Nitrogen and phosphorus cycling for aquaculture ponds with artificially-controlled drainage: Sources, sinks and treatment strategies

Nitrogen (N) and phosphorus (P) loss from freshwater aquaculture ponds has been an on-going challenge due to its direct link to lake/river eutrophication. However, characterizing N & P cycling and its response to treatment strategies for the ecosystems is difficult due to the complexity of artificially-controlled drainage and biogeochemical cycling. To address this challenge, this study specifically developed a nutrient dynamic model (Pond-NP) for aquaculture ponds to track the daily sources, sinks and biogeochemical cycling of N & P. A month sampling program was carried out at a typical Chinese mitten crab (Eriocheir sinensis) culture pond during 2020-2022 to evaluate the model performance. The model achieved a model fit of Nash-Sutcliffe efficiency >0.43. Our modelling practices revealed that the studied pond had a total inputs of 187.3 kg N/ha/yr and 29.8 kg P/ha/yr, with N & P use efficiencies of 35.4% and 28.1%, respectively. Among the used N & P, macrophytes had a larger (>51.2%) contribution than the cultivated animals, resulting in a limited improvement (<2%) of nutrient use efficiency via the stocking strategies. More input N & P was lost to sediment (31.4% and 63.4% of total inputs) than surrounding rivers (17.6% and 8.5% of total inputs). Ex-situ ecological treatment was estimated to reduce N & P significantly (>40%) for aquaculture tail water, and was thus recommended to control N & P loss together with sediment reuse. This study demonstrated the high value of our newly-developed model in quantifying N & P loss, and thus supporting the design of N & P mitigation strategies for aquaculture ponds.