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

River and stream water quality and ecology in the Wellington region: State and trends
9. Conclusions
Analysis of water quality, periphyton and macroinvertebrate data collected at
55 RSoE sites over the period July 2008 to June 2011 has found clear linkages
between river and stream health and catchment land use. Sites classified as
having ‘good’ or ‘excellent’ water quality (27 sites) and ecosystem health (27
and 36 sites for periphyton and macroinvertebrate indicators, respectively) tend
to be located on the upper reaches of rivers and streams that drain the forested
Tararua, Rimutaka and Aorangi ranges. As the proportion of pastoral and/or
urban landcover increases within a site’s upstream catchment, water quality
and macroinvertebrate health tend to decline while nuisance periphyton and
macrophyte growth increases. Of the 15 sites graded ‘poor’ for water quality,
five are located in urban areas and ten drain predominantly pastoral catchments
– of which most support at least some intensive agriculture. Similar patterns
were observed with periphyton growth and macroinvertebrate health, although
geological and climatic factors in part account for some sites (particularly those
in the eastern Wairarapa hill country) being assigned to ‘fair’ or ‘poor’ classes
for these indicators.
Analysis of New Zealand Freshwater Fish Database records also showed that a
significant relationship exists between fish community condition and upstream
catchment landcover in the Wellington region. Fish community condition is
significantly higher at sites located on rivers and streams draining
predominantly indigenous forest catchments than at those draining pastoral or
urban catchments. Based on limited monitoring, fish community condition at
some RSoE sites contrasts significantly with the water quality, periphyton and
macroinvertebrate indicators measured at these sites.
The majority of RSoE sites exhibited relatively stable water quality and
ecological health over the time periods examined (2006 to 2011 and 2004 to
2011, respectively). Generally speaking, the majority of statistically significant
trends tended to be indicative of improving water quality (predominantly
declining nutrient concentrations) but deteriorating ecological condition
(increasing periphyton cover/biomass and declining macroinvertebrate
community health). In most cases the reasons for the observed trends were
unclear. The presence of improving nutrient concentrations across a wide
spectrum of sites, including several reference sites, suggests that the
improvements are more likely related to natural factors such as climate
variability than changes in land use or land management practices. Similarly,
variation in river and stream flow probably influenced many of the trends
identified in periphyton and macroinvertebrate metrics.
Overall, while the absence of wide scale deteriorating trends in water quality
and ecological health is positive, many of the RSoE sites are considered
‘degraded’, with some very degraded when considered in the national context.
For example, most urban sites and several lowland pastoral sites recorded
nutrient concentrations well above their respective national median values for
similar urban and rural streams. The RSoE sites in poorest condition – in
particular those with small catchments dominated by urban or intensive
agricultural land uses – share in common one or more of the following
‘stressors’: nutrient enrichment, poor water clarity, nitrate or heavy metal
WGN_DOCS-#1100598-V3 PAGE 117 OF 160