Physical Environment of the West Coast Coastal Marine Area

2.5 Ecosystems Based on Depth, Substrate and Exposure

2.5.1 Spatial Distribution of Ecosystems Based on Depth, Substrate and Exposure
The primary drivers of depth, substrate, and exposure/energy (outlined in section 2.4 above) can be used to give a ‘three-dimensional’ picture of the physical nature of the West Coast marine and coastal environment, and thence the distribution of the main ecosystems. Figure 2.11 is a ‘two-dimensional’ attempt to show the widely varying spatial distribution of the main ecosystems along the depth and exposure axes, with the substrate dimension embodied partly in the size of the circles and partly in the ‘substrate descriptor’ for each ecosystem name (e.g. ‘sand and silt’ or ‘bedrock’).

Figure 2.11
Distribution of common examples of coastal and marine ecosystems on the West Coast, according to their depth, exposure/energy and substrate. Note: positions and sizes of each circle are indicative only.

It is important to note that the ecosystem types shown as named circles in the figure are simply illustrative of common examples that occur on the West Coast. They are not intended to be a comprehensive classification of the coastal marine area. In reality, while there are sometimes sharp physical boundary changes, there is more often just a gradual transition. For example, the intertidal depth zone steadily deepens out to the shallow subtidal and beyond, or some tidal lagoons flow out through river mouths to a sand or gravel beach.

2.5.2 Four Marine and Coastal Environmental Domains
The spatial distribution of the main ecosystems in Figure 2.11 admittedly looks complex. So to simplify the discussion to follow in Chapter 3 of the biological character of the West Coast’s marine ecosystems and habitats, the physical environment has been split into four broad ‘environmental domains’ (see Figure 2.12 and definition in box).

Environmental domains are areas with similar physical environmental conditions (as defined by factors including solar radiation, temperature, moisture and geological substrate) that have been demonstrated to have high correlations with plant and animal distributions.

These four environmental domains in the West Coast coastal marine area are the:

The boundary of one of these domains, the ‘estuarine environmental domain’, is outlined in Figure 2.11.


Figure 2.12:
Environmental domains of the West Coast coastal marine area

The names of the last three of the environmental domains listed above are related to the relevant depth zones (in section 2.4.1 above). One of the clearest physical divisions in the West Coast coastal marine area is between the ‘estuarine’ environments and the more ‘marine’ environment. In terms of their depth, the estuarine ecosystems shown in Figure 2.11 are mostly intertidal (but also include some shallow subtidal areas). However, they differ markedly from the open coast because they generally have a much lower level of exposure, and finer sediments. Compared to the intertidal beaches and rocky shores of the open coast, the estuarine environmental domain has lower salinity water, and riverine processes that are less affected by the influence of sea waves and coastal currents.

 
A tidal Flat estuary
(Three Mile Lagoon)
Photo: T. Hume, NIWA 		  A tidal lagoon
(Totara Lagoon)
Photo: D. Neale, DOC 		A river mouth (Whataroa River)
Photo: T. Hume, NIWA

Estuarine Domain
The West Coast has many enclosed estuaries and river mouths that connect with the sea through narrow entrances across barrier beaches. Intertidal mud and sand flats are prominent features of these areas, with permanent tidal channels that extend below the intertidal zone. Estuarine environments on the West Coast include broad tidal flat estuaries (e.g. Three Mile Lagoon), tidal lagoons (e.g. Totara Lagoon) and gravelly river mouths (e.g. Whataroa and Grey/Mawheranui Rivers). These are discussed in more detail in Chapter 3.

Intertidal (Open Coast) and Shallow Subtidal (Open Coast) Domains
The intertidal (open coast) domain extends from the MHWS line down to MLWS, and the shallow subtidal (open coast) domain reaches from MLWS out to about 30 metres depth. While the intertidal (open coast) domain can be quite different to the shallow subtidal (open coast) domain, the two are combined for the purposes of this section of the report. These two domains are areas where sharp physical changes occur across relatively short distances at a scale of metres or less. The effects of tides, currents and wave action are felt on the shore and seabed from the highest levels of the tide to depths of several tens of metres, and they dominate the physical dynamics of these domains. Breaking waves impart high levels of energy on the intertidal shore, reducing to lower levels with increasing water depth. The regular flood and ebb of the tide affects the environmental conditions and helps to determine the physical and biological patterns that are discussed in more detail in Chapter 3.

A typical rocky shore with reefs, off the Paparoa range coastline. Seal Island in the foreground.
Photo: D. Neale, DOC.

Because the West Coast faces generally towards the prevailing westerly winds and waves, most of the intertidal (open coast) and shallow subtidal (open coast) domains are very exposed to the waves and sea conditions of the open coast. The frequent sea storms and abundant supply of gravel from the rivers cause the West Coast’s ‘open coast’ shores to be heavily battered by waves and affected by sand and gravel scour.

Consequently, only a few areas on the West Coast shoreline are relatively less exposed to the direct impact of wave action and/or currents. These include sheltered bays and rocky shores with deep water immediately offshore (such as around headlands and islands). The lower energy conditions occurring in such places make them less affected by turbulence and sand scour.

 
Shallow subtidal reed near Cape Foulwind.
Photo: DOC Collection 		  A smal typical island with reefs and stacks. Wall Island off Tauranga Bay, near Cape Foulwind.
Photo: DOC.

The substrate at any particular place in these shallow marine domains is a distinguishing feature of the location’s physical nature. Relatively immobile hard rocky substrates (e.g. bedrock and boulders) contrast sharply with the mobile soft sediments (e.g. cobbles, gravel and sand). A combination of these sediment types often occurs at any particular site, but one or a few types are usually dominant The following discussion looks at the physical character (particularly substrates) of the two main types of open coast: rocky coasts and beaches.

a) Intertidal and Subtidal Rocky Substrates
Rocky shores and reefs are formed along the West Coast where mountain ranges approach the coastline. Examples are: the Kahurangi National Park mountains, Karamea Bluffs, Paparoa Range, Paringa-Moeraki coastal ranges, and Cascade-Awarua hill country. The geology, form and character of these rocky shores are quite varied, and they range in age from the Precambrian gneiss at Charleston to the Quaternary moraine deposits of central Westland and the Cascade Bluffs49.

The substrates of rocky coasts are mostly of bedrock and boulders and the type of rock can have an influence on the physical form and stability of the shoreline. Bedrock coasts are generally the most physically stable type of coastline. However, they can vary according to the geology of the rock, from resilient and steep granite coasts to more erodable sandstone and mudstone coasts. Bouldery coasts are less static, for the boulders can sometimes move under heavy wave action.

Only a few islands occur off the West Coast, but small rock stacks are a landscape feature in some areas. The largest and most biologically significant islands of the West Coast are the kilometre-long Open Bay Islands (Taumaka and Popotai), located some four kilometres off Haast50 (see Chapters 3 and 5 for a fuller account of islands and stacks).

(b) Intertidal and Subtidal Soft Substrates Uplift of the mountainous hinterland east of the alpine fault, coupled with the region’s high rainfall, has led to constant and on-going erosion of the land. It has been estimated that anywhere from 68 to 127 million tonnes of sediment is carried down the West Coast’s rivers and glaciers to the sea every year51, giving this region one of the greatest terrestrial erosion rates in the world. Most of this sediment is sorted into two main locations:

The longshore drift of sediments on West Coast beaches is mostly caused by wave action (which prevails from the southwest) which typically yield high volumes of sediment northwards, with net rates for most beaches probably in the range of 0.1 to 1 million cubic metres per year52.

 
A steep cobble beach, Ngaskawau
Photo: T. Hume, NIWA 		  A gently-shelving sandu beach, Kohaihai.
Photo: T. Hume, NIWA

West Coast beaches vary greatly in their physical composition, character and functioning. From a ‘bird’s eye view’, their forms are largely determined by the presence or absence of rocky headlands and major rivers. ‘Pocket’ beaches, such as those nestled between headlands on the Kahurangi, Paparoa and Paringa coasts, contrast sharply with the long beaches found where broad river plains meet the sea (such as those in the Karamea, Foulwind, Greymouth-Bruce Bay, and Haast localities).

When viewed in profile (horizontally), beach forms are predominantly a result of the shape and size range of their sediments. Consequently, there is a wide range of profiles, ranging from the gently shelving sand beaches of the Karamea and Cape Foulwind plains to the steep cobble beaches near Granity, Barrytown and Greymouth. Along considerable lengths of the West Coast, especially in areas between about Paparoa and Jackson Head, the beaches are not dominated by a single sediment size class, but are instead composed of a mixture of sand, gravel and cobbles.

Despite the prevalence of strong onshore winds, West Coast sand dunes are mostly low in height due to the coarse, heavy and moist nature of the beach sediments, which are unable to be moved far inland by the wind.

A mixed sand and gravel beach, Gillespie Beach.
Photo: T. Hume, NIWA


Deep Nearshore Domain
The deep nearshore domain extends from depths of about 30 metres (the outer edge of the shallow subtidal depth zone) out to the 12 nautical mile limit (the limit of the territorial waters). This domain mostly comprises the bed and waters of the continental shelf (which extends out to depths of about 200 metres), and some deeper areas of continental slope and submarine canyons lying within New Zealand’s territorial waters.

Off the northern part of the West Coast, the continental shelf (‘the shelf’) reaches out beyond the 12 nautical mile limit to the broad Challenger Plateau. In the southern part, the shelf narrows greatly toward the south, dropping off down the continental slope quite steeply in some places, to the depths of the oceanic environment of the Tasman Sea. The shelf becomes virtually non-existent at the southern end of the West Coast and off the steep coast of Fiordland (Fig 2.7). Between Hokitika and Big Bay, the West Coast shelf is dissected by submarine canyons, the largest of which originates near the Hokitika and Cook/Weheka Rivers.

The shelf is mostly covered with a deep bed of sediments ranging from fine muds to coarse sands and gravels. Not surprisingly, sedimentation rates on the West Coast’s shelf bed are among the highest in New Zealand, estimated by Norris (1978) to be 1.3 mm/year. The patterns of sedimentation for this coast have been summarised by Carter (1975), Price (1983) and others. Most sediment is supplied by the major rivers and it is mainly deposited on the beaches and continental shelf, or is lost offshore via the Hokitika and Cook Canyons and northwards alongshore toward the Challenger Plateau and Farewell Spit/Cook Strait regions. The sediments tend to be finer further offshore, but beds of gravels left by rivers during times of lowered sea levels (lag deposits) may be exposed in some locations.

Bathymetric charts show the more complex form of major nearshore features (e.g., the narrow shelf and the presence of canyons) in the south compared to the north. However, the substrates and other characteristics of the seabed south of about the Cook Canyon have not been documented in much detail.

Most studies report a predominance of fine sediments over most of the West Coast portion of the continental shelf. However, there are also some reports in published charts and literature of reefs and ‘foul ground’ (areas that are difficult or unable to be bottom-trawled, due to the predominance of snags, reefs or otherwise uneven seabed). In particular, Stevenson (2004) has mapped at a broad scale the distribution of foul ground identified by the NIWA trawl surveys, and these are shown in Figure 2.13. Other nearshore submarine reefs occur off the West Coast in depths greater than 30 metres. These include the Kahurangi Shoals in the north of the region, and some unnamed reefs off southern South Westland. 		 
Figure 2.13
Foul ground’ areas off the West Coast, as identified from NIWA fishery trawl
surveys.
Source: Stevenson 2004, RNZN (various dates).

49 Thornton 1985
50 e.g. see Neale (2006e)
51 Griffiths & Glasby 1985
52 Benn & Neale 1992

Figure 2.11Figure 2.12Figure 2.13Submission Form