Relation between phosphate and organic matter in fish-pond sediments of the Deroua fish farm (Beni-Mellal, Morocco): implications for pond management

An experimental approach of the phosphate exchange across the water-sedimen t interface in fish ponds of the Deroua farm (Beni-Mellal, Morocco) is need ed to understand the phosphate dynamics in these ponds in relation to their water quality. During this study, we conducted experiments to determine th e P-fractions of the different pond sediments and to estimate the release f rom these sediments of phosphate available for algal uptake. We also determ ined the amount of phosphate needed to saturate the sediments of two differ ent fish ponds under anoxic and oxic conditions. Phosphate release from sed iments comes mainly from Fe(OOH)approximate toP and is more important in po nds lined with sheets. The accumulation of organic matter in sediments favo urs the installation of anoxic conditions and enhances the phosphate releas e from CaCO(3)approximate toP, labile in these pond sediments. Under experi mental conditions, org-P plays a minor role in the P-release. Oxic conditio ns, to the contrary, have an inhibitory effect on the P-release from sedime nts. About 80-98% of the P-adsorbed onto different pond sediments was recov ered in the inorg-P-fractions. Aeration induces the oxidation of FeS to Fe( OOH) which can adsorb phosphate from solution. Besides, the presence of bac teria in pond sediments was essential to promote phosphate release under an oxic conditions by controlling the oxidation state of iron and the minerali zation of the organic matter. Sheet-lined ponds, when insufficiently dried, accumulate a large quantity of organic matter in their sediments. After a decrease in pH, P is released from CaCO(3)approximate toP and enhances the phytoplankton productivity responsible for renewed accumulation of organic matter. Org-C concentrations in sediments over 20 mg g(-1) d.w. favour the formation of toxic factors (Fe2+, Mn2+, NO2- and H2S) harmful for carp grow th. An extended period of drying efficiently enhances the mineralization of organic matter.