Biological Environment of the West Coast Coastal Marine Area: an outline of Marine and Coastal Ecosystems and Habitats

3.3 The Estuarine Domain

The estuarine domain occurs along the entire coastal landscape of the West Coast and can be divided into three main ecosystem types based on their landform and degree of tidal influence:

• Tidal flat estuaries
• River mouths
• Tidal lagoons.

3.3.1 Tidal Flat Estuaries
Tidal flat estuaries occur in low-lying land behind a barrier beach/dune system. They are dominated by broad expanses of mud and sand flats that are exposed at low tide and flooded at high tide. Generally they have a high saltwater content because of the relatively low amounts of freshwater entering this type of habitat. Tidal flat estuaries are most common in the central and northern parts of the West Coast and are mapped in Figure 3.1 (which also shows the distribution of river mouths and tidal lagoons).

A typical tidal flat estuary on the West Coast would have the following features:

• a mouth dominated by tidal currents (flowing in on the flood tide and out on the ebb tide), and sediments similar to the adjoining beach. These areas are a migration route for ‘freshwater’ fish that spend a period of the life in the sea (e.g. the five whitebait species and flatfish) and other species (including larval stages). The mouths of estuaries are often mobile and can sometimes be temporarily blocked by the interaction of sea conditions, beach accretion and river flows;
• fine sand flats and permanently submerged channels inside the mouth area and in the middle reaches of the estuary. This is where beds of bivalves (e.g. cockles and clams) tend to occur, along with crabs and worms that together form a rich food source for fish and wading birds;
• mudflats towards the upper reaches of the estuary where tidal influence and water movement are reduced. The beds in this area are dominated by animals such as estuarine snails;
• saltmarsh areas (usually growing on mud or fine sand) that are dominated by rushes, reeds and turfs that provide for habitat cover for birds and insects;
• estuary margins that grade from intertidal wetland habitats to dryland areas covered in forest, pasture or residences; and
• rivers and smaller waterways that feed into the upper reaches of the estuary.

A tidal flat estuary, Okarito Lagoon. Photo: T. Hume, NIWA

tuarine saltmarsh occurs in coastal wetlands throughout the West Coast, but the largest areas of saltmarsh are found on the tidal flats of the estuaries. These communities are typically dominated by tall rushes and sedges, as well as smaller turfing species, but their species and structure are mostly distributed according to their salinity tolerances15. Other common plants in this coastal habitat are sea rush, jointed rush, noded sedge, shore pimpernel, remuremu, manuka and a variety of coastal dune and turf plants.

Saltmarsh margin at Okarito. Photo: DOC Collection	Saltmarsh rushes at Okarito Photo: DOC Collection

Sea rush (wiwi) is a common estuarine plant on the West Coast. It often grows in dense stands up to 75 cm tall, characteristically mixed with other saltmarsh species including sea primrose, other rushes and introduced pasture at higher-shore levels16.

Estuarine and freshwater fish surveys have been undertaken by the Department of Conservation, NIWA, the Museum of New Zealand and other agencies17. Largely due to habitat diversity, it is difficult to assess relative abundances of the species that have been recorded. However, it appears that bullies, inanga, flatfish, estuarine stargazers, triplefins and yellow-eyed mullet are among the most abundant fish in estuary and river mouth areas. While the numbers of fish in the West Coast estuaries can be quite high, the species diversity of these sites is quite low in the national context, with quite a strong freshwater influence on the types of fish present.18

Kotuku (white heron) are found throughout Australasia and Southeast Asia, but a colony of about 30–50 pairs19 on the Waitangiroto River is the only breeding site for the New Zealand population of this species. The kotuku feeds in nearby tidal flat estuaries, such as Okarito and Saltwater Lagoons, during the spring-summer breeding season. It then disperses more widely throughout New Zealand for the rest of the year. It is ranked as a ‘nationally critical’ threatened species20, though numbers of the same species are stable overseas.

Tidal flat estuaries are occupied by numerous fish and waterbirds:
Kotuku (white heron) Photo: P McClelland, DOC	Inaga (whitebait) Photo: G Eldon, DOC Collection

Tidal flat estuaries support populations of lowland and coastal plants and animals that have otherwise been reduced by land development on the West Coast (and elsewhere throughout New Zealand)21. These include a range of species unique to this habitat – cockles and other shellfish, migratory wading birds, lowland freshwater fish, and saltmarsh vegetation. In all, some of these species are confined to a total area of only several hectares within the West Coast.

Saltmarsh provides habitat for a range of species including invertebrates such as estuarine snails and provides good cover for secretive wetland birds such as bitterns, crakes and fernbirds.

An exposed cockle bed in the Orowaiti Lagoon tidal flat estuary, where these shellfish reach densities of several hundred per square metre, and are a food source for fish and wading birds. Photo: M. Rogers, DOC collection

Tidal flat estuaries of particular note with above average rankings for their natural wetland values22 include:

• Karamea-Otumahana Estuary23 – A large and diverse wetland with extensive saltmarsh within conservation lands, and intertidal shellfish beds. This estuary is considered to meet stated criteria for international conservation importance24;
• Orowaiti Lagoon25 – A large estuary with partly developed margins but extensive shellfish beds, and saltmarsh within conservation lands;
• Buller River mouth wetlands26 (Lost Lagoon, Floating Basin and Bradshaw’s Creek). These are outside the coastal marine area as legally agreed between Councils and the Minister of Conservation under the Resource Management Act, but have features characteristic of estuaries;
• Okari Lagoon27 is a site of cultural importance; while less diverse than some other estuaries, it has some areas of saltmarsh and shellfish beds;
• Saltwater Lagoon Pouerua28 is a large estuary with natural forested margins and catchments. Pouerua is considered to meet stated criteria for international conservation importance29;
• Okarito Lagoon30 – the largest estuary in the West Coast, with diverse habitats of sand flats, channels, mudflats and saltmarsh. This lagoon is also considered to meet stated criteria for international conservation importance31; and
• Oparara and Three Mile32 Lagoons also have some notable features but are generally smaller and have less habitat diversity compared to other West Coast estuaries.

Wetlands of national importance to freshwater fisheries were ranked by Davis (1987)33. A number of West Coast estuaries considered to be ‘outstanding’ (an ‘A’ classification) were:

• Orowaiti Estuary and associated wetlands
• Saltwater Lagoon Pouerua<
• Okarito Lagoon
• Okuru/ Turnbull/ Hapuka Lagoon

Those ranked as ‘significant’ (a ‘B’ classification) were:

• Otumahana Estuary
• Okari Estuary
• Grey River Lagoon

3.3.2 River Mouths
The river mouth ecosystem type on the West Coast refers to those estuarine environments which occur around the mouths of the large rivers which rise in the mountainous hinterland. These rivers typically have a high content of freshwater, open expanses of waterway, coarse beds of sand and gravel, and a relatively small tidal influence. River mouths are common throughout the entire 600 km coastline of the West Coast, from Kahurangi Point to Awarua Point and are mapped in Figure 3.1.

River mouths provide habitat for many coastal birds, such as dotterels, oystercatchers, terns and gulls. They are also migration routes for ‘freshwater’ species such as inanga, other galaxid fish, lampreys and eels, they provide habitat for those species as well as others like flounder, kahawai, smelt and bullies. River mouths often form essential connections between the sea and wetlands.

An example of a river mouth with substantially modified margins, Hokitika River. Photo: D. Neale, DOC.	An example of a river mouth with less modified margins, Arawhata River. Photo: T. Hume, NIWA

River mouths of particular note with above average rankings for their natural wetland values34 include:

• Heaphy
• Karamea
• Little Wanganui
• Buller
• Taramakau
• Arahura
• Hokitika
• Mikonui
• Karangarua
• Waita
• Haast;
• Okuru;
• Waiatoto
• Arawhata
• Cascade.

A number of river mouths are connected to non-tidal freshwater wetlands that are considered to be of international conservation importance35 . They include (with the wetland):

• Waita River (Tawharekiri Lakes)
• Waiatoto and Arawhata Rivers (Burmeister Morass)
• Cascade River (Hermitage Swamp)
• Whataroa and Waitangitaona River (part of the Whataroa Ecological Region coastal wetland complex)

Figure 3.1 Estuarine ecosystems. Source: Neale 2006a, Neale unpublished DOC data.

3.3.3 Tidal Lagoons Tidal lagoons are relatively widespread on the West Coast and include the ‘hapua-type’ lagoons36, a brackish wetland usually enclosed in a backshore dune hollow. Tidal lagoons are often natural channels at the outlets of small to moderately-sized coastal streams or swamps. Their water levels typically rise and fall with the tides, but the intrusion of salt water is mostly confined to the immediate vicinity of their outlet to the open sea. In this report, “tidal lagoons” are distinguished from “estuaries” because they have a generally narrower form that tends to reduce the encroachment of saltwater and tidal influences (see photo of New River tidal lagoon). The steeper channel margins of a tidal lagoon mean that tidal flats and saltmarsh habitats are mostly absent or greatly reduced, and the tidal stream channel is the main feature of these wetlands. Care should be taken to avoid confusion where the term “lagoon” is sometimes applied in common usage to wetlands that are defined in this report as “tidal flat estuaries” (e.g. Okarito Lagoon). A tidal lagoon, New River/Kaimata. Photo: D Neale, DOC. Tidal lagoons occur throughout the West Coast, but tend to be less common in the north (see Figure 3.1), where land development has sometimes led to the loss of these habitats. Many tidal lagoons flow to the sea via the mouth of a larger river, whereas those flowing directly to sea are often associated with unstable mouths that occasionally migrate alongshore or temporarily block up. Up to about 30 hapua-type lagoons have been identified on the West Coast (Neale, 2006a) and some of these have been studied in detail, often as part of resource consent investigations. The channels and riparian margins of tidal lagoons provide habitat for fish (e.g. whitebait) and bird life. The diverse forest and wetland communities that occur there are used by a wide variety of native birds, fish and other animals on a seasonal or year-round basis. Park (1993) notes the ecological importance of coastal lagoons (both tidal and non-tidal) to the natural functioning of lowland ecosystems. He also notes their historic depletion resulting from land development that has altered the habitats, margins and catchments of many such places.

Tidal lagoons that have been given above average rankings for their natural wetland values37 in the West Coast region include:

• New River/Kaimata
• Totara Lagoon
• Te Rahotaiepa
• Hikimutu
• Waitangiroto
• Five Mile Lagoon
• Waikowhai Stream
• Ohinetamatea
• Hermitage Swamp (Cascade River).

A tidal lagoon, Waikowhai Stream. Photo: T Hume, NIWA

ikoriri Lagoon (Black Creek) provides a link to Shearer Swamp, a wetland of international conservation importance38.
Wetlands of national importance to freshwater fisheries were ranked by Davis (1987)39. West Coast tidal lagoons considered to be ‘outstanding’ (an ‘A’ classification) were:

• Hermitage Swamp

Those ranked as ‘significant’ (a ‘B’ classification) were:

• Ship Creek Lagoon;
• Dune lakes and swamp between Haast and Waita Rivers; and
• Haast wetlands.

3.3.4 Existing Protection in the Estuarine Domain
The three types of estuarine ecosystems are mostly within the coastal marine area, with their marginal lands under a variety of land tenures. Saltwater Lagoon is the only fully-protected tidal flat estuary, as it lies wholly within conservation lands. Okarito, Okari and Three Mile Lagoons, three of the largest estuaries, have mostly protected margins and islands. Other estuaries have variable amounts of protection on their riparian margins. Totara Lagoon, Te Rahotaiepa River, Waitangiroto River, Five Mile Lagoon and Ohinemaka River are tidal lagoons in the West Coast coastal marine area that lie mostly within Crown conservation lands. There are also fifteen river mouths (listed in section 4.8.1) that lie mostly within Crown conservation lands. Several other estuarine ecosystems are partly within conservation land or have mostly protected margins of shrubland and forest. Several tidal lagoons are closed to whitebaiting under the Whitebait Fishing (West Coast) Regulations 1994 (see Chapter 5).

A tidal flat estuary, Saltwater Lagoon Photo: T Hume, NIWA	Most estuarine areas on the West Coast are used by whitebaiters during the spring, such as here on the Taramakau River. Photo: S Nimmo.

3.3.5 Uses Associated with the Estuarine Domain
Estuaries, river mouths and lagoons are often places where the demands of land development and nature conservation are required to coexist. Their margins are often desirable sites for land development for housing, farming and other uses. Discharges often occur into the waters feeding some of these coastal wetlands, either as point discharges (sewage, storm-water, animal waste disposal) or as more dispersed surface runoff. Such uses are generally controlled under the Resource Management Act 1991.

The estuarine domain provides for a wide range of human activities. Estuary shellfish beds, whitebait and game birds are recreationally harvested and ecotourism, particularly bird-watching, is a feature of some estuaries like Okarito Lagoon. River mouths are a source of gravel aggregate. They provide for a wide range of recreational activities, including fishing and whitebaiting, boating, fossicking, birdwatching and sightseeing. Some rivers, especially those adjoining settlements, are subject to discharges from industrial, commercial, residential and stormwater sources. Tidal lagoons are used for recreational activities such as boating, kayaking, canoeing, birdwatching, whitebaiting and eeling. The margins of many are developed for farming, residential or other land uses.

The margins of some estuarine areas are developed for housing or used for recreation
Orowaiti Esturay Photo: D Neale, DOC	Hayaks at Okarito wharf Photo: S Nimmo

15 Jones & Marsden 2005
16 Jones & Marsden 2005
17 e.g. NIWA (database), Roberts et al 2005
18 M. Morrison pers comm 2006
19 C Wickes pers comm 2006
20 Hitchmough et al 2007
21 Morse 1981, Coker and Imbodem 1980
22 Partridge 2004
23 Neale et al 1993
24 Cromarty & Scott 1996
25 Rogers et al 1996
26 Neale 1995, Stevens 1995
27 Neale 1998a
28 Neale 1998b
29 Cromarty & Scott 1996
30 Goff et al 2001, Neale 1998b
31 Cromarty & Scott 1996
32 Neale 1998b
33 See Appendix 3
34 Partridge 2004, see Appendix 3
35 Cromarty & Scott 1996, see Appendix 3
36 Neale 2006a, Kirk & Lauder 2000, Hart 1999
37 Partridge 2004, see Appendix 3
38 Cromarty & Scott 1996, see Appendix 3
37 Partridge 2004, see Appendix 3
38 Cromarty & Scott 1996, see Appendix 3
39 See Appendix 3

Figure 3.1Submission Form

website designed by hairyLemon