River and stream water quality and ecology - Greater Wellington ...

River and stream water quality and ecology in the Wellington region: State and trends
4.1.4 Dissolved oxygen (% saturation)
Median dissolved oxygen (DO) values for the majority (49) of the 55 RSoE
sites complied with the RMA and RFP critical minimum threshold of 80%
(Tables 4.2 and 4.3), although most sites (50) did fall below 80% saturation on
at least one sampling occasion during the reporting period. The six sites that
recorded a median value below the 80% threshold were Ngarara Stream at
Field Way, Mangapouri Stream at Bennetts Road, Parkvale tributary at Lowes
Reserve, Mangaone Stream at Sims Road, Whareroa Stream at Queen
Elizabeth Park and Waiwhetu Stream at Wainuiomata Hill Bridge. Minimum
DO values were as low as 30–40% saturation at most of these sites, and even
lower for Ngarara Stream at Field Way (minimum and median DO values of
9% and 55% saturation, respectively). Actual DO concentrations (see Appendix
6) at this site were less than 3 mg/L on eight occasions and likely placed
significant stress on some aquatic life, particularly fish.
While dominant landcover does not seem to be a strong driver of DO saturation
(Figure 4.3), all six of the sites that failed to comply with the 80% minimum
threshold are located in catchments dominated by pastoral or urban landcover.
However, these sites are also located on typically small, slow flowing and low
elevation coastal streams which might naturally have lower DO levels (refer
Section 3.1).
Dissolved oxygen (% saturation)
120
110
100
90
80
70
60
50
0
RMA (1991) & WRC (1999) minimum threshold
0 10 20 30 40 50 60 70 80 90 100
% indigenous forest in upstream catchment
REC landcover class
Indigenous forest
Exotic forest
Pastoral
Urban
Figure 4.3: Scatterplot of median dissolved oxygen values (% saturation) for each
RSoE site against indigenous forest cover (%) in the upstream catchment, based
on monthly measurements between July 2008 and June 2011
Supersaturation was commonly recorded at most of the 55 RSoE sites at some
stage (Table 4.2). Very high saturation values observed at some sites during
stable flows in the summer months are a likely indication of high rates of
photosynthesis by periphyton and/or aquatic macrophytes. For example, a
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