Feature

Where Ocean Meets Coast

There's more than one body of water in an ocean, and the interactions between them are complex.

Dr Jonathan Sharples

New Zealanders have recently been made aware of how vulnerable the coastal marine environment is to the influence of the larger ocean. It is believed that the outbreak of toxic shellfish poisoning during the summer of 1992/93 was caused, in part, by the introduction of a species of plankton normally found offshore in the deep ocean.

One particular species from that event, Gymnodinium breve, caused respiratory problems for many people in Orewa on the northeast coast of the North Island. It also created havoc in the shellfish industry, as the mussels filtering the seawater containing the plankton became contaminated with harmful toxins. This plankton is still causing the closure of mussel farms around New Zealand.

Plankton are not able to travel far under their own power, so their distribution is determined principally by the movement of the water that they live in. Since the events of 1992/93, we have been trying to understand how this oceanic species of plankton was introduced into our coastal waters along the northeast coast and, in particular, how often this may be expected in the future. Answering these questions involves dealing with a difficult problem that is being tackled by oceanographers all over the world -- exactly how do the world's oceans affect the coastal environments, and how efficient is the exchange of water between the coasts and the deep ocean?

This perhaps seems a strange question. Looking out to sea from a beach you see an expanse of water stretching uniformly to the horizon. Surely the ocean reaches all the way up to the beach? However, if you sailed out from the beach and measured, for instance, the temperature and salt content of the water as you went, you would notice that the properties of the water change with increasing distance seawards.

Typically, the water nearer the coast is cooler and slightly less salty than that further offshore, because it is mixed with the fresh water from local streams and rivers. Continuing out to sea, a point is eventually reached where the temperature and salt content rapidly increase. This happens at the place where the seabed, which will have been deepening gradually with increasing distance from the beach, suddenly plummets to the depths of the ocean.

Shelf Break

This position on the seabed is called the "shelf break", usually occurs at a water depth of 150-200 metres, and marks the edge of the continental shelf. The region of water at this point, where the temperature and salt content suddenly change, is called the "shelf break front" by oceanographers. The word "front" is used to describe any region where quantities such as temperature and salt content change rapidly; meteorologists use the same term for regions in the atmosphere where the temperature changes very quickly.

What causes this front at the shelf break, and why is it important?

Relative to the waters of the deep ocean, waters on the continental shelf form only a very thin skin (one or two hundred meters thick, compared to three to five kilometres). As a result, the sudden step in the seabed acts as an effective barrier to the deep ocean currents flowing along the edge of the continental shelf, preventing the oceanic water crossing the shelf and reaching the coast.

Thus, on the continental shelf, the water tends to retain its lower salinity and lower temperature, and very little mixing occurs with the water on the ocean side of the shelf break front. This is analogous to the different weather experienced on either side of meteorological fronts shown to us in weather forecasts. Sometimes, however, this blocking effect of the shelf break front is temporarily broken. The problem for oceanographers is that, while we are aware that this can happen, we do not yet fully understand why it happens.

Much of the New Zealand coastline has a narrow continental shelf. In such areas, the contact between the deep ocean and the coast is potentially much more efficient because the oceanic water that crosses the shelf break front has only a short distance to travel before reaching the coast. The important point to note, particularly in the context of management of some of the effects of toxic plankton on New Zealand's shellfish industry, is that when this oceanic water reaches the coast it carries with it whatever species were living in it when it was offshore. We first need to try to observe such intrusions of oceanic water towards the coast, and then find out whether or not they can be implicated as having an effect on the coastal environment by, for instance, introducing toxic species of plankton.

Watching for Intrusion

Observing these intrusion events is, however, not easy. There are two principal methods traditionally available to oceanographers for taking measurements in the sea. We can visit the region with a research ship and make detailed measurements of the temperature, salt content, and plankton and nutrient concentrations over a large area. Alternatively we can leave recording current meters in the region for up to 12 months, recording hourly values of current speed and direction, and the water temperature.

Both of these methods have disadvantages. Observations from a research ship are carried out over a voyage lasting less than a month, which means we may not be guaranteed to "catch" an event of oceanic water breaking across onto the continental shelf. Current meters, on the other hand, can only measure temperature and water speed, which is a very limited subset of the information we require.

We are presently using both of these methods along the northeast coast. A series of six surveys of the area between Cape Brett and Hauraki Gulf have been completed, while at the same time seven current meter moorings have been placed in the region. For the six surveys we have used fast boats to allow us to cover the area in as short a time as possible (UDC Alert, operated out of Auckland, and Marlin Blue, operated by Norm McKinley out of Tutukaka).

During each survey we take a series of measurements at 40 positions between Cape Brett and Hauraki Gulf, from the coastline out to a depth of 300 metres. At each position the boat is stopped, and a set of scientific instruments (referred to as the CTD) lowered from the surface down to the seabed. The CTD continuously measures the water pressure (equivalent to the depth of the instruments in the water), temperature, salt content, and chlorophyll concentration as it moves down through the water.

The temperature and salt content can together indicate where the water has come from -- coastal or open ocean -- while the chlorophyll indicates the presence of phytoplankton (phytoplankton are marine plants that work in the same way as land-based plants, using chlorophyll for photosynthesis).

Data from one station, for example, in 200 metres of water near the Poor Knights Islands, showed a deep surface-mixed layer extending to a depth of 40 metres, indicated by the uniformity of all the variables (temperature, salinity, and chlorophyll) between the surface and 40 metres depth.

The thickness of this upper layer of the ocean depends on the recent strength of the wind: strong winds provide more turbulence, and so mix the surface water even deeper (around the New Zealand continental shelf this thickness can range between less than 10 metres and up to 200 metres).

Between about 40 metres and 80 metres the temperature changes rapidly, in a region referred to as the "thermocline". The importance of the thermocline is that it is a very stable region, through which it is difficult for turbulence to mix quantities like heat, nutrients, and phytoplankton. The chlorophyll results clearly indicate that most of the plankton are to be found in the thermocline, forming the mid-water chlorophyll maximum. This is a commonly observed feature in continental shelf and deep ocean waters during the summer months. It is caused by a slow leakage of nutrients into the thermocline from the deeper water. Within the thermocline a combination of very little turbulence and some light reaching down from the surface, allows the phytoplankton to use this nutrient and grow.

Few plankton are found in the surface water, despite the stronger sunlight, because all of the nutrients leaking upwards from the deep water are being used up by the plankton within the thermocline. By using sample bottles we can collect plankton from within the mid-water chlorophyll maximum, and preserve them for later identification under a microscope back at the laboratory. This is time-consuming work, but eventually we will have a picture of the distribution of the different plankton species superimposed on the information of the different water types. This will allow us to decide where some of the plankton originated, and at what times of the year we can expect them to arrive.

In addition to looking at the data from the surface to the seabed at one position, we can also look at the surface conditions over the entire region, to find out which parts of the coastline are being influenced by coastal water, and which by oceanic water.

In looking at an area north of the Hauraki Gulf, the conditions typical for winter vary from those found in summer. In winter, coastal water is found all along the coastline, and saltier, warmer water offshore towards the deep ocean. The transition region between these two types of water (the shelf break front) roughly follows the position at which the water depth is 150 metres. No part of the coastline in winter appears to be influenced by the oceanic water, a situation we have observed through autumn to late spring.

The summer conditions are, however, markedly different. North of the Hauraki Gulf, the water normally found further offshore moves onshore, and reaches the entire coastline: this is subtropical water from the East Auckland Current, an ocean current that flows southeastward along the northeast edge of New Zealand's continental shelf. This subtropical water apparently reaches only a short distance into the Hauraki Gulf, mainly through Jellicoe Channel. Any further progress into the gulf is strongly dependent on wind conditions.

Fishermen in the northeast are well aware of the periodic arrival of this ocean water at the coastline, as the warmer ocean water is also considerably clearer and more blue in appearance than the water usually found near the coast. Such summer intrusions of the blue water are relied upon for bringing species of fish, such as marlin and tuna, closer to shore. However, they are also important because whatever species of plankton have been growing in the water offshore, will also be introduced into the coastal region. Obviously, this has had, and may continue to have, a significant impact on how local aquaculture is managed, depending on the particular plankton species involved (only a few species are toxic).

At present, we believe that such summer intrusions of water from further offshore are likely to happen in most years, beginning in December and becoming well-established by late January, unless southwesterly winds (which push the water back offshore) are stronger than usual. It is important to note, however, that this has probably been a feature of this coastline for many years, and may be responsible for the diversity of many of the ecosystems along the northeast coast. Problems associated with toxic species of plankton arriving at the coast in the oceanic water are likely to reflect long-term changes in the offshore biology that, as yet, we know very little about.

Dr Jonathan Sharples is with NIWA (the National Institute of Water and Atmospheric Research Ltd).