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Chch waterway ecology & health investigated following quake

1 September 2011

Christchurch waterway ecology and health investigated following earthquake

Two reports commissioned by Environment Canterbury and Christchurch City Council that summarise how the February earthquake affected the ecology and health of Christchurch’s two main rivers and estuary have been released.

In all, eight studies were undertaken by NIWA, EOS Ecology, University of Canterbury and Aquatic Ecology to help determine the ecological effects of wastewater discharges and physical changes to waterways. The reports also provide advice on how to manage the effects and impacts of the earthquakes.

The studies focused on toxicity levels, aquatic invertebrate and fish communities, inanga (whitebait) spawning habitats as well as identifying ecologically important areas in the Avon/Ōtākaro and Heathcote/Ōpawaho rivers. They also looked at morphological effects, estuarine habitat and food safety of cockles in the Avon-Heathcote Estuary/Ihutai.

Ken Taylor, Environment Canterbury’s Director of Investigations and Monitoring, said the earthquake had both direct and indirect impacts on Christchurch’s waterways.

“The physical characteristics of some of our waterways have been altered following February’s earthquake. River bed levels have changed and banks have collapsed, altering the alignment of channels. There have also been significant changes to the bed of the Avon-Heathcote Estuary/Ihutai.

“The discharge of sewage, trade waste and sediment-laden stormwater into waterways has also resulted in physical changes to water and altered the water chemistry. This has impacted on the local ecology including vegetation, fish, invertebrates and micro-organisms,” he said.

Immediately following the earthquake large volumes of untreated wastewater were discharged into both rivers. It is estimated around 35,000 cubic metres per day of wastewater was discharged into the Avon/ Ōtākaro in March, with the volume decreasing significantly in the following months.

In late April to early May modelled and observed dissolved oxygen (DO) concentrations in some parts of the rivers were below guidelines for aquatic species. With higher discharge levels in February and March the DO concentrations were likely to be very low in some areas meaning fish kills were possible in both rivers. A study looking at survival of common invertebrate species found that the changes in water quality resulting from the wastewater discharges increased the mortality of sensitive species such as freshwater hoppers.

Studies investigating fish and invertebrate populations in the upper reaches of the Avon/ Ōtākaro, above the influence of most wastewater discharges, found there had not been major changes in the abundance or types of species present. Site-specific changes were found though, particularly where habitat changes due to sediment inputs had reduced the occurrence of sensitive species, such as caddisflies and bluegill bullies.

A survey of inanga spawning sites found that damage to sites on both rivers was highly variable, ranging from minor to severe. Inanga eggs were observed at only the Avondale site but this may not reflect earthquake effects alone,

The input of wastewater into the two rivers and the estuary has also been found to have had a significant impact on the Avon-Heathcote Estuary/Ihutai Bacteria levels increased dramatically after the February earthquake and the levels in cockles at the two river mouth sites were almost twice as high as any previous record when treated wastewater was discharged into the estuary, and more than four times as high as levels measured since the treated wastewater discharge was removed in March 2010. There has been consistent advice since the February 22 earthquake not to gather or eat shellfish from the estuary.

There have also been changes in bed height and estuary depth. The northern part of the estuary subsided between 0.2 and 0.5m and the southern part of the estuary rose by between 0.3 and 0.5m. The area affected by liquefaction mounds was found to be extensive, which affects estuarine biota by smothering and changing of dominant sediment particle sizes. The liquefaction sediments contained lower concentrations of heavy metals and organic matter than the pre-existing sediment, which may aid in estuary rehabilitation.

“The ecological and microbiological studies of our waterways following February’s earthquake will enable informed decisions to be made on a number of important issues. These issues include recreational water use, as well as works in and around waterways to stabilise banks and restore flow capacity.

“Environment Canterbury and Christchurch City Council will continue to work with Community Public Health and all of the agencies involved in the research to review urban water issues,” said Ken Taylor.

The study suggests a number of actions to mitigate the effects of the earthquake on waterways. For example it recommends measures to improve the spawning attractiveness of areas on the two rivers. This includes sediment trapping fences, weed control measures and sediment removal.

The study also identifies five ecological values that need to be considered should any major in-stream or bank-side remedial works be taken. These include trout and inanga/whitebait spawning areas, native fish population and areas supporting significant invertebrate and macrophyte communities.

For summary reports of the post-earthquake studies undertaken by NIWA, EOS Ecology, AEL and the University of Canterbury, with support from Environment Canterbury and CCC visit http://ecan.govt.nz/services/online-services/monitoring/pages/ecological-effects-christchurch-waterways.aspx .

Up-to-date information on public waterways affected by sewage discharges can be found at www.ecan.govt.nz/services/online-services/monitoring/ .

ENDS

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