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

Abstract

An experimental approach of the phosphate exchange across the water–sediment interface in fish ponds of the Deroua farm (Beni-Mellal, Morocco) is needed to understand the phosphate dynamics in these ponds in relation to their water quality. During this study, we conducted experiments to determine the P-fractions of the different pond sediments and to estimate the release from these sediments of phosphate available for algal uptake. We also determined the amount of phosphate needed to saturate the sediments of two different fish ponds under anoxic and oxic conditions. Phosphate release from sediments comes mainly from Fe(OOH)≈P and is more important in ponds lined with sheets. The accumulation of organic matter in sediments favours the installation of anoxic conditions and enhances the phosphate release from CaCO3≈P, labile in these pond sediments. Under experimental conditions, org-P plays a minor role in the P-release. Oxic conditions, to the contrary, have an inhibitory effect on the P-release from sediments. About 80–98% of the P-adsorbed onto different pond sediments was recovered in the inorg-P-fractions. Aeration induces the oxidation of FeS to Fe(OOH) which can adsorb phosphate from solution. Besides, the presence of bacteria in pond sediments was essential to promote phosphate release under anoxic conditions by controlling the oxidation state of iron and the mineralization 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 CaCO3≈P 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 growth. An extended period of drying efficiently enhances the mineralization of organic matter.