Thin-shelled pteropods are key to the food web

Acidified water a higher risk for West Coast

A regional report on acidified and low-oxygen sea water looks at why the West Coast is particularly vulnerable, and what can be done.

Salmon gobble up sea snails all the time, but something new is eating their shells.

Just millimetres long, most sea snails, or pteropods, are too tiny for the human eye, and their thin shells are almost see-through.

“They are magnificent little organisms,” says Thomas Pedersen, a University of Victoria oceanographer whose work on ocean chemistry is known world-wide.

In fact, once they see them magnified, many people call pteropods ‘sea butterflies’ — they seem to flutter through the ocean on tiny wings that poke out the open end of their shells.

The trouble facing pteropods is not the salmon, mackerel, herring, or even the whales that eat them up. That’s just life at the base of a food chain.

The trouble is that pteropods make their shells with a particular type of calcium carbonate—the same white stuff found in chalk, or stony corals—that dissolves very easily in waters acidified by extra C02.

And as a report published April 7 by Pedersen and 19 other scientists makes clear, ocean acidification is on the rise, especially here along the West Coast.

“A particular food source for salmon is particularly vulnerable,” said Pedersen.

“What does that mean for salmon, if we continue to pump C02 into the atmosphere?

Over the long term—the next few decades—it’s not going to be good news.”

Along with Debby Ianson, an oceanographer with Fisheries and Oceans Canada, Pedersen was one of two B.C. scientists asked to join a 20-person panel looking at West Coast ocean acidification and hypoxia (low-oxygen) back in 2013.

The resulting report is the most comprehensive look at the problem so far, and one the scientists hope will lead to a dedicated West Coast task force and monitoring network.

Pedersen calls ocean acidification the related effect of hypoxia the “evil twin” of global warming.

“Global warming gets a lot of attention, and so it should,” he said.

“But there’s another issue going on in parallel with global warming, and that’s a chemical issue.”

It’s easy see why ocean acidification is like an evil twin to global warming.

For one thing, Pedersen said, both share the same root cause — the “heck of a lot” of carbon dioxide that humans have put in the atmosphere.

In the air, C02 levels have shot from 280 to 403 parts per million from pre-industrial times to present day—all in just a century and a half.

“Mother Nature has never put such an increase on this planet in all of its geological history,” said Pedersen.

“That rate of change is extraordinary.”

About two-fifths of that C02 gets absorbed into surface seawater, where it becomes carbonic acid — the same weak acid found in club soda.

Oxygen gets used up in the process, and so do carbonate ions, which act as a basic building block for pteropod, clam, shrimp and scallop shells as well for fish skeletons and sea corals.

What makes the situation worse along the West Coast of North America is that coastal waters here have always been more acidic and low-oxygen compared with other coastal regions.

To understand why, Pedersen said it helps to imagine a train that starts on the surface of the sea between Iceland and Scotland and then dives down, rolling along the deep Atlantic ocean to Antarctica before it spins around, heads north into the Pacific and finally re-surfaces as it approaches the coast in the Gulf of Alaska.

The whole time it’s underwater, he said, the train is getting pinged by a rain of falling organic matter — dead plankton or feces or plant matter — that creates C02 and consumes oxygen in the train as it decomposes.

At the same time, another train travelling at mid-depth arrives on the West Coast from the seas north of Japan, and it’s doing the same thing.

“That’s our water,” Pedersen explained.

“By the time it gets to our shorelines, it’s more oxygen-depleted, and C02 rich.”

The first ‘train’, from Iceland, takes about 1,200 years, he added, while the one from north of Japan arrives every 30 to 50 years.

But neither train delivers CO2 anywhere near as quickly as humans have.

Another ‘evil’ trait of ocean acidification is the way it has crept up on people — three-quarters of all the published research is less than five years old, and it contains some unpleasant surprises.

While the recent collapse of some West Coast shellfish farms might be an early warning of its effects — a Nanaimo-area scallop farm lost a $10 million harvest in 2014 — new research shows that besides damaging their skeletal structures, ocean acidification also seems to disrupt the wayfinding ability of marine fishes.

Many fundamental questions about acidification and hypoxia remain, and long-term ocean data is hard to find.

Another ‘evil’ trait is that ocean acidification is non-linear — the trouble is not simply that seawater might slowly get more acidic, with a gradually worse effect on pteropods, shellfish or the things that eat them.

The way carbon chemistry works in the ocean, there is an acidity level at which there is a sudden switch, an “inflection point” where the type of carbonate creatures need for shells and skeletons doesn’t form the same way.

“The ocean has a natural ability to counteract that added acid,” said Pedersen.

“But at some point, you can overcome that and then bang, you’re in real trouble because you completely lose that carbonate ion.”

Of the panel report’s six major findings, four address the different ways that Mexico, the U.S. and Canada might deal with the problem.

Steps include controlling algae-nourishing pollution in coastal waters, helping ecosystems cope using protected areas and better fisheries and coastal management, as well as funding more scientific research and monitoring.

One example comes from the K’ómoks estuary on Vancouver Island, where C02-absorbing eelgrass is being transplanted from donor beds to previously disturbed areas.

Pedersen hopes the B.C. government will take up the call for a dedicated task force, as the state of California seems to be doing, but he not encouraged by the recent actions of Canada’s federal government.

“One of the things in Canada that I think is a discredit as a nation is that we’ve just come through 10 years of an anti-environmental government that simply refused to even consider the importance of issues such as monitoring the sea,” he said.

While the current government has signalled a change, Pedersen a lot of valuable time has already been lost.

“There’s been a lot of damage done to our capacity to understand our coastal waters.”