Carbon, nitrogen, and phosphorus release characteristics and underlying mechanisms in fish manure from recirculating aquaculture systems under alternating aerobic-anaerobic conditions

To mitigate the adverse environmental impacts of aquaculture operations, recirculating aquaculture systems (RASs) have emerged as promising alternatives to traditional aquaculture methods. Fish manure is the primary pollutant in RASs, and oxygen fluctuations significantly influencing the release of pollutants. However, the release characteristics and underlying mechanisms of carbon (C), nitrogen (N), and phosphorus (P) during alternating aerobic-anaerobic conditions in RASs remain poorly understood. This study conducted batch incubation experiments to examine the release dynamics of C, N, and P from fish manure under simulated alternating aerobic-anaerobic conditions. Results showed that the overlying water exhibited anaerobic and reductive conditions, with both COD and TOC concentrations decreasing over time. NH₄⁺-N levels significantly decreased from days 1–9, while NO₃^--N concentrations peaked on day 16. Furthermore, the alternating aerobic-anaerobic conditions significantly increased TP concentration in the overlying water. Soluble microbial byproduct-like substances in the overlying water transformed into humic acid-like substances over time. The relative abundance of Proteobacteria and Firmicutes decreased by 13.6 % and 6 %, respectively, while the relative abundance of Actinobacteria, Bacteroidota, and Chloroflexi increased by 7 %, 12.8 %, and 1.4 %, respectively. Overall, both abiotic and biotic factors influenced the release of C, N, and P from fish manure. The coupled effects of abiotic factors, specific bacterial communities, and functional genes played a critical role in the release and transformation of these elements. These findings provide new insights into the release behaviours and mechanisms of pollutants in RASs, contributing to improved environmental risk management in RASs.