This work aims to study the impact of the installation of seawater desalination plant in Agadir bay by drawing up the initial health state of two marine ecosystems Tifnit-Douira and Cap Ghir receiving desalination plants. Thus, a multidisciplinary study was conducted in the sentinel species Mytilus galloprovincialis, combining two complementary approaches: i) the chemical approach (physico-chemistry and chemical detection of pollutants); and (ii) the biological approach (ecotoxicological study of multi-biomarker response and reproductive biology). An inventory of macro-phyto/zoo-benthic species associated with mussel beds was also carried out to assess the biodiversity of these ecosystems.
Our results related to the physicochemical approach in the two studied stations reveal values that oscillate between: 16.24 and 21.61 °C for seawater temperature; 7.39 and 8.73 for pH; 43.15 and 65.16 mS/cm for conductivity and between 27.40 and 43.75 PSU for salinity. TDS and dissolved oxygen values vary between 21.14 to 31.88 and 4.33 to 8.14 mg/l respectively. These parameters follow monthly fluctuations in the two studied ecosystems due to the marine environmental responses to changes in daily and weekly climatic conditions and also to seasonal hydrodynamic factors (currents, swell and upwellings).
The study of metal pollution in both ecosystems has shown that their concentrations undergo monthly, seasonal and annual fluctuations depending on the dosed element. Cd, Pb and Cu recorded relatively high levels (2.28, 2.50 and 6.86 mg/kg respectively) with comparable annual profiles between the two stations. While Arsenic (As) oscillates between 7.97 and 12.60 mg/kg without reaching the toxicity threshold of 14 mg/kg. The measured values are significantly high, especially at Cap Ghir with a stability of the values throughout the study period. This attests to the presence of Arsenic in a natural way in the Atlantic marine ecosystem. The results obtained for the major metal elements studied showed maximums of 6.33, 145.51 and 285.74 mg/kg respectively for Mn, Fe and Zn. The revealed annual patterns appear similar between the two ecosystems with moderate seasonal fluctuations.
Biomarker response measures, Acetylcholinesterase (AChE), Glutathione-S-Transferase (GST), Catalase (CAT) and Malondialdehyde (MDA) in Mytilus galloprovincialis, have been shown to be present in measurable and inducible amounts. In addition, response levels fluctuate respectively between 1.94 to 8.85; 3.74 to 36.91; 3.52 to 17.94 and 1.13 to 5.91 nmol/mg protein. This is explained by the response of these molluscs, to variations in environmental conditions as well as to the presence of certain contaminants including heavy metals mainly Cadmium, and consequently to the physiological disturbances of the species during its development cycle.
The study of reproductive cycle in the mussel Mytilus galloprovincialis, testifies to a continuous sexual activity throughout the year with periods of collective egg-laying coinciding with spring and summer. The number of these collective gametic release varies between two to three periods depending on the environmental conditions, especially variations in seawater temperature. This results in a lack of collective sexual rest period in these mussel populations. The sex ratio study shows a balance between males and females of 1.14:1 in Tifnit-Douira, and it varies between 1.12:1 to 1.18:1 in Cap Ghir. The histological study allowed the detection of a single case of hermaphroditism, revealed for the first time in mussel populations in the Agadir bay.
The values of the condition index are high (>60) in Mytilus galloprovincialis of the studied stations during all seasons even during laying periods. The favorable conditions of the environment allow a continuous allometric and weight growth throughout the year. Regarding biological diversity, both stations have a very high diversity of Macro-Phyto/Zoo-benthic species. Indeed, the animal kingdom is rather dominated by Crustaceans and Gastropods, while Chromophytes and Rhodophytes are the most dominant in the plant kingdom.
All these results prove the well-balanced state of the environment in the two studied ecosystems Tifnit-Douira and Cap Ghir; and that permanent monitoring should be realized to protect their health state from the impact of desalination plants.
Keywords: Mytilus galloprovincialis, Agadir Bay, Cap Ghir, Tifnit-Douira, Desalination, Biomarkers, Marine ecosystem, Heavy metals, Reproductive cycle, Biodiversity, Biomonitoring.