Ocean acidification threatens NZ paua, mussels

Global warming's evil twin, the increasing acidification of carbon dioxide-saturated oceans is threatening New Zealand's corals, crustaceans and shellfish.

"Ocean acidification is accelerating, " Dunedin researcher Christina McGraw said.

Dr McGraw said this rapid change in ocean chemistry meant that severe damage was imminent.

Some of New Zealand's vulnerable organisms include economically important species such as mussels, oysters, and paua.

"These all have calcium carbonate shells, which are going to be increasingly hard for the organisms to construct as the carbonate concentration in the ocean continues to decline," the Otago University marine chemist said.

Nearly every marine animal with a shell is vulnerable to the impacts of ocean acidification, and some microscopic plankton, which form the base of marine food webs may be left with weaker and thinner shells.

Oceans store about 50 times more carbon dioxide than the atmosphere, and they have absorbed more than 30 percent of the carbon dioxide released by human activity. The extra carbon dioxide dissolved in the water had made it more acidic.

A doubling of acidity in seawater this century will halve the amount of calcium available in the water for shellfish to extract and use to make their shells, or in the case of New Zealand's sea urchins (kina), their calcium-based skeletons.

More worryingly, coralline algae -- a calcified seaweed which covers 80 percent of the Otago coastline and provides settlement sites for baby paua and kina -- is particularly sensitive to increased acidity.

Dr McGraw said these keystone algae species may have already passed their tipping point: "We may have already started seeing changes".

"If we combine the loss of coralline algae with the decreasing calcification, species such as paua and kina will take a double hit."

Impacts on mussels and oysters were expected to be seen by the middle of the century.

D r McGraw was speaking as the Monaco Declaration -- in which 150 ocean scientists from around the world voiced concern over increasingly acidity levels in oceans - was launched today.

She was one of three New Zealand scientists who signed the declaration -- the others were university colleague Eike Breitbarth and Kim Currie of the National Institute of Water and Atmospheric Research (Niwa).

Ocean acidification will have wider socio-economic implications, she said in an online press conference.

"Changes in the marine foodwebs and fish stocks may threaten the food supplies for millions of people worldwide," she said.

New Zealand has the world's fourth largest economic exclusion zone, and relies on six million square kilometres of seabed on its continental shelf, both inside and beyond the EEZ, for food and industry.

Dr McGraw said the imminent damage to ocean ecosystems from acidification would hurt both commercial and traditional harvests from NZ waters.

Marine farming promoted as becoming a billion-dollar industry by 2025 relies for 70 percent of its income on oysters and mussels -- both organisms vulnerable to the rising acidity levels.

Paua and pipi among the traditional Maori harvest are likely also to be hit.

"Unfortunately, the responses of New Zealand organisms to these increasingly acidic waters is not known," Dr McGraw said.

Their responses could not be accurately predicted from research overseas, and even within New Zealand there were likely to be regional differences.

Otago University chemistry, botany, and biology researchers are working with Niwa to mimic local seawater conditions from around the country in a collection of tanks so they can predict how growth patterns will vary.