Application de certains indices diatomiques à un cours d'eau marocain : Oued Hassar

Les eaux de l'Oued Hassar (Maroc) sont caractérisées par une forte minéralisation. La conductivité, variant entre 3470 et 7210 µS.cm-1, est due essentiellement aux fortes teneurs en chlorures. L'examen de l'Indice de Pollution Organique (IPO) montre que l'effluent d'eaux usées provenant de la ville de Mediouna constitue la principale source de pollution des eaux de l'Oued Hassar.Concernant l'étude des diatomées épilithiques, 61 espèces et variétés de diatomées ont été inventoriées. Ces taxons sont liés particulièrement à la salinité (Pleurosira laevis, Fragilaria pulchella,…) et à la charge organique (Nitzschia palea, Gomphonema parvulum,…).Quatre indices diatomiques ont été testés pour l'évaluation de la qualité des eaux de l'Oued Hassar : l'Indice Diatomique Pratique (IDP), l'Indice Diatomique Générique (IDG), l'Indice de Sládecek (SLA) et l'Indice Diatomique Trophique (IDT). La variation spatio-temporelle de ces indices reflète bien la dégradation de la qualité de ce cours d'eau, particulièrement en aval du rejet de la ville de Mediouna (IDP=3,86 ± 1,84 ; IDG=6,05 ± 1,95 ; SLA=4,35 ± 1,93 ; IDT=5,80 ± 1,31). Parmi ces indices testés, seul l'IDP présente une corrélation significative (0,63 ; α<5%) avec l'IPO (indice de référence) tandis que l'IDG, le SLA et l'IDT montrent des corrélations non significatives avec cet indice (0,38 ; 0,27 et -0,11 respectivement).The water quality in Moroccan rivers and streams becomes more and more injured because of the important amount of pollutants discharged in these aquatic ecosystems without any treatment in the most cases. A physical, chemical and microbiological analyses are regularly done by public institutions (office national des eaux potables, direction de l'hydrolique). However, these analyses become insufficient facing the big diversity of chemicals and the intermittence of pollution sources at the time. So, methods using a biotic indicators as a complement of the chemical analyses become a requirement in the water quality assessment. In the present study, besides the physical and chemical analyses, we have tested the use of benthic diatoms as indicators of the oued Hassar water quality and diatom indices as a tool for the evaluation of this quality.Oued Hassar is a canalized stream situated on the valley of oued Mellah river. It drains a part of the Berrechid groundwater as sources. Its main source (station SC) is situated to 3 km in the Northeast of Mediouna (155 m). Along this water course, in sections of various degrees of water pollution, six sites (SC, SB, HP, OM, SH and CA) were sampled on four different dates (august 1997 and january, june and august 1998). The station OM belongs to oued Mouileh stream which is a unique tributary of oued Hassar. The main source of pollution; the wastewater effluent of Médiouna agglomeration entered the stream between The oued Hassar source and the station SB (Sidi Brahim). This region has a Mediterranean semi-aride bioclimate. The influence of the ocean results in a summer temperatures generally varying between 25 and 30°C and winter temperatures varying between 12 and 19°C.The physical and chemical analysis data of the oued Hassar waters show particularly elevated values of water conductivity essentially due to the high contents in chlorides. These values were understood between a minimal value recorded at station SH in August 1998 (3470 mS.cm-1) and a maximal value signalled at station OM in January of the same year (7210 mS.cm-1). Concentrations in sulphates vary according to stations. The most elevated values were recorded in january 1998 at stations SC and OM (231 and 218 mg.l-1 respectively). The hardness values are very important especially at the source of oued Hassar (SC) where the concentration reaches 25,2 meq.l-1. This excessive mineralization of waters, particularly at stations SC and OM, is essentially due to the Permo-triasique saliferous sedimentary layers which crop out in the area.The oued Hassar stream receives, to 100 ms downstream the station SC, the wastewater effluent of Médiouna agglomeration, highly loaded with organic matters, ammonia, orthophosphates and suspended matters. The impact of this effluent results on the elevated values of these parameters downstream the wastewater input (SB). On January 1998 (rainy period), the important contents in orthophosphates and sulphates recorded at all stations were essentially due to the high amounts of superphosphates and ammonia sulphates used as amendment of the cultivated lands situated a long the sides of oued Hassar stream.Outside of this period, contents in orthophosphates and ammonia at stations SC and OM are very low. However, the elevated content in nitrates at the oued Hassar's source (maximal value: 93,3 mg.l-1) gives an idea on the deterioration of the groundwater quality.To put in evidence the evolution of the global organic pollution of waters, the "Indice de Pollution Organique" (IPO) have been calculated. The exam of this index shows that the wastewater effluent constitutes the main source of pollution. Indeed, water quality goes from a mean pollution state (3,88 ± 0,14) at the oued Hassar's source to a very strong pollution state (1,96 ± 0,31) at the station SB. Downstream the swampy zone, at the station HP, the water quality of oued Hassar improves remarkably (3,50 ± 0,74) due to the auto-purification phenomenon and the dilution of these waters by those coming from the secondary sources situated between stations SB and HP. At the stations SH and CA, the IPO mean values (3,50 ± 0,79 and 3,75 ± 0,41 respectively) show that the water quality remains rather identical as at the station HP.Concerning the survey of epilithic diatoms, 61 species and varieties of diatoms have been inventoried. These taxa are bound especially to the salinity (Amphora coffeaeformis, Fragilaria fasciculata, Fragilaria pulchella, Pleurosira laevis,…) and to the organic load (Gomphonema parvulum, Navicula subminuscula, Nitzschia palea,…). In the most collected samples, particularly at the stations SB, SH and CA, diatom communities are essentially dominated by N. palea and the complex N. inconspicua / N. frustulum. At the station SB, the relative abundance of N. palea reaches 71% in august 1997 and august 1998, and goes beyond 45% in January and june 1998. giving evidence of the important organic pollution. this species is substituted, at stations SH and CA, essentially by Navicula subminuscula, N. veneta and Nitzschia inconspicua / N. frustulum complex. Diatom communities at the stations SC, HP and OM don't show an exclusive dominance of one or two species, but several species dominate and form a more varied communities.Four diatom indices have been tested to assess the water quality in Oued Hassar (Morocco) : the "Indice Diatomique Pratique" (IDP), the "Indice Diatomique Générique" (IDG), the Sládecek Index (SLA) and the Trophic Diatom Index (TDI). These indices are different by the assigned indicative values and pollution sensitivity coefficients relative to every species and by the taxonomic list integrated in the calculation of every index. The spatio-temporal variation of these indices reflects clearly the deterioration of water quality downstream the wastewater input of Mediouna agglomeration (IDP=3,86 ± 2,30; IDG=6,05 ± 2,31; SLA=4,35 ± 2,62; IDT=5,80 ± 1,85) and its recovery after the swampy zone at the station HP (IDP=9,37 ± 2,35; IDG=11,83 ± 1,77 ; SLA=7,19 ± 3,05 ; IDT=6,76 ± 2,48). Among these indices, only the IDP presents a significant correlation (0,63 ; α