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

River and stream water quality and ecology in the Wellington region: State and trends
8.5 Monitoring limitations and knowledge gaps
Greater Wellington’s RSoE monitoring programme has provided a significant
amount of knowledge about the health of rivers and streams in the Wellington
region. However, there are some limitations associated with the monitoring to
date, as well as some more general knowledge gaps. The main limitations and
knowledge gaps are outlined below.
While the RSoE programme has remained relatively stable in terms of
monitoring sites and variables in recent years, there have been several
changes in analytical laboratory over the last ten years, the most recent
change being in July 2006 (for both water quality analyses and periphyton
and macroinvertebrate identification). There have also been some changes
in sampling personnel, most notably the use of external contractors
between July 2006 and December 2007. Despite a number of checks
being put in place to maintain consistency in sampling and analytical
methods, a review of the RSoE data collected during the preparation of
this report does indicate that there have been some effects on data quality,
particularly for some water quality variables. For this reason the length of
the temporal trend assessment period was restricted to the five most recent
years. While a five-year period is considered appropriate for analysing
recent trends in water quality data collected at monthly intervals
(eg, Ballantine et al. 2010), assessment over a longer period would also
have been desirable. Potentially the five-year period assessed here
prevented the detection of some significant changes in water quality
(Ballantine et al. 2010).
In addition to the change from external to in-house water quality sampling
personnel during the five-year trend period, changes in some of the RSoE
sampling runs meant that sampling times at some RSoE sites varied across
the reporting period. As noted in Section 4.2, these changes impeded
robust trend analysis for field measurements, particularly water
temperature, dissolved oxygen and pH which all exhibit diurnal variation.
The need for standardisation of field data collection, ideally to within +/- 1
hour, has been emphasised by Davies-Colley et al. (2011) in a recent
report setting out recommended variables and protocols to support the
establishment of a national river water quality monitoring programme.
This same report also recommended continuous monitoring of diurnally
fluctuating variables where possible.
While flow-adjusted trend analyses were possible for water quality data
from a much greater number of monitoring sites than in the last assessment
of trends in river and stream water quality (Milne & Perrie 2005), there are
still 11 sites where it has not yet been possible to derive an estimate of
flow robust enough to enable flow-adjusted trend analysis. This is a
significant limitation given that flow is known to influence some aspects
of water quality, particularly optical properties and total phosphorus and E.
coli concentrations (eg, three RSoE sites exhibited declining trends in
visual clarity based on ‘raw’ measurements but the lack of flow data
means that it is not known whether the trends are simply a result of
changes in flow). Further efforts are required to obtain robust flow
estimates for the affected sites; this may require manual flow gaugings at
WGN_DOCS-#1100598-V3 PAGE 115 OF 160