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

River and stream water quality and ecology in the Wellington region: State and trends
Mean and maximum mat periphyton cover demonstrated few statistically
significant correlations apart from moderate to weak correlations with water
temperature, shade, dissolved oxygen and dissolved inorganic nitrogen (Table
5.4). It is important to note that a strong correlation between periphyton
growth and a particular environmental variable does not necessarily mean that
there is a cause and effect relationship between the two. Although variables
such as water temperature, shade and nutrient concentrations have been shown
to directly affect periphyton growth (MfE 2000), other variables are unlikely to
directly influence periphyton growth. For example, the moderate correlation
between filamentous periphyton cover and pH is likely to be related to the four
RSoE sites with the highest mean filamentous periphyton cover being located
in eastern Wairarapa. These sites typically have high pH (median pH of 7.6–
8.0) due to their catchment geology (see Section 4.1.5) but proliferation of
filamentous periphyton is more likely to be a result of low summer-time flows
and infrequent freshes that are also typical of these sites.
The strong negative correlation between periphyton biomass/filamentous cover
and visual (water) clarity was unexpected because all other factors being equal,
higher water clarity and the associated increase in light availability to the
streambed should result in increased periphyton growth. It is likely that the
strong negative correlation is due to high water clarity being associated with
oligotrophic conditions present at some RSoE sites.
The relatively poor correlation between periphyton growth and average accrual
period (rs=0.38–0.02) was also surprising given that the frequency of flushing
flows is widely recognised as one of the key factors determining periphyton
growth (eg, MfE 2000). The poor correlation is likely to be related to the overriding
effect of land use related factors such as nutrient concentration and water
temperature at some sites. For example, although sites such as Motuwaireka
Stream at Headwaters and Waikanae River at Mangaone Walkway are
characterised by long accrual periods (average annual maximum accrual of 110
and 97 days, respectively) they have low periphyton biomass as they are
located in the headwaters of forested streams. Conversely, although
Ruamahanga River at Gladstone Bridge has the second shortest average annual
maximum accrual period (39 days) it has moderate periphyton growth – most
likely due to moderate nutrient enrichment found at this site.
Most RSoE sites assigned to the ‘excellent’ periphyton class (refer Table 5.3)
were characterised by very low median concentrations of TN and/or TP
(100), reflecting their location in catchments dominated
by indigenous forest (refer Figure 5.1). Two notable exceptions were
Whareroa Stream at Waterfall Road (Figure 5.4) and Parkvale Stream at Lowes
Reserve; these two sites recorded moderate to high median nutrient
concentrations but are also characterised by a high degree of shade (shade
scores of 19 and 18, respectively) which is likely to be the key factor
controlling periphyton growth at these sites.
PAGE 64 OF 160 WGN_DOCS-#1100598-V3