Waipoua River 20130219.pdf - Northland Regional Council

Dissolved oxygen is also important for freshwater invertebrates and fish, with some speciesbeing more sensitive to low oxygen levels than others. Dissolved oxygen levels vary withtemperature, biological activity and how quickly it transfers from the atmosphere. Biologicalactivity includes microbial activity by bacteria and primary production by plants and algae.Aquatic plants photosynthesise during the day (producing oxygen) and respire at night(using oxygen). Given the pristine state and lack of plants and algae in the Waipoua River itis likely that the fluctuation in dissolved oxygen throughout the day is reasonably smallcompared to an impacted lowland river dominated by aquatic plants.Between 2007 and 2011 the lowest dissolved oxygen recorded was 69.4% and the medianwas 101% (Table 5). Dissolved oxygen also scored Excellent for stream health. However,again it is worth noting that oxygen levels are likely to be lowest in the early hours of themorning and highest in the afternoon, which does not coincide with Council monitoring.Dwersteg (2007) recorded much higher dissolved oxygen levels at several sites in theWaipoua catchment, including at the SH bridge, in Waikohatu Stream and at the ford in thelower reaches, the highest of which was 124.5% saturation, which suggests that levels mayalso drop well below those recorded during RWQMN monitoring.Table 5: Water quality statistics for 2007-2011 for Waipoua River at the SH12 bridge, includingstream health score (A = Excellent, B = Very good) Source: NRC 2013.ParameterNo. ofsamplesMeanStdDevMed Min MaxNitrate, nitrite-N (mg/L) 54 0.03 0.03 0.02