River and stream water quality and ecology - Greater Wellington ...
River and stream water quality and ecology - Greater Wellington ...
River and stream water quality and ecology - Greater Wellington ...
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Chlorophyll a (mg/m 2 )<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
(5)<br />
(21)<br />
<strong>River</strong> <strong>and</strong> <strong>stream</strong> <strong>water</strong> <strong>quality</strong> <strong>and</strong> <strong>ecology</strong> in the <strong>Wellington</strong> region: State <strong>and</strong> trends<br />
(40) (55)<br />
0<br />
2004 2005 2006 2007 2008 2009 2010 2011<br />
Figure 5.8: Chlorophyll a concentrations at Ruamahanga <strong>River</strong> at Gladstone,<br />
based on annual periphyton biomass sampling between 2004 <strong>and</strong> 2011. The<br />
numbers in brackets indicate the accrual period (days) prior to sampling <strong>and</strong> the<br />
red horizontal line indicates the MfE (2000) 50 mg/m 2 threshold<br />
The reasons behind the increasing periphyton cover <strong>and</strong>/or biomass trends at<br />
such a large number of sites across a wide range of l<strong>and</strong>cover types are unclear.<br />
At two sites changes in sampling location may be behind the apparent increase<br />
in periphyton growth. For example, the increase in both annual mean <strong>and</strong><br />
annual maximum filamentous periphyton cover at Pauatahanui Stream at<br />
Elmwood Bridge is the likely result of shifting the site 40 m down<strong>stream</strong> in<br />
2008 to enable easier access to the <strong>stream</strong>. Habitat at this down<strong>stream</strong> site is<br />
more suitable (gravel/cobble) for periphyton growth compared to the former<br />
silt-dominated up<strong>stream</strong> site. Similarly, trends in periphyton growth at Hutt<br />
<strong>River</strong> at Boulcott may be related to the sampling site being shifted to a<br />
shallower site 100 m up<strong>stream</strong> in 2009 when the previous sampling site became<br />
too deep to access safely.<br />
There were no clear links between trends in periphyton growth <strong>and</strong> trends in<br />
<strong>water</strong> <strong>quality</strong> variables such as nutrient concentrations <strong>and</strong> <strong>water</strong> temperature<br />
that are known to be key drivers of periphyton growth. At most sites where<br />
clear increases in periphyton growth <strong>and</strong>/or biomass were detected there were<br />
no significant increases in nutrient concentrations; in fact, the temporal trend<br />
analysis presented in Section 4.2 indicates that significant decreases in TN<br />
<strong>and</strong>/or TP concentrations were recorded at some of these sites (although <strong>water</strong><br />
<strong>quality</strong> trends were assessed over a shorter period). The exception was<br />
Mangaroa <strong>River</strong> at Te Marua which exhibited a significant increase in TP<br />
concentrations.<br />
Variation in river flow over the 2003/04 to 2010/11 period may be a key factor<br />
behind the observed trends in periphyton growth. Due to the occurrence of<br />
frequent freshes during the summer months of 2003/04 <strong>and</strong> 2004/05,<br />
periphyton biomass samples taken from many RSoE sites during these years<br />
followed only a short accrual period <strong>and</strong>, in some cases, coincided with lower<br />
WGN_DOCS-#1100598-V3 PAGE 71 OF 160<br />
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(22)<br />
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