Contribution à l'étude de la relation entre les différentes formes du phosphore dans les sédiments d'une retenue de barrage eutrophe en climat méditerranéen (El Kansera, Maroc) Contribution to the study of the relationship between different species of phosphorus in an eutrophic reservoir sediments in mediterranean climate (El Kansera, Morocco)

Résumé

Le présent travail porte sur l'évaluation des différentes formes de phosphore, dans les sédiments d'un réservoir eutrophe en climat méditerranéen (El Kansera, Maroc) et la détermination des relations entre ces formes. L'application d'un schéma de fractionnement chimique du phosphore a permis de déterminer les proportions de quatre formes différentes : le phosphore lié au fer Fe(OOH)-P, le phosphore lié au calcium CaCO3-P, le phosphore organique soluble dans l'acide POSA et le phosphore organique résiduel POR. La forme biodisponible a été déterminée par ailleurs, par des bioessais en utilisant une Chlorophycée (Scenedesmus crassus) isolée à partir des eaux de la retenue du barrage étudié.

Une étude par analyse factorielle des correspondances a mis en évidence l'influence de la nature des sédiments étudiés et de leurs propriétés physico-chimiques sur la répartition des formes du phosphore dans les sédiments. En période de basses eaux, les sédiments sont caractérisés par la prédominance des formes inorganiques ; alors qu'en période de hautes eaux, la forme POSA est la plus abondante. La confrontation des résultats obtenus par la méthode des bioessais avec ceux relatifs aux extractions chimiques séquentielles de phosphore a montré par ailleurs, que la forme biodisponible du phosphore est fortement corrélée aux formes inorganiques {Fe(OOH)-P et CaCO3-P} et indépendante vis à vis des formes organiques {POSA et POR}.

Mots-clés:

• Phosphore,
• phosphore biodisponible,
• extractions chimiques,
• bioessais,
• sédiments,
• retenues de barrages eutrophes

Abstract

We have studied in this work the different forms of phosphorus in the sediments of a eutrophic reservoir in a Mediterranean climate (El Kansera – Morocco). The studied sediments have been collected at eight sampling stations located at different points in the middle of the lake during two sampling sessions in contrasting weather conditions: the first sampling occurred in November 1992, a period of autumn mixing and low water levels, whereas the second took place in January 1994 during a high water level period, just after the first winter floods of the Beht river whose waters are retained in the studied reservoir.

The evaluation of the different forms of phosphorus was carried out using both a chemical extraction method and a bioassay method. The application of the chosen chemical extraction scheme allowed the determination of the proportions of four different phosphorus forms: iron-bound {Fe(OOH)-P}, calcium-bound {CaCO₃-P}, acid-soluble organic {POSA} and residual organic {POR}. The bioavailable form {Pbio}, representing less than 32 % of the total sediment phosphorus content, has been evaluated using a Scenedesmus crassus algal strain isolated from El Kansera reservoir waters.

According to the results of our analyses, the concentrations of iron-bound phosphorus are higher overall than those of calcium-bound phosphorus. The sediments collected during the low water period contain spatially stable values of POSA (0.03 – 0.09 mg.g^-1). These values are lower than those determined in the sediments collected during the high water period (0.24 – 0.42 mg.g^-1). This form represents less than 11% of the total phosphorus in the low water period whereas it represents more than 25% and up to 56% of the total phosphorus in the filling period. This result can be explained by the freshness of the organic matter supplied by the continuous water flow in the high water period and the degradation of this organic matter during the low water period. Moreover, the anaerobic conditions of the hypolimnetic layers and the low external supply during the low water period favour organic phosphorus mineralisation.

A Factorial Correspondence analysis study has shown the influence of the nature and the physico-chemical properties of the studied sediments on the distribution of the different phosphorus forms. In the low level period, the predominance of the inorganic forms is obvious while POSA is the most abundant form in the period of rising water levels. This differentiation can be explained by the different transport dynamics of suspended organic or inorganic particles according to the water level and also to the external or internal supply of the organic matter.

We have also determined the statistical relations among the estimated different forms of phosphorus in El Kansera reservoir sediments. This statistical analysis was carried out to establish the correlation between the bioavailable phosphorus determined by the bioassay method and the phosphorus forms determined by chemical speciation. The goal of the study was to determine, according to statistically significant correlations, the chemical forms of the bioavailable phosphorus contained in the studied sediments.

The comparison between the results we have obtained by the bioassay method and those obtained by sequential chemical extraction has shown that the bioavailable form of phosphorus is strongly correlated with the inorganic forms {Fe(OOH)-P} and {CaCO₃-P}. The high correlation found between the iron-bound form of phosphorus and the bioavailable one is in good agreement with previous work described in the literature that has shown that an important part of the iron-bound and aluminium- bound phosphorus is bioavailable, whatever the extraction method may be.

It has also been demonstrated that the forms {Pbio}, {Fe(OOH)-P}, {CaCO₃-P} and {Pinorg}are notably independent from the {POSA} and {Porg} forms, the correlation coefficients being relatively low. The residual organic form of phosphorus {POR} does not seem to be linked to any variable. Therefore, we can say that this form is not bioavailable for the Scenedesmus crassus species.

Keywords:

• Phosphorus,
• bioavailable phosphorus,
• chemical extractions,
• bioassays,
• sediments,
• eutrophic reservoirs