Nitrogen budget and effluent nitrogen components at an intensive shrimp farm
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This study examined the operation of an intensive tropical shrimp farm in relation to the nitrogen (N) budget and the N components of the discharged effluent. Weekly samples were taken for total N (TN) concentration from the farm intake and discharge water over a 10-month period. A N budget was calculated based on TN data, continuous datalogger records of water exchange volumes, and farm records for feed addition, harvest and sediment removal. TN levels in the intake water were low throughout the 10 months and only contributed 5% of the N input to the farm. Most of the N (90%) entered the farm ponds as formulated shrimp food. Within the ponds, 22% of the input N was converted to harvested shrimp, 14% remained in the sediment, while most of the remainder (57%) was discharged to the environment. Only 3% of input N was unaccounted for, and assumed to be lost to the atmosphere via denitrification or volatilization of ammonia. More intensive sampling of effluent (three times a day) was done over 7-day periods in February (late summer) and July-August (winter) to provide detailed information on the N composition. All parameters varied substantially both within and between days. Forty-two to forty-five percent of the discharged N was in particulate form, mostly phytoplankton. The dissolved N fraction had two main components: dissolved organic N (DON), which comprised 37-43% of TN, and total ammonia N (TAN), which comprised 12-21% of TN. Comparison between the results of this and previous studies suggests that little progress has been made in improving nitrogen utilization efficiency of intensive Penaeus monodon shrimp farming over the past decade. Thus a major challenge facing this industry is to improve both environmental and economic performance by developing and implementing an integrated approach to reducing nitrogen waste.