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

River and stream water quality and ecology in the Wellington region: State and trends
be significantly impacted by physical barriers (eg, culverts, weirs) that prevent
upstream fish migration.
8.1.2 Temporal trends
Temporal trend analysis performed on both raw and flow-adjusted physicochemical
and microbiological water quality data found that water quality was
typically stable at the majority of RSoE sites between July 2006 and June 2011.
A total of 78 ‘meaningful’ trends (ie, statistically significant and a relative rate
of change >1%/year) were identified across nine core variables (equating to
14.1% out of a total of 411 possible trends), with half of these trends associated
with just three variables: total phosphorus (TP, 15 sites), nitrite-nitrate nitrogen
(NNN, 13 sites) and total nitrogen (TN, 11 sites). In almost all cases, the
trends in nutrient concentrations were decreasing. Such trends were present at a
number of sites, including both nutrient-rich (eg, Mangaone Stream at Sims
Road Bridge) and nutrient-poor (eg, Hutt River at Te Marua) sites. The reasons
for the improvements are not known, but given that reference sites were well
represented, these ‘improving’ trends may be related more to natural factors
(eg, climate variability) than changes in land use/management. More
importantly, while a number of ‘degraded’ sites showed improvements, these
sites – as well as many sites that did not exhibit any meaningful trends – still
recorded median nutrient concentrations well above ANZECC (2000) TVs.
A few sites exhibited trends indicative of deteriorating water quality – such as
an increase in DRP concentration (Mangaroa River at Te Marua and
Whangaehu River 250 m from confluence) and a decline in visual clarity
(eg, Horokiri Stream at Snodgrass and Whareroa Stream at Waterfall Road). In
most cases, the causes for these trends are not known; as noted in Section 4.2.2
and discussed further in Section 8.3.2, declining water clarity in the Horokiri
Stream may be associated with soil/stream bank erosion and sediment runoff
from forestry activities.
The results of trend analysis conducted on water quality data from NIWA’s
monitoring sites located on the Ruamahanga (three sites) and Hutt (two sites)
rivers for the same five-year period were not too dissimilar from the findings
for RSoE sites located on these rivers. In fact, analysis over a longer time
period (1989 to 2007) by Ballantine and Davies-Colley (2010) suggests that
water quality in both of these rivers has remained relatively stable for some
time. Where trends were identified, these were typically of a very small
magnitude (eg, an increase in flow-adjusted NNN concentration of 0.006
mg/L/year was observed for the site Ruamahanga River at Wardells).
Periphyton cover/biomass and macroinvertebrate community health were also
stable at the majority of RSoE sites. However, and in contrast with water
quality, where trends were present in one or more periphyton or
macroinvertebrate metrics (19 and 15 sites, respectively, for the period 2004 to
2011), these were overwhelmingly indicative of deteriorating ecosystem health
and tended to occur mainly at ‘impacted’ sites (ie, sites located in pastoral and
urban catchments). The majority of the observed trends in these metrics did not
coincide with one another, nor with any trends in water quality. It is likely that
variation in climatic factors – in particular the frequency of flushing flows –
PAGE 102 OF 160 WGN_DOCS-#1100598-V3