhla (Sud du Maroc 23°30'N-16°O) a été appréhendée sur le mois de janvier à février 2010 par modélisation hydrodynamique bidimensionnelle (2D). Cette étude vise à mieux comprendre la circulation marine de la baie et son mode de fonctionnement hydrodynamique. Les marées, principalement la marée semi diurne de type M2, dominent les courants de marée au sein de la baie.

An oceanographic survey was carried out in the Nador lagoon located on the Moroccan Mediterranean coast, in July 2012. The objective is to study the hydrological and sedimentary state of the lagoon after the establishment of the new pass, in 2011. The distribution of the physicochemical parameters of the environment allowed the identification of a spatial variability in relation to the general circulation in the lagoon and the presence of the treatment station on the continental shore. The sedimentary state also shows the intensity of the sea-lagoon exchange which is elevated in the center compared to the NO and SE extremities. Thus, maintaining the environmental balance of this ecosystem depends on the degree of sea-lagoon exchange and the control of the impact of anthropogenic activities.