Discharge of sediments, nitrogen (N) and phosphorus (P) during the emptying of extensive fishponds: effect of rain-fall and management practices

In 1997 and 1998 we assessed the input and output of suspended matter, nitrogen and phosphorus during the exploitation period of a fishpond in the Domaîne Départemental de Lindre (Moselle, north-eastern France). Special attention was given to the emptying period which was studied under different meteorological conditions (rainy and dry periods). The pond has a surface area of 2 ha, a volume of 16 000 m3 and is used for the production of various Cyprinidae (roach, bream, chub, pike, perch, carp). During the emptying operation, 630 and 2830 kg ha−1 of suspended matter, 10.8 and 36.5 kg ha−1 of Total-nitrogen and 1.2 and 5.1 kg ha−1 of Total-phosphorus were discharged from the pond during drought and heavy rainfall (50.5 mm in 166 h), respectively. Except for the emptying period, the pond retained 6850, 117 and 6.6 kg ha−1 of suspended matter, nitrogen and phosphorus, respectively. In the course of the emptying period, more than 32% of the suspended matter was discharged as was 8.5% of the nitrogen and 25% of the phosphate. For the whole exploitation period 4020 – 6220 kg ha−1 of suspended matter, 80–106 kg ha−1 of nitrogen and 1.6–5.5 kg ha−1 of phosphorus accumulated in the pond.