Ecosystems are natural systems that include the organisms that live in a particular area, together with the physical features of the environment in which they live (for example, ‘rocky reef ecosystems'). This section talks about the types of ecosystems found in Tai Tokerau, and the kinds of things that make them special compared to other areas in New Zealand. We start by outlining the physical environment (e.g. the shape of the shore, waves, currents, climate etc.), and then discuss the living things that are found there.
New Zealand is a group of islands with no other land close by to shelter our shores from exposure to waves from the ocean. Most people in Tai Tokerau will have noticed that the waves on the western coast are generally bigger than those on the eastern coast. This is because most of the waves impacting on the New Zealand coast come from a south-westerly direction, and the western coast of Tai Tokerau is exposed to waves from this direction. The western coast of Tai Tokerau is therefore described as a moderate to high wave energy environment. In contrast, the eastern coast of Tai Tokerau is a lee shore, sheltered from the prevailing waves from the south-west by the land mass of Tai Tokerau. This coast is exposed to waves from predominantly northerly and easterly directions. These waves consist of ocean swells caused by sub-tropical storms, and waves generated by wind. The eastern coast can be described as a low to moderate wave energy environment (Laing, 2000).
The roughness of the sea, as indicated by the height of the waves, varies with the season. Maximum wave heights occur from May to August and minimum wave heights occur in December and January (Laing, 2000).
As well as the broad difference in wave action between coasts, there are also differences in wave exposure along the same coasts. These differences arise from the shape of the coastline, which can provide localised shelter from waves. For example, the waters in the harbours of the western coast of Tai Tokerau (such as the Kaipara and Hokianga Harbours) are more sheltered than those on the exposed open coastline (such as Ninety Mile Beach). The map in Figure 2 shows the shape of the coastline, and the major harbours of Tai Tokerau. You can see from this map that except for the harbours, the western coastline is relatively straight, although it curves gently to the north-west along Ninety Mile Beach. Between the Kaipara Harbour mouth and Maunganui Bluff, and along Ninety Mile Beach, this open coastline is comprised of mostly sandy beaches backed by sand dunes. Between Maunganui Bluff and Ahipara, there are sandy beaches between rocky headlands and inter-tidal reefs.
You can also see from Figure 2 that the shape of the eastern coast is much more irregular than the western coast. This coast consists of sand or gravel beaches, interspersed with rocky headlands and inter-tidal reefs. Islands are common inshore along this coastline, and there are also lots of harbours and estuaries. On the eastern coast, the orientation of the beaches (that is, the direction that the beach faces), and their fetch lengths, are highly variable. ( Fetch length is the distance that the wind can blow straight across water without being impeded by land).
In addition to ecological differences between the western and eastern coasts arising from physical differences, the general sea conditions and proximity of shelter make the eastern coast more attractive for boating than the open western coast, particularly as the western harbours tend to be protected by sand bars that can be hazardous.
Figure 2: Map of Northland showing the outline of the coast, and the main harbours.
A generalised cross-section of the land and sea-bed in Figure 3 shows how the land mass of New Zealand sits on the continental shelf. The sea-bed slopes relatively steeply off the edge of the continental shelf down to the floor of the ocean basin, and this region is called the ‘continental slope'.
Figure 3: Diagram of a generalised cross-section showing the slope of the sea floor off the coast of Tai Tokerau.
The continental shelf surrounding New Zealand varies in width from about five to 500 kilometres. Figure 4 below shows the water depth of the sea surrounding Tai Tokerau. The extent of the continental shelf is approximately indicated by the 250 m depth line. On the eastern coast the edge of the continental shelf is closer to the Tai Tokerau coast in the north (at North Cape and Cape Brett) than in the south (at Bream Head and Takatu Point), while on the western coast the width of the continental shelf does not vary as much from north to south.
Figure 4: Bathymetric chart of Northland. Note the extent of the continental shelf which is bounded to seaward by the 250 m depth contour highlighted in red. (Map adapted from Brodie, 1964).
Major Currents
There are two major ocean currents that influence the Tai Tokerau marine environment (See Figure 5 below). Both originate from the Tasman Front, which is the major current that flows in an easterly direction to the north of New Zealand (Carter et al. 1998). The West Auckland Current flows down the western side of Tai Tokerau, and the East Auckland Current to the east.
Different bodies of water (or water from different sources) may have different surface temperatures. Satellite images can be used to measure the Sea Surface Temperature (SST for short) around the coast of New Zealand. These images suggest that the West Auckland Current flows along the edge of the continental shelf and that inshore of this there is a northward-flowing current (Uddstrom and Oien, 1999). Sea surface temperatures suggest that the West Auckland current appears to be strongest in autumn and winter but weak or absent in late spring and summer (Uddstrom and Oien, 1999). However, the currents on the western coast of Tai Tokerau have not been studied much compared to those in other areas in New Zealand, and further research is required to fully understand water movement on this coast. (Some research in this area is currently being undertaken by the National Institute of Water & Atmospheric Research (NIWA)).
The East Auckland Current has a much larger flow than the West Auckland Current (Bury 2001), and its pattern of movement with respect to the edge of the continental shelf offshore from the coast has significant implications for the ecology of eastern Tai Tokerau. The edge of the continental shelf, which prevents the oceanic currents from coming onto the shelf itself, generally prevents the mixing of water on the shelf with that of the East Auckland Current off the edge of the shelf (Sharples et al. 1998). These two water masses thus tend to develop and retain significantly different physical, chemical and biological characteristics. (For example, the temperature of the water in the East Auckland current can be different from that of coastal waters, and there are different species of plankton and fish present). However, under wind and weather conditions that can occur in late spring and summer, a weak transfer of water across the shelf edge may occur, with oceanic water moving onto the shelf, and coastal water moving into the deep ocean. In regions where the continental shelf is narrow, such as on the north-eastern coast of Tai Tokerau, this cross-shelf exchange of water has the potential to introduce oceanic species (that is, organisms than normally live out in the deeper ocean rather than in waters close to the coast) into nearshore coastal waters (Sharples 1997, Sharples et al. 1998). People in Tai Tokerau often talk about the arrival of the “blue water” in summer. This is the annual intrusion of water from the East Auckland current onto the continental shelf.
Coastal Sediments
The composition of the sediment on the coast and the bottom of the sea varies throughout Tai Tokerau both with respect to the size of the particles in the sediment (e.g. coarse sand, fine sand, silt etc.), and what the sediment particles are made of.
The size of the particles of sediment found on the shore tends to be related to the roughness of the sea in the area. In sheltered areas where the water is generally very calm, the sediment particle size is small. This is because when the water is calm, fine particles can settle out of the water down to the bottom. In more turbulent waters, water movement keeps fine particles suspended in the water, so the sediment on the bottom is comprised of the coarser particles that are heavy enough to settle out. So for example, the sediment in sheltered harbours and estuaries is much finer (e.g. silt or mud) than on beaches exposed to rough waves.
You may have noticed that beaches in different areas of Tai Tokerau have sands of different colours. This is because the composition of sand varies from place to place. Sand may contain fragments of shell that have been broken up by wave action. It may also contain minerals (such as iron in the black sands of Muriwai Beach on the western coast). Fragments of rock that arise from the weathering of cliffs and headlands can also make up a very significant part of sand, so the material that forms sand may reflect the local geology. However, in some cases, (such as in the Hauraki Gulf) sand from some distance away may also be transported to the coast by rivers. In addition, sediment such as mud or clay can be washed by rain from the land directly into the sea, or into rivers or streams that carry it into the sea. As discussed above, this fine material tends to settle out in areas where the water is calm, and thus forms a higher proportion of the sediment in sheltered waters.
The sediment types on the open coast of Northland have been determined by sampling of marine beaches and foredune (i.e. sand dunes closest to the sea) sand and analysing their sand grain size, mineral content, and the proportions of minerals present (Schofield, 1970). Open west coast beaches of Tai Tokerau are composed of clean quartz-based sand. ( Quartz is a mineral comprised of silicon and oxygen, i.e. silicate). The sand is firmly compacted with little or no silt or clay. (Because the coast is rough, these fine particles are kept in suspension in the water). The quartz content of the sand averages about 56% of the sediment (with the shell removed), and has a low heavy mineral content of about 5% compared with the black sand found to the very south of the region (i.e. Muriwai Beach) (Applied Geology Associates, 1982). Erosion of the cliffs on the west coast provides a continuing sediment source.
The eastern coast is a little more complex in that it has three different main sand types. From Parengarenga Harbour to Doubtless Bay the sands are rich in silica (quartz). At the entrance to Parengarenga Harbour the sand averages 95% quartz, 5% rock fragment and less than 1% heavy minerals (Applied Geology Associates, 1982). This sand is very white in colour. Further down the east coast from Doubtless Bay to Ngunguru, the sediment has a higher rock fragment content (of greywacke rock), with approximately equal proportions of rock, quartz and feldspar sand and shell fragments (Applied Geology Associates, 1982) . ( Feldspar is a group of minerals consisting of silicates of aluminium). From Ocean Beach to Takatu Pt the sand is feldspar (55 to 76%) with quartz (19 to 33%). Mafic minerals (silicate minerals relatively high in heavier elements e.g. iron) and rock fragments constitute the remainder (Applied Geology Associates, 1982) .
>> Proceed to Chapter 3, "Climate" (Continuation of Section 3.2)