The Auckland region supports 21 native fish species (Stevenson & Baker, 2009).
Fourteen of these fish are diadromous, undergoing migrations between fresh and saltwater as part of their life cycle. Increased stormwater runoff associated with developed catchments can have a direct physical impact on aquatic environments. This direct physical impact is due to increased volume and contaminants, affecting both the diversity of habitats and the health of fish and aquatic invertebrates that use these areas for some (or all) of their life cycle. Water quality is a significant determinant of species richness in Auckland stream environments (Allibone et al., 2001).
Auckland streams cater for complex life cycle stages of aquatic fauna, both common and rare. Streams, wetlands and estuaries are recognised as highly productive environments for threatened and endangered species (Becker et al., 2001). WSD also promotes vegetated buffers for aquatic environments (streams, wetlands, spring seepages, etc.) from potential land use effects. In addition to assisting with local climatic effects and protection of habitats, these riparian buffers also help to moderate stormwater runoff quality and quantity effects. The riparian zone is the area of land adjacent to streams and wetlands that is the transition between land and water (Becker et al., 2001).
WSD promotes the protection and enhancement of urban streams from the impacts of developing catchments. The benefits of this approach include providing ecosystem services and their associated social, cultural and economic values, and increasing habitat diversity and biodiversity.
WSD methods to promote aquatic ecosystem health include:
- Keeping and enhancing streams and other freshwater environments
- Preserving and restoring riparian vegetation along banks, natural floodplains, wetland margins and the groundwater zones under streams
- Linking areas of riparian vegetation to create riparian corridors.
Promotion of continuous stream corridors
Greenways (also known as lineal parks), wildlife corridors and riverways are lineal open spaces linking natural, cultural and recreational areas in coincidence with streams or other lineal landscape features. Greenways provide the framework to protect, conserve and link natural resources and open spaces. In Auckland, greenways are being used to provide important cycle and walkways.
Continuous stream corridors do not need to comprise entire stream lengths (although this is ideal). 'Islands' of continuous habitat along streams also provide meaningful habitat, providing stepping stones for migratory animals, and acting as repositories for ongoing and future recolonisation of species in the catchment. In Auckland, stream corridors are marginal areas for development due to flooding constraints, steep slopes and poor aspect. However, stream corridors do have significant value as open space linkages between coastlines, ridgelines and volcanic cones. They can have diverse and continuous ecologies, and where they are suitably enhanced they can attenuate and treat stormwater runoff.
Some of the mechanisms to promote greenways are:
- Collaboration between landowners and community groups
- Providing greenways when preparing master and structure plans
- Auckland Transport and Local Boards partnering on the Greenways Project.
Headwater streams
In total length, almost 70% of Auckland watercourses are first order (or headwater) streams. When combined with second order streams, that percentage increases to almost 90% (O'Brien, 1999). Second order streams are those formed by the junction of two first order streams; a third order stream is formed by the junction of two second order streams, and so on (Figure 10).
Headwaters can include isolated pools, spring seepages, wetlands, and ephemeral and intermittent streams. These often support invertebrate taxa not otherwise present in permanent streams (Parkyn et al., 2006). These headwater areas also sustain downstream habitats through nutrient and carbon inputs, insect drop, flow moderation, and by through their contribution to base flows.
The large number and small size of headwater streams make them vulnerable to modification from land use change and development. It has been common for ephemeral and intermittent streams to be piped or filled as part of development. This large scale modification and culverting of smaller headwater streams throughout Auckland represents a significant risk to freshwater ecology, and the loss of these systems could have a dramatic effect on larger order watercourses downstream. When considering aquatic resource protection, it is important to consider the entire catchment and to recognise that all streams regardless of size are integral components of a catchment system. More information on Auckland's headwater streams can be found in ARC Technical Publications 310-313 Small Headwater Streams of the Auckland Region (2006).
WSD methods to maintain headwater streams include:
- Planning the road network to work with the stream network, minimising crossings and the need for culverts
- Identifying intermittent and permanent streams at the project onset, and preserving a buffer around them in the development layout
- Planting around intermittent and permanent streams.
Wetland environments
Wetlands are permanently or intermittently wet areas, shallow water and land-water margins that support a natural ecosystem of plants and animals adapted to wet conditions (RMA 1991, s2.1). Wetlands can take the form of spring seepages, swamp forest, bogs, fens, marshes or floating rafts. All of these diverse wetland types can be constructed and/or restored to provide for the values that wetlands contribute.
Wetlands provide for highly diverse species assemblages and they perform a very important role in the catchment by providing water quality treatment at the plant-soil-water interface, and by attenuating peak flows. WSD strongly promotes the restoration of natural wetlands for their inherent stormwater management processes. The application of wetlands to stormwater management is discussed in further detail in Section E.