Conference 2003
Greening the City:
Bringing Biodiversity Back
into the Urban Environment
Abstract:
Aquatic
Macrophyte Planting: a Novel Way to Improve the Effectiveness of
Stream Enhancement Programmes?
Alastair
Suren, Scott Larnard & Marty Flanagan (NIWA, Christchurch)
Among aquatic ecosystems,
urban streams have undergone some of the most severe and diverse
human-caused modifications, with large changes to hydrological,
geomorphological, and chemical conditions, and resultant changes
to biological health. Techniques for restoring or rehabilitating
urban streams are at an early developmental stage. Four topics
in particular have received considerable recent attention: channel
rehabilitation, bank stabilization, riparian revegetation, and fish
(salmonid) population recovery. Rehabilitation of benthic biota
and in-stream processes such as primary production has generally
been neglected. People undertaking stream restoration programmes
make an implicit assumption that physical rehabilitation will be
followed by natural colonization of native species, and recovery
of in-stream processes.
Submerged and amphibious
macrophytes are important components of many stream ecosystems.
Macrophytes generate particulate nutrients, provide habitat for
epiphytes, invertebrates and fish, and modify streamflow, sediment
structure, and sediment and water chemistry. Re-establishment of
stream macrophytes following rehabilitation projects in agricultural
areas has been associated with improved water quality and increased
biodiversity. These observations suggest that native macrophytes
could be an important element of urban stream rehabilitation, and
their active introduction may help improve biodiversity of restored
stream sections.
In February 2002, the
Christchurch City Council commenced a stream enhancement project
at Papanui Stream, with the aim to create conditions favouring a
stable, self-sustaining ecosystem. A wooden-lined box drain was
removed from a 330 m stream reach and a new channel excavated.
The streambed was lined with geo-textile and gravel, then cobbles
were added to form riffles. Channel construction was completed in
August 2002.
Three native macrophyte
species (Callitriche petriei, Myriophyllum triphyllum,
and Potamogeton cheesmanii) were raised in a nearby rural
stream from locally sourced cuttings, and introduced into the lower
200 m section of the enhanced reach of Papanui Stream during October
2002. An upstream 100 m section was left unplanted as a control.
Our objectives in this study were to assess the survival and growth
of native macrophyte species transplanted into a newly-constructed
stream channel and to evaluate the effects of transplanted macrophytes
on stream community structure.
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