Assessing the threat of marine debris
For several years, Mira Solvang, a marine ecologist at the Institute for Coastal Research in Bergen, has been turning over a gloomy question: is the state of our ocean rubbish merely grim, or a good deal worse?
Solvang sits on a panel of researchers called the Ocean Litter Assessment Group, whose remit takes in, among other topics, the worsening problem of marine debris — the drifting and sunken waste most people simply call ocean rubbish. Study upon study has rung alarm bells over the state of the seas, and in a recent article in the journal Coastal Science Letters, Solvang and her fellow authors set out to establish how many of these supposed hazards genuinely stand up.
One thing she has noticed repeatedly is that a study will wrap up by guessing at the larger implications of its results. Research may show, say, that a certain fish is swallowing floating scraps of plastic, then warn that the species as a whole could be sliding toward extinction. 'The problem was that these assumed risks had never really been checked,' Solvang says. 'We simply didn't have much to go on.'
Her group worked through more than 130 pieces of research on the consequences of marine debris, all published before the close of 2015. For each, they asked two things: which hazards its authors had examined — 428 supposed hazards in all — and what the evidence had in fact shown.
In about four out of every five instances, the feared consequences of marine litter proved real enough. In the rest, the group found that shortcomings in how the studies had been designed and conducted undermined their conclusions — a missing control group here, unsound statistics there.
Tellingly, just one carefully built study failed to turn up the effect its authors had been looking for — an inquiry into whether oysters came to harm after taking in microscopic particles. The plastic, the scientists observed, passed from the oysters' stomachs into their bloodstream and lingered for weeks; the shellfish, though, seemed none the worse for it.
Oysters, in other words, may take rubbish in their stride; even so, the review painted a far clearer picture of the many kinds of trouble marine debris really does cause.
Among the threats the analysis confirmed, most could be laid at the door of plastic rather than materials such as wood or metal. And it was mainly the more sizeable fragments that did the damage — animals ending up entangled in castoff material, or eating it and injuring themselves badly.
A large share of marine debris, though, is 'microplastic' — fragments no bigger than five millimetres across. Some starts out as an ingredient in cosmetics and toiletries, some as fibres loosened from synthetic garments in the wash, some as the eroded remains of larger objects. Against the many studies of full-sized debris, Solvang's team found little work on how these tiny particles behave. 'With microplastic, many questions remain open,' she says, noting that more has appeared since 2015, when the group closed its survey.
The paths by which debris ends up killing sea creatures are likewise full of unknowns, she adds. Much of the research has fixed on a lone animal — or even its tissues and cells — rather than on entire populations. On top of that, the plastic levels used in laboratory experiments are frequently far higher than anything the ocean itself holds. So nobody can put a figure on how many fish, turtles or seabirds plastic pollution might kill, or say how the loss of one species would tell on the predators above it and on the wider ecosystem.
'We have to start asking questions that carry more ecological weight,' Solvang maintains. More often than not, scientists have no idea how a catastrophe — say, a tanker spilling its whole cargo of oil across a vast stretch of sea — will affect the environment until the event is already behind them. 'We leave the important questions too late,' she says. Yet if researchers can grasp the way marine litter eats away at ecosystems little by little, they may still stop matters from worsening.
Framing the questions well also helps governments, and the public, decide where their attention belongs. The threats that grab the most notice are not always the real priorities. The very name 'Great Pacific Garbage Patch' — a mass of debris in the northern Pacific — tends to summon up some huge floating island of trash. The reality is far less dramatic: much of the waste sits beneath the surface or is too small to see, and a boat could cross the whole area without a single piece coming into view. A Norwegian outfit called Blue Current is now planning to sink machinery into the Patch and similar spots to vacuum up plastic. A recent paper, though, ran simulations suggesting that placing the same devices nearer the coast would curb pollution far more over the long run.
'Putting a few of these misconceptions right really does count,' Solvang says. Whether in the media or within the scientific community, she remarks, 'all those images of stranded and tangled-up animals convince people that plastic is killing off everything in the ocean.' A patient reading of the research already out there helps ecologists tell the problems that truly call for action from those they would do better — as with the oysters — to take in and let pass.
