Nitrification remains the most predominant method of ammonia removal wastewater treatment. However, it is well established that seasonal temperature variations have detrimental effects on nitrification which is known to completely cease at temperatures below 5°C in suspended sludge systems. This is of considerable concern to lagoon facilities where nitrification remains unreliable as wastewater temperatures as low as 0.5°C have been observed during winter months. Unlike lagoon facilities, moving bed biofilm reactors (MBBR) technology is capable of achieving significant ammonia removal rates at very cold temperatures. However, additional methods to enhance nitrification are necessary to meet mandated effluent targets. In suspended growth systems nitrifier enrichment was determined to be possible through low dissolved oxygen acclimatization which reduces the endogenous decay rate of the nitrifiers thus promoting enrichment. However, there is a lack of knowledge and research as it relates to applying the low DO effect to achieve nitrifier enrichment in biofilms. This research aims to explore enriching nitrifying biofilms under long-term low DO concentrations and study the effects that the enriched biofilms have on nitrification at very low temperatures. The research concluded that nitrifier enrichment is possible through DO limitation. The study of the reactor performances to temperatures as low as 0.5°C demonstrates a non-linear decline in removal rates between 19.5°C and 0.5°C. When acclimatized to low DO the threshold temperature was observed at 2.5°C below which, removal rates declined significantly. When acclimatized to high DO the threshold temperature was observed at 5°C. The biofilms in the high and low DO reactors showed distinctly different responses to DO. The biofilm mass in low DO reactor was consistently greater than the biofilm mass in the high DO reactor which confirms the hypothesis of the study that long-term low DO is able to inhibit nitrifier endogenous decay and thus result in nitrifier enrichment in the biofilm. In this study, an Arrhenius correction coefficient of 1.150 was found for the transition from 5°C to 0.5°C for the high DO acclimatized MBBR system. A second coefficient of 1.130 was found for the transition from 5°C to 0.5°C in the low DO acclimatized nitrifying MBBR system.