Seasonal variation in biological oxygen demand levels in the main stem of the Fraser River, British Columbia and an agriculturally impacted tributary

The Fraser River basin is one of British Columbia's most diverse and valuable ecosystems. Water levels and temperatures along the Fraser are seasonally variable, with high flow during the spring freshet and low flow during winter months. In the Fraser River, dissolved oxygen (DO) concentrations impact many aquatic species. Biological oxygen demand (BOD) measures the amount of oxygen consumed by bacteria during the decomposition of organic matter and is an indicator of water quality in freshwater environments. We compared BOD, DO, and pH during winter (November 2011) and summer (July 2012) in the main stem of the Fraser River at Fort Langley and a tributary in an agricultural area of the Fraser Valley, Nathan Creek. In November the BOD of the main stem of the Fraser River was 2.36 mg/L, pH 7.26, and DO 9.13 mg/L. BOD and DO of Nathan Creek was not significantly lower at 1.68 mg/L and DO 8.28 mg/L, however, the pH was significantly lower (p=0.001) at 6.75. In July, the Fraser River had significantly higher BOD levels than in winter at 4.43 mg/L, but no significant change in pH and DO. Nathan Creek BOD was significantly higher than it was in winter and higher than the main stem at 7.34 mg/L, with no significant change in pH and DO. There were strong seasonal differences in BOD in the Fraser River and Nathan Creek, with the highest levels seen in July. The higher BOD seen in Nathan Creek in July may be an indication of agricultural impact. Although all BOD values fell in the range of 1-8 mg/L and are considered to be relatively unpolluted.