Oceans

Oceans

You can go to a coffee shop in Oslo, which is at 60 degrees north, and sit out there in your shorts and t-shirt and have a coffee in the summer, and the reason you can do that is because the Gulf Stream transfers an inordinate amount of heat to Europe. It’s basically why Europe survives.

Lionel Carter explaining the thermohaline circulation that warms Europe, broadcast in Our Changing World, National Radio, 9 February 2006

Elizabeth Batham (1917-1974) began her scientific career in botany, but marine science was to become her great love. Her doctoral research on anemones, which she completed at Cambridge University after the Second World War, is still referred to in modern textbooks today.

Born in Dunedin, Batham pioneered marine biology, but one of her greatest achievements was the establishment of the Portobello Marine Biological Station on the Otago Peninsula. She assumed control of the derelict station in 1951 and worked single-mindedly towards its refurbishment.

At the time, there was no road to the station and the only access was by ferry from Port Chalmers. When this service was cancelled, Batham travelled by canvas-covered canoe or on foot, carrying supplies over the paddocks.

Batham was in her 50s when she learned to scuba dive, considering it a necessary skill for her research, but ill health forced her to give up her directorship at Portobello in 1974. A few months after her retirement, she disappeared and was later presumed to have drowned off Seatoun, in Wellington.

Ocean conveyor

The world’s oceans are a major player in weather and climate. Moved along by tidal forces and wind, they transport heat around the continents via large global circulations. Another force that makes the oceans move is the differential in the water’s density, which is determined by temperature (thermo) and salinity (haline). Such density differences drive the world’s largest global deep-water current, the thermohaline circulation or global ocean conveyor.

Marine scientist Lionel Carter, of the National Institute of Water and Atmospheric Research, says the circulation is at its largest when it passes New Zealand off the Chatham Rise, on its way between Antarctica and the Pacific Ocean. The cold and dense water gradually warms and rises to the surface as it travels via the Indian Ocean to the Atlantic. In the North Atlantic, it forms what is known as the Gulf Stream, a warm current that passes Europe before it cools and the waters sink again. It takes 1500 years for the oceans to complete one such cycle.

Disruptions of this global heat transport can have major consequences. The last time the ocean conveyor shut down was about 11,000 years ago, during a period called the Younger Dryas, when excess fresh water and ice spread into the North Atlantic from a large Canadian lake. Average temperatures in the North Atlantic fell abruptly by nearly 5°C and remained low for more than a millennium.

EL niño and La niña

El Niño is a disruption of the ocean-atmosphere system in the tropical Pacific, with important consequences for weather around the globe, including increased rainfall across the southern tier of the United States and in Peru and drought in the West Pacific, sometimes associated with bush fires in Australia. In normal, non-El Niño conditions, the trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the West Pacific, so that the sea surface is about half a metre higher around Indonesia than around Ecuador. La Niña is characterised by unusually cold ocean temperatures in the equatorial Pacific.

Ocean fertiliser

About half of New Zealand’s Exclusive Economic Zone (EEZ) lies in nutrient-rich subantarctic waters. However, phytoplankton, the microscopic plant cells that live in the surface layer, do not grow to their full potential in these waters due to a lack of dissolved iron, which they need for photosynthesis and growth.

This iron limitation of phytoplankton is found in other regions, such as the Southern Ocean and the Gulf of Alaska, where iron-infusion experiments have resulted in large phytoplankton blooms visible from space. An international, NIWA-led voyage to the remote Southern Ocean, known as the Southern Ocean Iron Release Experiment or SOIREE, also showed that the addition of small amounts of dissolved iron over a 50km2 area caused a bloom of phytoplankton that covered 1100km2.

During another voyage, which was part of the international project SOLAS (Surface Ocean Lower Atmosphere Studies), a team of NIWA scientists studied the effects of iron addition in the southwest Bounty Trough, about 250km east of the Otago Peninsula.

The phytoplankton response there was modest compared with other iron-fertilisation experiments, and seems to have been limited by other factors, including limited light or lack of other important micronutrients.

Marine treasure trove

Estimates of the numbers of species on Earth vary widely, from four million to more than 100 million. In 1995, the United Nations Environment Programme conservatively estimated that there may be around 13.6 million species, with the highest diversity in the sea. New Zealand has the fifth-largest EEZ, at four million km2 or 15 times the land area, and discovering new marine species is routine. Taxonomists report a new species of fish about every two weeks, and they discover other new species faster than they can name and classify them.

By Veronika Meduna

Medals and awards

Elizabeth Batham: FRSNZ

Further reading and Websites

Elizabeth Joan Batham biography – Dictionary of New Zealand Biography website

Peter Batson, Deep New Zealand: blue water, black abyss, Canterbury University Press, 2004

Neil Andrew, Malcolm Francis (eds), The Living Reef: the ecology of New Zealand’s rocky reefs, Craig Potton Publishing, 2003

Image courtesy of the National Institute of Water and Atmospheric Research.

Find Out More