by Helen Scales
In parts of the oceans, winds blowing across the sea surface cause deep, cold waters to upwell into the shallows. These deeper waters are naturally rich in carbon dioxide and undersaturated with carbonate ions. Nina Bednaršek has led studies of sea butterflies in two upwelling regions. The first, in 2008, was in the Scotia Sea that stretches between Tierra del Fuego, at the tip of South America, and the island of South Georgia in the Subantarctic. The second, in 2011, was along the western seaboard of North America, between Seattle and San Diego. At both sites, Nina found sea butterflies with signs of damage and shell decay similar to those seen in animals that have been through acidification experiments in labs. Her findings have been interpreted as a worrying sign of things to come.
Will it matter if sea butterflies start to disappear from the oceans? Will declines or shifts in their range send ripples of change through the rest of the open ocean ecosystems?
One way that a loss of sea butterflies would potentially matter is because they play a part in drawing carbon away from surface seas down into the deep and away from the atmosphere. They do this via carbon locked up in organic matter, mostly their faeces.
Clara Manno was the first to identify sea butterfly droppings. They are compact pellets, oval in shape, greenish brown and quite easy to spot once you know how. She calculated that a single sea butterfly produces around 19 droppings per day, and they sink rapidly through the water column. Sifting through sediment samples gathered from the Ross Sea off Antarctica, she calculated that almost a fifth of all the organic carbon sinking into the depths – the so-called organic carbon pump – consisted of pteropod poo. Add their abandoned mucous webs, plus their dead bodies that get dragged down by their shells, and it means sea butterflies could drive half of the organic carbon pump in some polar waters. It’s very difficult to predict exactly how things would change if sea butterflies were to begin abandoning acidifying waters in the Arctic and Antarctic. Other planktonic species could conceivably move in and take their place in the ecosystem, but there is always the chance that they would be less effective at removing carbon from the atmosphere and pulling it into the deep sea. If the organic carbon pump were to weaken, it would add yet another twist to the tangle of problems caused by climate change.
Without sea butterflies, there would also be a lot of hungry sea angels out there, as they eat little besides sea butterflies. Seabirds and fish also eat sea butterflies (although not exclusively); they in turn are eaten by bigger fish, as well as whales and seals, making sea butterflies a potentially crucial link in ocean food webs, including ones in which people are involved; there’s a series of short hops from plankton to sea butterfly to salmon to dinner plate. If sea butterflies vanish or shift their ranges, it’s possible the animals that eat them will also have to move, or find something else to eat, or go hungry. Exactly how important sea butterflies are as food for other animals, and whether ecosystems would be disrupted without them, is not clear. Much more research is needed.
It’s true that sea butterflies can be extremely abundant and, when they are, other animals will often zero in and stuff themselves. Silke described to me a day during her time in Svalbard when a huge flock of sea butterflies drifted into the fjord; hundreds of kittiwakes and fulmars sat on the sea, merrily picking at the submerged feast. Other researchers have counted 10,000 sea butterflies in a single cubic metre of water, but such high densities only occur in patches that come and go.
A major challenge that lies ahead for ocean acidification research will be to move on from single-species studies. It’s all very well knowing how individual animals react when exposed to acidifying seawater, but what happens when hundreds and thousands of organisms are all interacting, eating each other and competing for space and food? There’s one thing everyone agrees on when it comes to understanding the impacts of ocean acidification: it’s complicated. And most complicated of all will be predicting how entire ecosystems are going to respond. But researchers are finding ways.
How to probe an ecosystem
The departure lounge at Gran Canaria’s Las Palmas airport overlooks the runway, and beyond it the Atlantic stretches out to the horizon. For two months in the late summer of 2014, if passengers glanced up from their Starbucks coffee and gazed through the huge glass walls, they might have caught a glimpse of science in progress.
Nine orange structures nod gently in the sea. Each is a ring of floating pipes sticking up into the air and supporting the top end of a giant, tubular plastic bag; two metres (six feet) wide and 15 metres (50 feet) long, it hangs down into the water. Umbrellas keep the rain off, and rows of spikes stop birds from landing and pooping on them. Down on the seabed, piles of iron railway wheels are used as anchors to hold the equipment in place. These structures act as giant test tubes, designed to test the effects of ocean acidification, not just on single species but on the profusion of life that makes up an open ocean ecosystem.
There’s a bunch of clever things about these test tubes, which go by the name of KOSMOS, or the Kiel Off-Shore Mesocosms for future Ocean Simulation (mesocosm simply being a larger version of a microcosm, an encapsulated miniature world). For starters they are portable; they can be taken apart and shipped around the world, to repeat experiments in different sites, although this doesn’t come without its challenges. In Sweden, the KOSMOS tubes were frozen in by sea ice and the previous spring, in Gran Canaria, some were torn to shreds by huge waves whipped up in a storm.
The KOSMOS tubes also benefit from being very big. To do something like this on land would be laborious and far more expensive; it would involve building huge tanks and pumping in seawater, causing who knows what confusion and damage to minute sea life in the process. Much better to take the test tubes to the ecosystem, rather than the other way round. The sides of the giant plastic bag are carefully lowered to enclose 55,000 litres of seawater and everything in it. Then the stage is set to manipulate conditions inside the tubes, in this case to pump in carbon dioxide at varying concentrations, to mimic the effects of ocean acidification.
Once that’s done, the contents of the tubes are sampled every day or two. Water samples are extracted from the water column in each tube, traps at the bottom catch sinking particles and plankton nets are dragged through. Sampling all nine tubes can take hours, out in the dazzling, subtropical sunshine, but the really hard work has still to begin.
Back in the PLOCAN labs, the samples are divided up between researchers who eagerly whisk them off and plug them into an array of complex analytical devices, incubators and microscopes. By the time I pay them a visit, the 40-strong KOSMOS team has already had a few weeks to smooth out the kinks in their protocols but, even so, I’m amazed at how seamlessly the whole project is running.
Everyone knows what they’re doing and the order in which things need to happen, as if they are part of their own well-functioning ecosystem. And what’s more, they are all still smiling despite the long hours, roasting air temperatures, questionable coffee dispensed by the machine in the corridor and, for many of them, the repetitive, mind-numbing tasks – like counting sea butterflies.
Silke Lischka’s job, along with her assistant Isabel, is to sort through all the debris caught in the sediment traps at the bottom of the mesocosm tubes and, as she and I had done, scour the plankton net samples. They do things a little more systematically, though; each of them has a counting chamber made from clear resin block with a long, narrow groove in it, the same width as the microscope’s field of view, into which they pour the samples. They work their way along this elongated drop of water, counting sea butterflies as they go. Imagine an underground train driving slowly past while you stand on the platform counting all the people inside; you’re much less likely to miss anyone, or count twice, compared to standing by a crowded swimming pool and doing a head count. A tally of sea butterflies is kept using an old-fashioned, mechanical counter with typewriter keys that clack when they’re pressed and give a satisfying ding when they reach 100.
To get throug
h all the samples takes hours, glued to a microscope, sometimes through long, sleepless nights. But I get a strong sense that everyone involved, especially Silke, knows why this is all worthwhile. They are contributing their part to a big, complex picture, probing the ecosystem from top to bottom, from the uptake and use of nutrients and the release of gases into the air, to viruses, phytoplankton and zooplankton. It’s too early to say how the experiment is going, how the sea butterflies and all the other parts of the ecosystem are responding to different carbon dioxide levels, but by the end of the project, and following a great deal of data-crunching, the team will take a step back and trace a labyrinth of invisible connections.
Similar studies have been carried out already in Arctic waters and off the coast of Scandinavia, but this is the first time the KOSMOS mesocosms have been deployed in open seas. Beyond the edge of continental shelves, these clear, blue, nutrient-poor waters are representative of what two-thirds of the oceans look like. Understanding what happens here is a major part of predicting how life across the planet will respond to ocean acidification.
Head of the KOSMOS project is Ulf Riebesell from GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany. I catch up with him after he has stayed up all night working on the latest stage of the experiment. The idea is to simulate the upwelling events that regularly take place when a steady current sweeping in from the north stirs up eddies in the island’s wake, drawing deep water to the surface. The team used an enormous plastic bag to collect 80,000 litres of seawater, weighing 80 tonnes, from seven miles offshore and 650 metres down; the collecting bag took three hours to reach the surface, where it bobbed like a bloated whale. After they were set back a day by a broken water pump, the deep water was injected into the mesocosm tubes and all finally went according to plan. Now the team are on standby, waiting to see what effects unfold. They expect the nutrient-rich deep water will kick-start a phytoplankton bloom, and with it a feeding frenzy that will sweep through the rest of the ecosystem.
‘It’s like a big rain shower over a desert,’ is the way Ulf describes the upwelling event to me. He is still wide awake and brimming with enthusiasm when we sit down to chat about the project. There is one thing in particular I want to ask him about, something that has been bothering me for a while: the passing of time.
Why time matters
Most ocean acidification studies take place over the course of hours and days and a few, like KOSMOS, keep going for months. But out in the real world, there is a hundred years to go before ocean pH is expected to reach extremely low levels. In that time, will marine life be able to adapt to the creeping changes in the world around it?
This is a major limitation of most ocean acidification studies; critics point out that they take place too fast, and don’t continue for long enough, to truly mimic acidification in the oceans. A few longer-term studies hint that there is scope for adaptation, but perhaps only up to a point. Ulf’s research group has bred phytoplankton called coccolithophores for 1,800 generations in high carbon dioxide conditions. These microscopic algae live inside clusters of calcium carbonate discs – collectively called the coccosphere – making them likely victims of acidification. However, over time, the laboratory population became more robust to falling carbonate saturation and lower pH.
The experiment acted as a form of artificial selection. The high carbon dioxide treatment slowed the growth rate of some coccolithophores, probably because they needed more energy to keep building their carbonate skeletons. Meanwhile some of them were more robust and were able to maintain their growth rates, perhaps even growing faster. These individuals were the ones that reproduced more rapidly, passing on more of their genes to the next generation. Slowly, in laboratory conditions, the acidified coccolithophores became adapted to their shifting water chemistry.
It remains unknown exactly how the coccolithophores’ physiology changed; it’s possible that as generations went by, they became more adept at ramping up metabolic rates and pumping ions around to maintain pH balance. Or, there may be some other as-yet unidentified mechanism that allows them to survive.
Could other calcifiers adapt to acidifying waters like coccolithophores? To repeat the experiments on anything that lives longer than these microbes would take an insanely long time. Coccolithophores have a generation time of a single day. It took five years to study them for 1,800 generations. For organisms like sea butterflies that have generations lasting a year, these experiments become quite unthinkable. Plus, it’s well known that organisms with short generation times evolve quickly, compared to species that take longer to mature and reproduce (this is because the genomes of short-lived species are copied more frequently and errors quickly build up in their DNA, leading to more genetic variation that natural selection will act on). Coccolithophores are also highly abundant, with up to 10 million of them in a litre of seawater. It means that coccolithophores are inherently more adaptable to environmental changes than larger, rarer species like sea butterflies. And like Steeve Comeau’s naked sea butterflies, the big unknown is whether carbon-resistant coccolithophores would survive out in the oceans, where there are masses of other species all competing for resources and space.
Even those organisms that can change their ways and adapt to a high carbon dioxide world may eventually still lose out. As the oceans continue to acidify, the cost of concentrating carbonate ions and building skeletons and shells will keep on steadily rising until calcifiers can simply no longer afford to make their homes.
‘There are certain limits you can’t pass,’ Ulf tells me.
The century of acidifying seas that lies ahead will be unavoidably long and slow compared to the short-term studies aiming to forecast the future (after all, the only way to really know how the oceans are going to respond is to sit back and watch what happens in real time, but that’s hardly the point of studies like this). However, the rate at which ocean acidification is now taking place is a mere beat of a sea butterfly’s wings compared to the millennia that rolled by in previous climate change events, the ones that came and went before modern humans showed up. Sceptics point to these past events, to times when carbon dioxide levels were naturally high without humanity’s input. And look – look at all those things that were alive back then and are still here now. There are still corals and plankton and all those molluscs with shells. They didn’t melt away before, so why should we believe that will happen this time?
Things are different now. Given enough time, and a slow enough pace of carbon enrichment, the oceans themselves respond to ocean acidification and lessen its effects. Deep down on the sea floor there are vast deposits of calcium carbonate sediments, made from the fossilised remains of calcifying creatures – mostly coccolithophores and foraminifera – that lived and died over millions of years. In the past, carbon dioxide levels have risen in the atmosphere as is happening now, but from other sources besides human activities. The pH of shallow seas fell and, over the course of many centuries, those surface waters sank down, until they reached the deep carbonate sediments, causing them to dissolve and release carbonate ions. It meant that levels of carbon dioxide in the atmosphere were decoupled from the saturation of carbonate ions in the seas; while atmospheric carbon dioxide increased, it didn’t drag down carbonate saturation with it. In essence, the oceans had their own colossal mechanism that buffered against acidification, which explains why many creatures with chalky skeletons were able to survive previous climate change events. In the past, calcifiers were protected from acidification by their ancestors – calcifiers of earlier eons – whose remains accumulated on the seabed. But now that link has been broken. The problem is, the oceans’ inbuilt balancing mechanism takes 1,000 years or more to work, because that’s how long it takes shallow waters to spread through the deep ocean. This time around, we don’t have 1,000 years to wait.
Anthropogenic climate change is taking place much faster than anything the planet has experienced before, and the oceans can no longer keep pace with carbon emission
s. The rate of uptake of carbon dioxide into the oceans far outstrips their ability to buffer against falling pH. Now, carbon dioxide levels and carbonate saturation are locked in relentless decline; side by side they drop together. The oceans today are slaves to the atmosphere.
A major talking point for climate change – and a target for sceptics – is the issue of how much experts agree on the facts. Increasingly, scientists worldwide are standing up and making it abundantly clear that they do agree, by and large, on the causes of climate change and the global troubles that could lie ahead. On a similar note, do experts agree that ocean acidification is happening, and that it’s a problem for the seas? A 2012 survey of experts suggests that consensus is strong, at least when it comes to the bigger issues.
Jean-Pierre Gattuso from the Laboratoire d’Océanographie in Villefranche, France, led a survey asking 53 ocean acidification experts how much they agreed, or disagreed, with a list of statements. Almost all the experts agreed – without question – that ocean acidification is currently in progress, that it’s measurable, and that it is mainly caused by anthropogenic carbon dioxide emissions ending up in the oceans (many pointed out that in coastal waters other pollutants, such as excess nutrients, can also affect pH).
As is the scientist’s prerogative, many respondents picked apart the questions being asked, pointing out problems with the wording: ‘What do you mean by “most”?’, ‘What does “adversely affect calcification” mean?’
In many cases, they emphasised the lack of certainty, the lack of long-term studies and the variable responses of different organisms. Without more data, it’s difficult to be sure how food webs and fisheries will fare in a more acidic world. However, experts did agree that calcifiers, with their chalky skeletons and shells, are the marine species most likely to lose out.