Plastic bags, aluminium cans, and fishing debris not only
clutter our beaches, but accumulate in open-ocean areas such as the "Great
Pacific Garbage Patch". Now, a paper by researchers at the Monterey Bay
Aquarium Research Institute (MBARI) shows that trash is also accumulating in
the deep sea, particularly in Monterey Canyon.
Kyra Schlining, lead author on this study, said, "We
were inspired by a fisheries study off Southern California that looked at sea floor trash down to 365 meters. We were able to continue this search in
deeper water—down to 4 000 meters. Our study also covered a longer time period,
and included more in situ observations of deep-sea debris than any previous
study I'm aware of."
How the study was
To complete this extensive study, Schlining and her
co-authors combed through 18 000 hours of underwater video collected by MBARI's
remotely operated vehicles (ROVs). Over the past 22 years, technicians in
MBARI's video lab recorded virtually every object and animal that appeared in
these videos. These annotations are compiled in MBARI's Video Annotation and
Reference System (VARS).
For this study, research technicians searched the VARS
database to find every video clip that showed debris on the sea floor. They then
compiled data on all the different types of debris they saw, as well as when
and where this debris was observed.
In total, the researchers counted over 1 500 observations of
deep-sea debris, at dive sites from Vancouver Island to the Gulf of California,
and as far west as the Hawaiian Islands. In the recent paper, the researchers
focused on seafloor debris in and around Monterey Bay—an area in which MBARI
conducts over 200 research dives a year. In this region alone, the researchers
noted over 1 150 pieces of debris on the seafloor.
The largest proportion of the debris—about one third of the
total—consisted of objects made of plastic. Of these objects, more than half
were plastic bags. Plastic bags are potentially dangerous to marine life
because they can smother attached organisms or choke animals that consume them.
Metal objects were the second most common type of debris
seen in this study. About two thirds of these objects were aluminum, steel, or
tin cans. Other common debris included rope, fishing equipment, glass bottles,
paper, and cloth items.
The researchers found that trash was not randomly
distributed on the seafloor. Instead, it collected on steep, rocky slopes, such
as the edges of Monterey Canyon, as well as in a few spots in the canyon axis.
The researchers speculate that debris accumulates where ocean currents flow
past rocky outcrops or other obstacles.
More in deeper parts
The researchers also discovered that debris was more common
in the deeper parts of the canyon, below 2,000 meters (6,500 feet). Schlining
commented, "I was surprised that we saw so much trash in deeper water. We
don't usually think of our daily activities as affecting life two miles deep in
the ocean." Schlining added, "I'm sure that there's a lot more debris
in the canyon that we're not seeing. A lot of it gets buried by underwater
landslides and sediment movement. Some of it may also be carried into deeper
water, farther down the canyon."
In the same areas where they saw trash on the seafloor, the
researchers also saw kelp, wood, and natural debris that originated on land.
This led them to conclude that much of the trash in Monterey Canyon comes from
land-based sources, rather than from boats and ships.
Although the MBARI study also showed a smaller proportion of
lost fishing gear than did some previous studies, fishing gear accounted for the
most obvious negative impacts on marine life. The researchers observed several
cases of animals trapped in old fishing gear.
Other effects on marine life were more subtle. For example,
debris in muddy-bottom areas was often used as shelter by seafloor animals, or
as a hard surface on which animals anchored themselves. Although such
associations seem to benefit the individual animals involved, they also reflect
the fact that marine debris is creating changes in the existing natural
Impacts last years
To make matters worse, the impacts of deep-sea trash may
last for years. Near-freezing water, lack of sunlight, and low oxygen
concentrations discourage the growth of bacteria and other organisms that can
break down debris. Under these conditions, a plastic bag or soda can might
persist for decades.
MBARI researchers hope to do additional research to
understand the long-term biological impacts of trash in the deep sea. Working
with the Monterey Bay National Marine Sanctuary, they are currently finishing
up a detailed study of the effects of a particularly large piece of marine
debris—a shipping container that fell off a ship in 2004.
During research expeditions, researchers occasionally
retrieve trash from the deep sea. However, removing such debris on a large
scale is prohibitively expensive, and can sometimes do more damage than simply
leaving it in place.
Schlining noted, "The most frustrating thing for me is
that most of the material we saw—glass, metal, paper, plastic—could be recycled."
She and her coauthors hope that their findings will inspire coastal residents
and ocean users to recycle their trash instead of allowing it to end up in the
ocean. In the conclusion of their article, they wrote, "Ultimately,
preventing the introduction of litter into the marine environment through
increased public awareness remains the most efficient and cost-effective
solution to this dilemma."