The Difference between Dippers and Divers: Plastic Pollution in Deep-Diving Seabirds

The Study:

Tavares, D. C.; de Moura, J. F.; Merico, A.; Siciliano, S. Incidence of marine debris in seabirds feeding at different water depths. Marine Pollution Bulletin 119, pp 68-73. 2017. DOI: 10.1016/j.marpolbul.2017.04.012

Plastic Beneath the Surface

The growing problem of plastic in the ocean has received more and more attention in recent years. A recent study estimated that more than 9 billion tons of plastic is present on our planet, and as it doesn’t degrade, the only places it has to go are to landfills as trash and out to the environment as litter. Ultimately, much of this plastic will end up in the ocean, where it will stay for hundreds, if not thousands, of years.

Debris floating at the sea surface is easy to see, but what lies beneath? Source: Photo by Cesar Harada on Flickr, courtesy of the Algalita Foundation.

Plastic pollution is commonly imagined to be an issue that affects the sea surface. This is because plastic is buoyant, and we’ve all seen the upsetting images of large rafts of garbage floating on the surface of what should be pristine ocean waters. To make matters worse, many scientists are now concerned about large masses of plastic sinking into deeper ocean waters. Previous work by scientists collecting seawater samples in the North Atlantic showed that the mass of plastic in seawater does indeed decrease exponentially with depth below the sea surface. However, they’ve also found that smaller plastic pieces are more susceptible to sinking below the surface than larger pieces.

Large, easily visible pieces of plastic are usually found at the ocean’s surface, but high numbers of tiny plastic pieces are escaping into deeper waters. Once there, the plastic could be mistaken for food by deep-diving creatures. This is worrisome because ingestion of plastic can harm marine wildlife, possibly causing starvation or internal damage.

Scientists now know that plastic ingestion is widespread among seabirds, which mistake the small pieces of plastic for food. To date, most research studying harmful effects of plastic ingestion on seabirds has focused on birds that feed at the sea surface. In the study highlighted in this post, researchers from Rio de Janeiro, Brazil set out to analyze stranded birds along the 400 miles of Brazilian coastline to learn whether plastic pollution could also be harming diving birds that go deep to feed.

The plight of albatrosses, which are both surface skimmers and divers, on the Midway Atoll has brought attention to the harm being done to seabirds that unwittingly ingest plastic. Source: Chris Jordan via Ars Electronica on Flickr.

Searching for Seabirds

To find out whether diving birds on the coast of Brazil were ingesting plastic particles, researchers needed to see what these birds were eating. Every day from 2010 to 2013, stranded seabirds were collected along 400 miles of Brazilian coastline as part of a beach monitoring program. Analyzing stranded dead animals is a common method used by scientists monitoring the health of sea animals, as it doesn’t require living animals to be captured or sacrificed. It’s also less costly and time-consuming for researchers than embarking on expeditions to capture specific species of live birds.

The researchers ended up with 623 birds, representing 22 different species. They dissected the animals to search for any evidence of plastic in the intestinal tracts. The plastic pieces they found were counted and classified based on type of material (hard plastic, soft plastic, rubber, foam, nylon line, etc.).

The Unexpected Difference Between Dippers and Divers

Based on previous research, the scientists expected that birds feeding at the ocean surface would be more likely to ingest plastic. However, they were surprised to find just the opposite: A greater number of birds that fed in intermediate waters (3-6 meters, or about 10-20 feet, below the sea surface) and deep waters (20-100 meters, or about 66-328 feet below the sea surface) had plastic in their bodies, while plastic was rarely found in birds classified as dippers, which feed at the surface.

In an unexpected twist, researchers found that the deepest diving birds were most likely to have ingested plastic debris. In fact, they found a significant correlation between diving depth and debris in the intestinal tract: the deeper a bird species typically dives, the higher the probability that it had ingested marine debris. Hooks and nylon line, along with hard and soft plastic, were the most common items in the birds. Some of the bird species that most commonly ingested plastic were the black-browed albatross, which dives to about 5 meters (16 feet) as well as the sooty shearwater and white-chinned petrel, which dive up to 47 meters (154 feet).

The researchers found that black-browed albatross (left), sooty shearwater (center), and white-chinned petrel (right) were the species that had most often swallowed plastic debris. Source: Left: Wikimedia Commons; Center: Wikimedia Commons; Right: Don Loarie on Flickr.

The Significance of Sentinel Species

This study suggests that birds feeding at the sea surface might not be the most sensitive to the problem of plastic debris pollution. Scientists often identify “sentinel species” that are expected to be especially sensitive to certain environmental stressors. These species act like the canary in the coal mine: they show effects of the problem before it becomes as serious for other species. To find a good sentinel species for microplastic ingestion, it’s important that we identify the creatures being more seriously harmed. Unexpectedly, these might be the birds that dive down deep to find their dinner.

Along with highlighting the importance of studying how plastic might be impacting deep diving birds, the authors also observed that plastic was found more frequently than any other type of manmade debris (wood or metal, for example) in the birds, with hard plastic pieces, soft plastic pieces, and nylon line being the most common materials. The researchers note that increasing awareness in local communities, along with more rigid regulations on water quality, could help protect the seabirds of Brazil from continuing to consume harmful plastic shards.

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