Effects of lining and drying on the chemical composition of sediments and influence of organic carbon on carp growth in aquaculture ponds

Chemical characteristics and P‐fractionation of sediments from the Deroua fish culture farm (Beni Mellal, Morocco) were analysed in 1998 and 1999. The high concentrations of organic N and organic C in the sediments reflect the high productivity of phytoplankton in the lined ponds. Despite the fact that these ponds were not frequently fertilized, inorganic P was more important in the sediments. The P released from CaCO₃≈P was the principal source of P for phytoplankton. The mineralization rate of organic C during the drying period was about 87 µg g^?1 day^?1 in the sediments. The mineralization rate of organic N seems to be less, compared with organic C. The experimental results indicate that the C:N ratio and texture of sediments could explain the immobilization of organic N. All forms of P remained almost unchanged after the drying period except for Fe(OOH)≈P. The ferric (oxy) hydroxides protect NaOH_ext‐P from the bacterial lyses and stabilize therefore their concentration. The inorganic P represented about 88% of Total P in the sediments and may influence the mineralization rate of organic P. The growth of silver carp were reduced in presence of high concentrations of organic C in sediments (> 30 mg/g). Compounds toxic to fish such as Fe²⁺, Mn²⁺, H₂S and CH₄ were then released in the water column.