Mass balance of fishponds: are they sources or sinks of phosphorus?

Fishponds exhibit high natural retention potential for phosphorus, which enters the ponds from non-point, diffuse and point sources, as well as from aquaculture management. Results of phosphorus mass balance monitoring of nine large fishponds (60-449 ha) over 2010-2014 revealed total phosphorus retention ranging between -66 % (sink) and +52 % (release) of the P loads from inflows, i.e. specific P retention varied between -7.83 g m(-2) (sink) and 1.00 g m(-2) (release) of surface area, per one fish production cycle. Retention of P was eliminated by application of fertilizers (with simultaneous fish feed application) during fish production process and/or by massive P release from fishpond sediments after previous heavy loads. P retention could be increased by preference of surface water outflow instead of bottom discharge and/or also by minimizing of emissions of suspended sediment particles during a fish harvesting. The real role of fishponds in transport processes of P throughout a watershed could be eruditely assessed only by following newly proposed method called new approach when P input (via inflows) and P output (via outflows) are compared with natural fishpond ability to retain P, which is determined by a simple model proposed by Hejzlar et al. (2006). For evaluation of effect of actual fishery management, the culture-balance method is suitable too. If the P inputs (stocked fish, fertilizing, feeding) and outputs (harvested fish) derived from fishery practice were in balance, the P retention did not decline. However, in condition of inputs > outputs, the P retention appreciably ceased or was totally eliminated. It was concluded that fishponds could serve as an important sink of P transported throughout a watershed even under semi-intensive fish (common carp) production condition.