Ocean acidification – the past, present and future clam

Time for more ocean acidification bad news. This report describes the results of a novel study that looked at what happens when you raise two different species of “commercially important” – I think that means we eat them - bivalves not only in water with predicted future levels of pH, but also in a tank with pre-industrial levels of CO2.

The surprise is not so much that the clams and scallops did worse under future conditions, but they did considerably better in the past conditions than they do today:

At the 750ppm level, basic shell structures like the hinge were severely malformed, while the surface of the shell had holes that were apparent when it was examined via scanning electron microscopy. There was also a significant drop in the viability of the larvae, and those that did survive were developmentally delayed compared to those raised at today’s concentrations. Matters got worse at the higher levels.

The interesting twist in the new work is that the authors also run the experiment under preindustrial CO₂ levels of about 250ppm (actual levels were closer to 280ppm). For both species of shellfish, the mortality was much lower and development proceded more quickly. For the quahog, viability doubled (from 20 percent to 40 percent), while for the bay scallop, viability went from 43 percent to 74 percent. The animals made major developmental milestones more quickly—metamorphosis at day 14 occurred in half the animals at preindustrial CO₂ levels, but that dropped to less than seven percent at modern levels.

In other words, it may be that even the current decrease in pH may be adversely affecting bivalves.

Overall, they suggest that population crashes in bivalves have been ascribed to a number of stresses, like overfishing and pollution, but it’s possible that ocean acidification has also been at work in these cases. Given that the Earth has experienced higher CO₂ levels in the past, why are they being hit so hard now? According to the paper, it’s actually been over 24 million years since levels are likely to have been this high, and many shellfish have diversified more recently than that; any changes in CO₂ in the intervening time have also been far more gradual than the current pace.

Not great news.