| Summary: | The use of constructed wetlands to treat wastewater from fish farms has the potential to reduce the environmental impact of the aquaculture industry. In order to gain a greater understanding of the processes occurring within a newly constructed wetland at a commercial marine fish farm, nitrogen removal and bacterial communities were studied in model wetlands and the fish farm wetland. The limits of nitrification capacity in a vertical trickle flow model system were tested by dosing with aquaculture wastewater supplemented with increasing amounts of ammonium chloride. Greater than 97% ammonia removal was observed with ammonia concentrations of up to 358ppm. At higher concentrations of ammonia a lower percentage of ammonia removal occurred, and nitrite accumulation was observed. A decrease in the number of detected bacterial OTUs (as measured by 16s rRNA T-RFLP) was detected concurrent with the reduction in the percent of ammonia removed. T-RFLP analysis of the ammonia mono-oxygenase gene showed a clear successional pattern of three different ammonia oxidizing bacterial OTUs (belonging to the Nitrosomonas Nm143 lineage N.oligotropha/N.ureae lineage and N.aestuarii/N.marina lineage). Lab-scale wetlands were used to investigate the effect of flood/drain cycles on nitrogen removal. When the multiple cycles were used, the concentrations of ammonia and organic nitrogen were lower after treatment in the flood/drain wetlands than in permanently submerged wetlands. However, the concentrations of nitrites and nitrates were higher in the flood/drain wetlands. Elevation of nitrate concentration could be prevented by shortening the drainage period. Subsequent work on the fish farm wetland also showed that flood/drain cycles improved ammonia removal, but reduced nitrate and nitrite removal. The total bacterial communities in the submerged wetlands showed a greater degree of similarity to each other than those in the flood/drain wetlands. The ammonia oxidizing bacterial communities in the flood/drain wetlands were dominated by bacteria belonging to the Nitrosomonas aestuarii/N.marina lineage, and the submerged wetlands were dominated by a bacterial OTU that was unidentified by T-RFLP. Nitrosomonas aestuarii/N.marina was the dominant ammonia oxidizing bacteria during the first 17 months of operation of the fish farm wetland. The abundance of other ammonia oxidizing OTU showed seasonal variation. The total bacterial community did not show clear temporal or spatial patterns of variation. Effective nitrogen removal was seen in the wetlands with the exception of one pair of cells which began to experience elevated ammonia concentrations after about 15 months. It was shown that introducing flood/drain cycles to this wetland could rapidly improve performance and prevent ammonia accumulation.
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