Relargage du phosphore à l'interface eau-sédiment dans des étangs de pisciculture de la station Deroua (Béni Mellal, Maroc) Phosphate release at the water-sediment interface in a fisheries pond of the Deroua fish farm (Béni Mellal, Morocco)

Résumé

De l'urée et du super phosphate triple ont été appliqués aux étangs d'alevinage de carpes chinoises de la station Deroua pour stimuler la croissance des algues. Toutefois, une forte proportion de ce phosphore est fixée par les sédiments et sera libérée en grande quantité dans les étangs lorsque les sédiments sont en conditions anaérobies.

Nous avons étudié le relargage des orthophosphates par les sédiments provenant de deux étangs différents. L'un (A₁) en terre ce qui permet la percolation des éléments nutritifs vers les couches profondes, l'autre (C₁) dont le fond recouvert d'une fine membrane en polyéthylène, est parfaitement imperméable et empêche toute perte. Par ailleurs, nous avons étudié les effets de l'oxygène dissous, du pH, des nitrates et du glucose sur le relargage des orthophosphates par les sédiments.

Les résultats obtenus montrent qu'en anaérobiose, la solubilité du phosphore augmente dans l'eau interstitielle. Les orthophosphates libérés, dont la concentration peut atteindre 873 µg/l, proviennent de la fraction minérale en particulier du Fe(OOH)=P. Le sédiment C₁ qui accumule de grandes quantités de matière organique libère plus de phosphore que le sédiment A₁.

L'addition des nitrates limite la mobilisation du phosphore à partir des sédiments. Le pH acide provoque la dissolution du phosphore lié au calcium alors qu'un pH alcalin provoque celle du phosphore lié aux hydroxydes de fer et sa précipitation partielle sur les carbonates présents dans les sédiments.

Mots-clés:

• Maroc,
• Deroua,
• orthophosphates,
• relargage,
• anaérobie,
• percolation,
• polyéthylène

Abstract

In the Deroua fish farm (Béni-Mellal, Morocco), a series of ponds was lined with polyethylene to prevent water seepage (lined ponds, C). Another series of ponds was not lined (unlined ponds, A). The lined ponds do not need much fertiliser to enhance phytoplankton growth. They receive a small quantity of urea and triple super phosphate and accumulate high levels of organic matter in their sediments. The unlined ponds receive a large quantity of fertilisers. In the ponds, an important increase in phytoplankton growth occurred particularly after fertilisation with triple super phosphate. The phosphate (P) released from the sediment is the essential source of P for phytoplankton when the ponds are not fertilised.

Sediment samples used in this study were collected in June 1999 from the A₁ and C₁ ponds of the Deroua fish farm. The bottom sediments were collected with a core sampler to provide samples from the 0-5 cm layer. The samples were mixed and analysed for pH, total Kjeldahl nitrogen (NTK), nitrates (N-NO₃^-), ammonium (N-NH₄⁺), total iron (tot-Fe) and P-fractionation in eight replicates, using the Golterman method (1996).

In this work, experiments were conducted to evaluate the P released from sediments of lined and unlined ponds under controlled conditions. The samples included: untreated sediments, sediments treated with hydrogen peroxide (H₂O₂), and sediments treated with chloroform (CHCl₃). H₂O₂ was used to destroy the organic matter of sediment. To inactivate bacteria, CHCl₃ was added to the sediments. CHCl₃ does not modify the mineral and organic structures of sediments but does reduce the bacterial biomass. Urea and triple super phosphate were added to Chinese carp rearing ponds at Deroua fisheries farm to stimulate algal growth. However, phosphate bound to the sediments was released into the water column during anaerobic conditions. The released phosphate from lined pond (C₁) enhanced the algal production.

We have tested the fertilisation capacity of A₁ and C₁ ponds as well as the effects of different factors such as dissolved oxygen, pH, nitrate, and organic matter (glucose) on the phosphate released from the sediments. The results show that under anoxic conditions, phosphate solubility increased in the interstitial water. The released phosphate (873 µg/l) comes from the mineral fraction, particularly from Fe(OOH)=P. The P released from the A₁ untreated sediment under oxic and anoxic conditions was low in comparison to the P released from the C₁ untreated sediment and did not exceed 9.1 µg.g-¹ d.w. (Figure 1). The removal of organic matter with H₂O₂ had a positive effect on the P released from the C₁ sediments only after the second day of incubation. The inactivation of bacteria with CHCl₃ had an inhibitory effect on the P released from the C₁ sediments under oxic and anoxic conditions; however for the A₁ sediments, this inactivation of bacteria had no effect on the kinetics of P release. Phosphorus release from the C₁ sediments was important under anoxic conditions; the P came from Fe(OOH)=P after the reduction of Fe(OOH).

The addition of nitrates decreased the mobilisation of phosphate from the sediments. Acid conditions increased the dissolution of calcium-bound phosphate, while alkaline conditions increased the dissolution of iron-bound phosphate and its re-precipitation onto carbonate present in sediments. High concentrations of organic matter (glucose) increased oxygen demand and favoured the development of reducing conditions. The phosphate was released from Fe(OOH)=P.

The conceptual model of P released in fish ponds shows that, in semiarid climates and in calcareous areas, the polyethylene lining of ponds prevents not only the seepage of water but also the percolation of nutrients. Consequently, the P was more available for algal uptake in the lined ponds. More phosphate will be released concomitant with the process of the ponds becoming anoxic, as nitrate concentrations and the pH of sediments decrease.

Keywords:

• Morocco,
• Deroua,
• orthophosphates,
• release,
• fertilisation,
• oxic,
• anoxic,
• lined pond,
• production,
• fish