Physical Environment of the West Coast Coastal Marine Area

2.4 Characterising the Marine and Coastal Environment

Over the years several science-based classification systems have attempted amongst other things, to spatially classify the different estuarine, coastal and marine environments of the West Coast45. In the absence of a finalised national marine classification being developed for use with the Marine Protected Areas: Policy and Implementation Plan (MPA Policy)46, this section characterises different domains within the West Coast marine and coastal environment according to several important physical factors. The West Coast Marine Protection Forum can consider the physical patterns and processes explained above, and the characterisation based on depth, substrate and exposure outlined below, but will be guided mostly by the national classification under the MPA Policy when it is released.

Recent work relating to the MPA Policy by DOC and MFish officials on behalf of Ministers47 indicates that there are three important physical elements which help in describing and mapping the country’s marine habitat diversity. They are:

Accordingly, information on the West Coast’s marine and coastal environment is presented below in terms of depth, substrate and exposure. However, it is emphasised that the way this has been applied does not necessarily relate to the number of ecosystems, habitats, or areas the Forum will recommend for protection.

In the West Coast coastal marine area, water depths obviously range from shallow at tidal limits on open coasts and at river mouths, out to much greater depths across the continental slope and submarine canyons offshore. Substrates can vary according to their texture, geology and shape. The degree of exposure to energy sources such as waves, tides, and currents can have an important bearing on the character of a coastal area. Depth, substrate and exposure are to some extent interrelated and in combination they can lead to the formation of a variety of landforms, ecosystems, and habitats.

2.4.1 Depth
Depth has an important influence on the character and biology of the marine environment and the depth zonation of shores has been a key concept in marine science for a long time. The physical and biological changes are very obvious as one goes from the terrestrial and freshwater margins of the marine environment out to the ocean depths.

The coastal marine area of the West Coast covers a wide range of depths from shallow terrestrial and freshwater environments to ocean floors at over 2000 metres depth. The following list describes the different depth zones, with a brief definition of what each term means. They are illustrated in Figure 2.10.


Figure 2.10
A schematic diagram of depth zones of the West Coast, based on a profile off the Karangarua River mouth.

2.4.2 Substrate
The term ‘substrate’ refers to the surface of the seabed or land. Substrate also influences the marine biology of an area: that is, different species can often be found to live on different substrates. One of the most obvious substrate contrasts is that seen between ‘hard’ rocky shores and ‘soft’ sandy beaches. Hard substrates usually comprise:
These two substrates are typically dominated by species that need to attach to the hard and immobile surfaces of the rocks (such as mussels and seaweeds).
Soft substrates are those that are more readily moved by the action of waves and currents near the coast and at most continental shelf depths. They include (from largest to smallest size range) These substrates often support species (such as surf clams and worms) that can burrow into unconsolidated sediments that move with the waves and tides.
Substrate types can vary widely according to: There is some information on where these different sediment types occur on the bathymetric data on the regional map in Figure 2.13 later in this chapter and on the more detailed maps shown in chapter 5.

2.4.3 Exposure
‘Exposure’ refers to the levels of energy that a site is subjected to by natural processes. Exposure (or energy) tends to have a more subtle effect on the biology of an area than depth and substrate. While the exposure of a site is usually considered in relation to waves, the exposure or energy levels might also be influenced by other features, such as currents, tides, winds, gravity and infrequent catastrophic events (e.g. flood, storm surge, or submarine landslide). A convenient way to consider the relative exposure of different sites is by dividing them into

The exposure of a substrate to wave energy tends to be greatest on intertidal marine shores, diminishing both landwards (onto the land and up rivers) and seawards (into greater depths). A clear example of these variations in exposure is given by contrasting an estuarine sand flat with an intertidal sandy beach. While both may be at the same elevation and of the same sediment type, the greater exposure to waves in the beach system makes it (among other things) steeper and more mobile than the estuarine flat.

Other Factors
Other physical factors can also affect the character of the coastal and marine environment. Two factors that have received particular attention in the scientific literature of the West Coast48 are salinity and the ways that the coastal and ocean currents affect the coastal marine area.

Exposed Paparoa Coastline
Photo: D. Neale, DOC 		Sheltered coastline, Jackson Bay/Okahu
Photo: N. Shears, Auckland University

45 e.g. King et al 1985, McEwen 1987, Francis 1996, Neale & Nelson 1998, DOC 2004, Roberts et al 2005, Walls 2006a, b
46 DOC & MFish 2005
47 Walls 2006a


48 e.g. Heath & Ridgway 1985, Moore & Murdoch 1993, Heath 1982, Stanton 1976

Figure 2.10Submission Form