Effect of stopping adding brown sugar on water quality and nitrogen budget in biofloc systems cultured with Litopenaeus vannamei

Based on a 30-day culture of Litopenaeus vannamei and its pond water with biofloc technology (BFT), we set up brown sugar continuous addition (BS) group and non-added brown sugar (NBS) group to explore the effect of stopping adding brown sugar on the water quality and nitrogen budget in L. vannamei biofloc (BF) culture system. The total ammonia nitrogen (TAN) and nitrite nitrogen (NO₂ ^– -N) were monitored within 28 d, and the total nitrogen (TN) of shrimp body and feed were measured before and after the experiment. The results show that the concentrations of TAN and NO₂ ⁻-N in water of BS and NBS groups remained low levels. During the test, the concentrations of TAN in these two groups maintained at 0.02~0.06 mg·L⁻¹, and after the 7^th day, that of  NO₂ ⁻-N was lower than 1.00 mg·L⁻¹.The results indicate that the main nitrogen income was feed, which accounted for 78.8%; the main nitrogen output was water TN, which accounted for 45.06% and 52.55% in BS and NBS groups, respectively; the nitrogen output of harvested shrimps accounted for 21.49% and 25.43%, respectively, and the nitrogen utilization efficiencies of feed in the two groups were 18.14% and 23.14%, respectively. Thus, it is concluded that the removal effects of TAN and NO₂ ^– -N by microorganisms in water body will not be affected if brown sugar is stopped in a stable BF culture system.