Main Navigation

• Home
• Executive Summary
• 1.0 Introduction
• 2.0 Methods
• 3.1 The Oceanography of the New Zealand Marine Ecoregion
  • References and further reading
• 3.2 Cetaceans, seals, and seabirds
• 3.3 Fishes
• 3.4 Benthic invertebrates, algae, and plants
• 3.5 Information gaps
• 3.6 Future steps for the future of Biodiversity
• 4.0 Discussion
• 5.0 References
• Appendix 1
• Appendix 2
• Appendix 3
• Appendix 4

3.1 The Oceanography of the New Zealand Marine Ecoregion

Print this page

New Zealand is a relatively long, narrow archipelago that lies athwart the West Wind Drift and forms the western boundary to the South Pacific Ocean south of 34°S. The position of New Zealand and the shape of the bathymetric platform ensure that currents are guided by the topography (Carter et al. 1998). This results in shelf edge currents, uplift over shallow features, and oceanic eddies that interact with coastal waters. Oceanic water maintains an intimate contact with coastal waters. Offshore currents are particularly important in maintaining shelf-break fronts, e.g. the East Auckland Current and the Southland Current/Subtropical Front.

The South Pacific western boundary current, the East Australian Current, separates from the coast of Australia and a flow of water crosses the Tasman Sea forming the Tasman Front. A portion of this warm, salty Subtropical Water (STW) flows adjacent to the northeast New Zealand landmass to form the East Auckland Current, which transports water southwards along the northeast continental shelf (Stanton et al. 1997, Stanton and Sutton 2003). Most of this flow deflects south around East Cape as the East Cape Current before forming the northern side of the Subtropical Front (STF) (Tilburget al. 2001, Carter et al. 1998, Heath 1985). Three permanent eddies lie offshore of this boundary current: the North Cape, East Cape, and Wairarapa Eddies (Roemich and Sutton 1998). Currents probably uplift over certain bathymetric features such as Mernoo Gap, East Cape Ridge, Puysegur Bank, and Three Kings Rise (e.g. Bradford and Roberts 1978, Bradford et al. 1991).

New Zealand intersects the circumpolar STF which separates the subtropical gyres from the Southern Ocean, i.e., warm, salty STW from cold, fresh Subantarctic Water (SAW). The STF passes south of Australia and Tasmania and approaches New Zealand at around 45°S off Fiordland (Heath 1985). The front deviates south, along the continental margin, before following the shelf break northwards along the east coast of the South Island, where it is locally known as the Southland Front and has an associated current called the Southland Current. The Southland Current advects mainly SAW with peak surface speeds of 20-30cm per second (Chiswell 1996, Sutton 2003). The STF turns east along the crest of the Chatham Rise at 43.5°S where it is constrained by the shallow bathymetry to a limited depth of 300-350m and a narrow width of approximately 100 km (Sutton 2001). Although the southern limits of the South Island are at latitudes normally associated with SAW, in fact the entire coastal region is bathed in water of STW origin, with the transition to SAW (i.e. the STF) occurring at the continental shelf break around the southern extreme of the South Island.

New Zealand's semidiurnal tides (M²and N₂) have a complete 360° range of phase around New Zealand (Walters et al. 2001). The semidiurnal tides have been characterised as a coastally trapped Kelvin wave travelling anticlockwise around the shelf. Tidal elevations increase towards the coast with a degenerate amphidrome situated at the centre of New Zealand (Heath 1985). A by-product of this geometry is that tides are always 180° out of phase through Cook Strait, resulting in very high tidal velocities through the strait. High tidal velocities also occur north of Cape Reinga and in Foveaux Strait. These areas of strong tides are associated with tidal mixing (Bowman et al. 1980).

New Zealand is located on the pole-ward boundary of the South Pacific subtropical gyre in the southwest Pacific Ocean. For this reason shelf edge (at 200m) nitrate concentrations are modest (about 5-15mmol m^-3) in comparison with many regions of the world (Conkright et al. 2002) and are similar in range to the northeast Atlantic Ocean, although this statement is based on very limited local data. The absolute concentrations of dissolved inorganic nutrients in oceanic water around New Zealand depend on the water mass involved. Coastal water is mainly of subtropical origin, although SAW lies adjacent to the southeast South Island slope. These two water masses have different nutrient characteristics that result in a north to south gradient in mean nitrate at 200m at the shelf break (Ridgway et al. 2002). The distribution of mean nitrate ranges from 10mmol m^-3in the northwest to 16mmol m^-3in the southeast. The distribution on the west coast ranges from 10mmol m^-3 in the northwest to 12mmol m³in the southwest, with a minimum off the central west coast of =6mmol m^-3.

STW has a typical mix of nutrients (Tomczak and Godfrey 1994). Nitrate (NO₃) and dissolved reactive silica (DRSi) are depleted more or less together (Zentara and Kamykowski 1981). SAW, as well as being low in iron and copper (Sedwick et al. 1997, Croot and Hunter 1998), has an excess of NO³relative to DRSi (Zentara and Kamykowski 1981). The interaction of SAW and STW at the STF, especially over the Chatham Rise, results in this region being highly productive (Bradford-Grieve et al. 1999). In addition, atmospheric transport of iron from arid and semi-arid parts of Australia may be a source of iron to surface seawater in this region (Kieber et al. 2001; Boyd et al. 2004). It is thought that freshwater is generally not an important source of nutrients on the open coast, given the degree of dilution (e.g. Hawke and Hunter 1992). Deep winter mixing and upwelling of deep waters are more likely to be dominant in enhancing nutrients in surface waters.

The nutrient content of the oceans has implications for ecosystem structure and function. For example, the Southern Plateau behaves like a system that is low in nutrients (despite high NO₃) and primary production, with phytoplankton dominated by very small cells. It is a low total biomass, low productivity system with high transfer efficiency (Bradford-Grieve et al. 2003). In this region there is very little organic matter arriving at the sea floor and most of the production occurs in the water column. This system is apparently tightly coupled. On the other hand, productivity is much greater in the STF over the Chatham Rise, phytoplankton cells are much larger, and there is much more sedimentation of organic matter to the sea floor (Nodder and Northcote 2001).

It is not only the water mass that has significance for nutrient supply to phytoplankton. Mesoscale processes that influence the supply of nutrients to surface waters and seasonal patterns of heating and cooling (as reflected in the depth of the surface mixed layer) are significant contributors to the patterns of phytoplankton distribution as seen from space as sea colour (Murphy et al. 2001).

The dynamical signature of the warm-core eddies down the east coast of the North Island results in deep, surface winter mixing (greater than would occur in the adjacent water from which the eddies formed (Bradford et al. 1982, Bradford and Chapman 1988). These eddies contribute to enhanced winter nutrient renewal, lower winter phytoplankton biomass, and are probably responsible for the generally extensive spring phytoplankton bloom in the region (Murphy et al. 2001). Not only do these eddies change the seasonal pattern of nutrient renewal but they can also retain larvae of coastal organisms (Chiswell and Booth 1999) or entrain coastal water taking it offshore (Bradford and Chapman 1988).

Winds are important in driving upwellings that bring nutrient-rich water to the surface. In the Southern Hemisphere winds blowing parallel to a coast cause surface waters to move to the left of the wind. Thus a northwesterly wind blowing along the coast north of Auckland (Sharples and Greig 1998) and a southwesterly wind blowing off the west coast of the South Island (Stanton and Moore 1992) will move surface water away from the coast and bring nutrient-rich deeper water to the surface inshore. A conspicuous summer feature of western Cook Strait is a large upwelling plume that originates from Kahurangi Point (Bradford-Grieve et al. 1986) that is responsible for considerable biological enhancement in the region.

The seasonal pattern of mixed-layer depth varies from north to south with water mass and with the large-scale circulation (Longhurst 1998). The seasonal pattern of mixed layer depth interacts with the nutrient characteristics of the water masses, freshwater inflow, and light penetration (mainly through the depth of the sunlit surface layer) to determine the seasonal patterns of primary production, nutrient depletion, and ecosystem structure.

A comprehensive overview of the distribution of plankton is available only for phytoplankton as observed from space through the time period 1997-2000 (Murphy et al. 2001). STW to the north and in the Tasman Sea has a classical cycle of spring and autumn chlorophyll blooms consistent with production being co-limited by nitrate and light. Chlorophyll-a concentrations varied annually between about 0.1 and 0.4mg m^-3to the north and in the Tasman Sea, but east of New Zealand the mean maximum was 0.8mg m^-3. SAW has a low-amplitude annual cycle of chlorophyll abundance that peaks in early autumn, consistent with production being limited predominantly by a combination of iron and light. SAW chlorophyll-a concentrations varied from 0.1 to 0.3mg m^-3and rarely exceed 0.4mg m^-3. Chlorophyll-a is generally greatest in the STF and has the greatest variability with concentrations varying from =0.1mg m^-3to =1mg m^-3. Through winter, elevated chlorophyll concentrations over the Chatham Rise often occur in a narrow band south of the Rise and tend not to extend as far as the Chatham Islands. At other times of the year there was a broader and more complex region of increased chlorophyll stretching across the whole length of the Rise and beyond the Chatham Islands. Chlorophyll concentrations were higher in the STF east of New Zealand (mean 0.6mg m^-3) with some evidence of a spring and autumn peak in the seasonal cycle compared with the STF to the west (mean of 0.4mg m^-3) and without a consistent seasonal pattern across the years analysed.

There is no similarly extensive picture of the quantities of zooplankton present in the New Zealand region. Bradford and Roberts (1978) show that, using limited data for the whole New Zealand region, there is a significant positive correlation between zooplankton biomass and surface chlorophyll-a. Nevertheless, no such relationship is evident for STW analysed separately. It is clear that the zooplankton biomass picture is much more complicated than can be interpreted from the limited data that we have for the New Zealand region. Any understanding of the spatial and temporal distribution of zooplankton in the New Zealand region will have to take into account the dynamics of the planktonic production system, ecosystem structure, and trophic relationships among plants and animals in the system.

Contributors: Janet Bradford-Grieve and Philip Sutton.