Title: The effect of temperature and oxygen levels on biofilter activity

In Ukraine, recirculating aquaculture systems (RAS) may be useful for the cultivation of high value species such as the Black Sea sturgeon (Acipenseridae), rainbow trout (Oncorhynchus mykiss) or common carp (Cyprinus carpio) fingerlings. Biofilters are an integral part of RAS. The primary objective of this study was to evaluate the effects of temperature and oxygen levels on biofilter activity. Furthermore, it was examined if simple test kits could be used for accurate measurements of TAN, NO2 and NO3. The results indicated that, when the measured in a spectrophotometer, the kits provided reliable and accurate measurements of nitrogen compounds in freshwater and sea water. A second objective of the study was to evaluate the effects of different temperatures and oxygen levels on biofilter activity. A biofilm of nitrifying bacteria was allowed to settle in the biofilters for 4 weeks at 19 ±1˚C, salinity of 12‰ and continuous aeration. Three treatments of biofilters were tested: (1) Reduced temperature, where the temperature was reduced to 8.3 ±0.3˚C with continuous aeration; (2) Reduced oxygenation, where the aeration of biofilters was suspended and (3) control group that was maintained at the same conditions as during the conditioning period. The results of this study showed that low temperature reduces the nitrification rate. Nitrification activity was also reduced in the low oxygen biofilters, but the response was probably rather due to insufficient mixing of the water when the aeration was suspended than lack of oxygen.