“The ocean is an energy that drives weather, including the higher precipitation in extreme weather events like super-storm Sandy, the severe droughts and so on associated with climate change. To understand weather and climate, we must understand the oceans.” Movie director James Cameron
Life on Earth is heavily reliant on what happens in our oceans. Most marine life, in turn, relies on a regular and ongoing supply of phytoplankton.
These microscopic marine plants lie at the base of the marine food chain and form the staple diet of minute ocean creatures called zooplankton. Without enough zooplankton in the seas, most marine life would not survive.
Small wonder then that marine scientists are concerned that phytoplankton levels are in sharp decline around the world. During the 20th century, global concentrations are estimated to have fallen by more than 12 per cent on average, with the latest projections suggesting further falls of more than 50 per cent by the year 2100.
“The decline is determined by many factors,” says environmental scientist Dirk Olonscheck of the Potsdam Institute for Climate Impact Research in Germany.
Previous studies have investigated a variety of physical mechanisms, he explains. These include factors ranging from higher sea-surface temperatures to reduced mixing of water layers – which means phytoplankton receive fewer nutrients for photosynthesis.
These mechanisms could be partly responsible for declining phytoplankton levels in the oceans, Mr Olonscheck says. “But they cannot explain the magnitude of the decline suggested by observations.” For example, scientists do not understand how various phytoplankton species react to higher sea surface temperatures. Nor do they understand how zooplankton, which graze on the phytoplankton, respond to higher temperatures.
“The metabolic theory of ecology states that metabolic processes such as photosynthesis and respiration are temperature sensitive and should increase at different rates in response to a warming environment,” he explains. “It is likely that higher temperatures increase grazing pressure, leading in turn to a reduction in phytoplankton concentrations.”
To set matters straight, the German scientists modelled a range of temperature sensitivities. “Our results show that the biological response of plankton to a warming ocean can be even more important than changes in the ocean’s physical structure,” he says.
In particular, levels of chlorophyll a, which phytoplankton use to synthesise sunlight, are in decline. (Chlorophyll is a green plant pigment that absorbs the solar energy needed to produce carbohydrates from carbon dioxide and water.)
“The decrease in global oceanic chlorophyll a is further evidence for global ocean warming,” Mr Olonscheck says.
Testing the waters
Phytoplankton levels can be assessed by measuring chlorophyll concentrations. Since 1899 this has been done using an oceanographic instrument known as a Secchi disk.
Mounted on a pole or line, the disk is lowered slowly into the water. The depth at which a pattern on the disk is no longer discernible is a measure of the water’s transparency. “By using computer models, this measurement can be related to the surface chlorophyll concentration,” Mr Olonscheck says.
Additionally, direct measurements are made by scrutinising data from remote-sensing satellites, as well as by analysing the organic matter found in seawater. “Combining these different measurement sets is statistically challenging and is a source of uncertainty in predicting long-term trends,” Mr Olonscheck says.
Carbon emissions are leading to acidification and de-oxygenation of the oceans. These processes are happening at a rate much faster than in the past, and so scientists expect significant changes in ocean flora and fauna. Might these one day lead to the extinction of some marine species?
“This is well possible,” says environmental scientist Richard Twitchett, of Plymouth University in Britain.
The fossil record, he explains, shows that the biggest of the world’s major five mass extinctions are associated with global warming, as are a number of smaller events.
This is because as global temperatures rose, oxygen levels fell, thus depriving marine life of a vital necessity. During the Jurassic extinction alone, more than a quarter of ocean genera, or groups of closely related species, vanished, as did about 5 per cent of biological families, comprising several genera.
“Of course, there have also been ‘warm’ periods in recent Earth history when there were no elevated extinction rates – including some very short, sharp global warming ‘events’ that caused no significant extinctions,” Professor Twitchett says. “Key factors are the magnitude, rate and duration of warming.”
Studies by ecologists and climate modellers also predict a degree of extinction, he notes.
Some scientists warn that the Jurassic extinction may not be readily extrapolated to the present epoch. So how much marine life might be at risk?
“We can’t extrapolate directly to modern ecosystems because the starting conditions – such as the species present, distribution of continents, ocean circulation and atmospheric conditions – are different,” Professor Twitchett says. “Indeed, even if we could rerun the Jurassic experiment again, the outcome for any one species might be different.”
Another problem with direct extrapolation, he adds, is the influence of human activity: “This provides another impact on modern ecosystems that did not previously exist.”
Few experts would disagree that global climate change is affecting today’s marine life. “This is not hypothetical – negative effects are coming to light every day,” says marine biologist Pam Allen, of the Australian Marine Conservation Society.
There are several reasons for this, she explains. As climate change causes oceans to acidify, the carbonate shells of animals such as shellfish and corals become thinner. Warming oceans, combined with overfishing, lead to blooms of jellyfish so large that fish populations cannot compete.
“The effects on coral reefs are evident – warmer waters resulting from climate change are causing bleaching events on a massive scale,” Ms Allen says.
She finds it hard to predict the proportion of marine life that is likely to be affected by climate change. “Ocean animals are intimately intertwined, just like on land, and unravelling food webs and ecological associations are a life’s work.”
The parlous state of our oceans has prompted some savvy scientists to devise plans of action.
One promising strategy involves installing long tubes beneath the ocean waves to draw up the cold, nutrient-rich water on which phytoplankton depend.
In addition to absorbing carbon dioxide, plankton emit the chemical compound dimethyl sulphide; this encourages the formation of clouds that, by reflecting the sun’s rays, help prevent heat from reaching the Earth’s surface.
The extra plankton near the surface would also increase the number of fish. Finally, the upwelling of cold water should cool the ocean surface, leading to fewer cyclones that build over warmer seas.
The idea comes from a natural model: the sporadic natural upwelling of deep waters off the continental shelf, which are richer in nutrients, dramatically increases phytoplankton levels. This, in turn, boosts fish stocks.
An example is the Humboldt Current off the Peruvian coast, which supports one of the world’s largest fisheries of sardines, anchovies and jack mackerel.
Such geo-engineering solutions, as they are collectively known, might soon be needed. A recent climate study, spanning 2000 years and published in the British journal Nature Geoscience, claims that the Earth was warmer from 1971 to the turn of the century than at any other time in about 1400 years.
During the past century, average global temperatures were about 0.4 degrees higher than over the previous five centuries. Antarctica was the only region not to follow this disturbing trend.
Learn more about the work of the Alfred Wegener Institute at: awi.de/en
Get to grips with the microscopic world of phytoplankton at: www.whoi.edu/main/topic/phytoplankton
Read the latest report on climate change by the Intergovernmental Panel on Climate Change at: ipcc.ch/report/ar5/wg1/
Learn more about climate change science in the Australian curriculum textbook Oxford Big Ideas, Science 10 (Oxford University Press, 2012)
AusVELS Science: Biological sciences: ausvels.vcaa.vic.edu.au/Science/Curriculum/F-10
F-10 Physics (under sub-strand “physical sciences” in AusVELS): ausvels.vcaa.vic.edu.au/Science/Curriculum/F-10
Read article at The Age