Carbon dioxide is not only warming the atmosphere but also acidifying the oceans.
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Our oceans are in trouble. Alongside long-recognised problems like pollution, overfishing and warming, the oceans are now known to be acidifying at such a fast rate that biologists the world over believe that some groups of marine organisms may be unable to evolve or adapt. The decline of some species, like the Pacific oyster in the American Pacific Northwest, has already been attributed to ocean acidification. Now New Zealand scientists are looking at the effect on local species.
The atmospheric and ocean systems are inextricably interrelated, with a constant exchange of CO2 between the two, says Cliff Law, a Niwa biogeochemist who recently chaired a workshop on potential environmental and socio-economic impacts of ocean acidification on New Zealand. The oceans have acted as a buffer to climate change, not just by absorbing some of the heat from the warming atmosphere, but also by providing a “sink” for 45% of the additional CO2 pumped into the atmosphere since the start of the Industrial Revolution.
When tiny aquatic plants called phytoplankton photosynthesise, they fix atmospheric CO2 into their cells. Then, when they die, a proportion of that carbon sinks into the deep ocean where it remains isolated from the atmosphere for 100 to 1000 years.
But as well as being taken up by phytoplankton, atmospheric CO2 dissolves in seawater. And as it dissolves, the ocean becomes more acidic: the pH level of the ocean has already fallen over the past 150 years from 8.2 to 8.1 (on a scale of 0 to 14). This may not sound like much, but on this logarithmic scale it equates to a 30% increase in the concentration of hydrogen ions.
IPCC scenarios suggest that projected increases in CO2 could send the ocean’s pH levels down to 7.8 – equal to a 150% increase in concentration of hydrogen ions – by 2100. “The rate at which this process is happening may have really serious implications for a number of the species in the ocean,” says Law. The combined effect of an increase in acidity and dissolved CO2, and a decrease in available carbonate – another result of an increase in CO2 in the oceans – will “affect just about every physiological process there is in the ocean, including respiration, photosynthesis and metabolic rates”.
Law is working on Marsden and Ministry of Fisheries-funded projects to study the effect of ocean acidification on bacteria, phytoplankton and zooplankton, which sit at the base of the marine food chain. Although many phytoplankton species – the ones that don’t make carbonate shells – might benefit from an increase in atmospheric CO2, the future is uncertain for carbonate-dependent shell-makers. And preliminary results from a 10-year sampling project suggest that pteropods – a zooplankton with a carbonate shell that is an important foodstuff for juvenile fish – are already becoming scarcer in sub-Antarctic waters.
Law’s colleague, marine ecologist Vonda Cummings, has been looking at the impact of ocean acidification on Antarctic bivalves. Bivalve molluscs, which use calcium carbonate (CaCO3) to make their shells, are one of the groups thought to be most susceptible to ocean acidification; species that live in cold water, which has naturally low levels of carbonate, are likely to feel the effects of ocean acidification first.
Working in temperatures of -1.9°C, Cummings’s team of scuba divers collected samples of two species – a geoduc and a scallop – from McMurdo Sound, Antarctica. Back at Niwa’s laboratory at Mahanga Bay in Wellington, the shellfish were raised in tanks of icy cold seawater at a range of pH levels.
“We found that the animals at the lower pH actually lost shell weight, which meant the shells were dissolving under the more acidic conditions, or they weren’t generating new shell as quickly as they would have under normal conditions.”
In a project funded by the Ministry of Fisheries, Cummings’s team is now looking at how seawater pH and carbonate content affects fertilisation, larval development and growth and survival rates of New Zealand paua.
The impact of ocean acidification on species like paua, geoducs and scallops is not just an issue for commercial and recreational fishers. These molluscs play important roles in the marine ecosystem and provide food for species above them in the food chain. The effects of acidification are potentially ocean-wide, and international researchers are investigating the likely effects on a range of species, including fish and crustaceans. “You don’t have to have a CaCO3 skeleton to be affected by low pH,” says Cummings.
Globally, scientific effort is focused on understanding ocean acidification and anticipating the impact of a rapidly acidifying ocean over the coming decades. “There is a level of urgency about the issue,” says Law. “A major knowledge gap is the knock-on effects of ocean acidification on ecosystem services, such as commercial fish stocks, oceanic control of atmospheric CO2 and climate, and the recreational and aesthetic facilities of the coasts and oceans that we take for granted.”
Can we slow or stop the process? “People have talked about liming the ocean, putting more carbonate into the sea,” says Law, “but you’re talking about the equivalent of the White Cliffs of Dover being broken up and dispersed around the ocean.” And that short-term solution would require a large amount of energy. Law has a better solution: “Reducing our carbon emissions is the only way that we’ll stop or slow ocean acidification.”
HOW MANY SPECIES?
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“Knowing the number of species on Earth is one of the most basic yet elusive questions in science,” say the authors of a paper published in PLoS Biology in August. They calculated the number to be about 8.7 million – 6.5 million species on land and 2.2 million in the sea. However, they assessed only one branch of the tree of life, the eucaryota, which includes everything from slime moulds to flagellates and comprises all plants, animals and fungi. The other major branches of the tree of life – bacteria and archaea (primitive single-celled micro-organisms) – each comprise millions more species. Of the eucaryota assessed in this survey, the authors estimate 86% of species on land and 91% of marine species are yet to be described. Describing these remaining species, they predict, “may take as long as 1200 years and would require 303,000 taxonomists at an approximated cost of US$364 billion”. But at the rate at which species are becoming extinct, they say, many will be gone before they are described.
MEAT, NOT MURDER
Some European laboratories are months away from growing synthetic meat for the table, says New Scientist. “And not just the usual steaks and burgers either. Meat from exotic animals could one day widen our culinary choices.” Production of the meat starts with muscle stem cells, which can be obtained easily without killing the source animal. There are no reports yet on what the lab-grown clumps of animal muscle taste like, though New Scientist writer Andy Coghlan says the muscle-like strips grown in a university laboratory look “anaemic and unappetising”.
NAVIGATING BY THE STARS
From September 11, Wellington’s Carter Observatory is running a month-long programme, “1000 Years of Navigation”, focusing on stories of navigation and migration using the stars.