Balances of phosphorus and nitrogen in carp ponds

The impact of carp pond effluents on natural waters was investigated in the German federal states of Brandenburg, Saxony and Bavaria, and in Hungary. Data from 38 ponds (size = 0.25-122 ha) were available for the calculation of inlet-outlet differences. An average difference of 0.51 kg phosphorus (P) ha(-1) year(-1) was obtained. This means that every hectare of pond surface releases 510 g P less than it receives from the incoming water. This result was independent of the amount. of fish harvested (less than or equal to 1500 ha(-1) year(-1)). The average retention of P (P-balance) was 5.71 kg P ha(-1) year(-1). Phosphorus retention increased with increasing intensity of production. Nitrogen (N) retention increased with production intensity from 78.5 kg ha(-1) year(-1) in German standard ponds to > 290 kg N ha(-1) year(-1) in pig-cum-fish ponds in Hungary. A predominantly mineralized sludge suspension is released during harvesting at loads below 1% of the retention capacity of the pond. Under usual pond management regimes, the sludge load during harvesting ranged from 50 to 200 L ha(-1) equivalent to 0.3-9.3 g dry matter ha(-1). The present study suggests that ponds are not a burden on the environment. By contrast, these water bodies improve water quality. Therefore, pressures to reduce the intensity of pond production cannot be justified on the basis of supposed impacts on water quality. However, even if loads during harvesting are low compared with the retention capacity of the pond, more effort should be carried out to reduce the pollution of streams by pond outlets downstream. This can be done by limiting pond drainage to periods when the suspended material has settled or by short-term sedimentation of the sludge in a settling pond downstream of the rearing facility.