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

River and stream water quality and ecology in the Wellington region: State and trends
Figure 5.4: RSoE sites Whareroa Stream at Waterfall Road (left) and Awhea River
at Tora Road (right)
Most of the RSoE sites assigned to the ‘poor’ periphyton class recorded very
high median concentrations of TN (>1 mg/L) and/or TP (>0.04 mg/L). Awhea
River at Tora Road (Figure 5.4) and Huangarua River at Ponatahi Bridge were
two exceptions, recording only low or moderate median nutrient
concentrations. However, these two sites are characterised by both high water
temperatures (median values >14 o C) and long accrual periods (mean annual
maximum accrual period is estimated to be 105 days at both), a reflection of
their location in the eastern Wairarapa which typically experiences dry
summers. Both sites also have minimal shade.
The other exception in the ‘poor’ class was Mangaroa River at Te Marua. This
site recorded only moderate concentrations of TN and TP and a low median
water temperature. The reason for high periphyton growth at this site is
unclear but could be related to the relatively long accrual periods this site can
experience (average annual maximum accrual period of 89 days).
5.1.6 Macrophytes
Nine RSoE sites have substrate dominated by soft sediment (eg, silt and sand)
which makes them more suitable for growth of macrophytes than periphyton.
Although monitoring of macrophyte cover was not undertaken during the July
2008 to June 2011 reporting period (monitoring only commenced in July
2011), general observations made at the time of routine monthly water quality
sampling suggest that extensive macrophyte cover is frequently present at four
soft-bottomed RSoE sites: Mangapouri Stream at Bennetts Road (Figure 5.5),
Mangaone Stream at Sims Road Bridge, Whangaehu River 250 m from
confluence and Whareama River at Gauge.
Extensive macrophyte growth has also been observed at some hard-bottomed
RSoE sites, many of which have substrate at the lower size limit of that
suitable for periphyton growth and/or experience long accrual periods. These
sites include Pauatahanui Stream at Elmwood Bridge, Makara Stream at
Kennels, Wainuiomata River upstream of White Bridge, Parkvale Stream at
Weir and Taueru River at Gladstone Bridge. Most of these sites are located
near the bottom of low lying catchments dominated by pastoral land use.
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