Often invisible plastic waste in the ocean can harm birds, fish and other marine inhabitants. But not all organisms suffer because of it. Sea skaters use the waste as a deposit for their eggs.
Plastic has become an ever-present part of daily life. Plastic bottles, bags, furniture, electrical appliances, food packaging, construction materials - much of it ends up in the sea. Lighter than water, the plastic floats and is carried by ocean currents. All in all, there is an estimated 100 grams of plastic per square kilometer of ocean, which adds up to a volume of over 40,000 tons of plastic waste.
That's bad news for the many water birds, fish and turtles that eat the plastic and die because the cannot digest it. But there is also a sea creature that takes advantage of the plastic: the sea skater (Halobates sericeus). Like its freshwater relatives, this insect is about a centimeter long and stands on the surface tension of water. It skates back and forth above the sea. It lays its eggs underwater. But to do this, it needs a suitable substrate, such as any solid, floating debris.
Various organisms can use plastic as a habitat
Plastic replaces wood
The substrate does not have to be plastic: Traditionally, sea skaters lay eggs on algae, wood or floatable squid backbones known as cuttlebone.
"A very important substrate is pumice, which is released during volcanic eruptions, and is very similar to plastic in its characteristics. It is inorganic, it does not decompose easily and it's an ideal substrate to attach eggs to," said biologist Martin Thiel, who researches this particular habitat. Wood, for example, was much more important as a substrate in earlier centuries when large amounts of timber were still transported on waterways.
The industrial development of the 20th century led to barrages and dams that retain almost all the wood once used by the insects. But in the same period, the amount of plastic waste has dramatically increased. Today there is nearly 100 times as much plastic as there was in the 1970s. Researchers from the Institute of Oceanography at La Jolla University in California have now shown that the sea skater has much more room for its eggs. A lot of plastic collects in convergence zones where various currents come together.
While the sea skater once again has plenty of room for its eggs, that does not necessarily mean that it can also reproduce indefinitely. The limited food supply in the Pacific Ocean limits its growth. In addition, the sea skaters compete with the other inhabitants of the convergence zones.
"On this floating debris, there is a very particular community of organisms that has adapted to the conditions there," said Thiel, who reviewed the research of his Californian colleagues.
Advantageous for fishers
Each piece of plastic is first occupied by algae. Then other organisms establish themselves - like barnacles that are, in fact, shellfish. And then come the predators. The sea skaters are joined by slugs looking for nutrients on which they can prey.
"There are also moss animals and many other organisms that take hold on plastic. It goes all the way up to many little fish, some larger fishes and even tuna," Thiel said.
The plethora of life forms drawn to the floating ecosystems also have their uses for humans. Fishing fleets place satellite emitters on rafts and let them float. Along with other floating debris, the rafts eventually end up in the convergence zones filled with fish, and the fishers can go off in search of tuna.
Turtles that consume plastic can die from its effects
Plastic has clear downsides, though. Through the constant ebb and flow of the tide and the aggressive UV rays from the sun, the plastic gets worn down into smaller and smaller pieces until it can barely be seen with the naked eye. Most plastic in the oceans is now a microplastic of this form. And a great deal of plastic waste is already microscopically small when it lands in the ocean to begin with.
"Most are synthetic fibers from textiles, components from cleaning products, cosmetics and catalysts used in chemical production. Debris worn away from tires and from countless other grinding processes gets trapped in rainwater and moved to rivers and eventually into the ocean," explained Heinz-Dieter Franke, who works as a researcher at the Alfred Wegener Institute on the Heligoland archipelago. Franke added that since this plastic is invisible to the human eye, the public has long ignored the problem.
The smaller the plastic particles are, the more dangerous they can be for ocean inhabitants. Products of plastic decomposition can remain for a very long time in water. And surrounding toxins can also lodge themselves on the surface of plastic particles.
The invisible debris can easily be swallowed, broken down during digestion and make their way into other parts of organisms and, ultimately, into the food chain at large.
"As plastic particles decompose more, there's a greater risk of them affecting the hormonal health of organisms and leading to bigger problems," Franke said.
Although the problem of plastic waste in oceans has long been known, researchers are still unsure about what exactly changes biologically in areas where there is a lot of plastic floating around. Questions about the role bacteria plays in further decomposing plastic remain unanswered.
"We have to keep in mind that this is an environment located very far out in the ocean and that makes it extremely difficult to research. We know very little about the ecology of this unique habitat," Thiel said.
It is clear, though, that some ocean inhabitants profit from the changes, and others do not. A possible increase of the sea skater populations could be good for the small fish that prey on them. But for larger plankton-eating organisms, it represents a negative development, since sea skaters consume plankton.
"The flow of energy and materials is being shifted," Franke said. "What that means exactly depends on which viewpoint you adopt."
For those who fish, it is positive when certain fish populations increase. But "that is looking at things too strongly from the human perspective," which cannot necessarily serve as a scientific criterion, Franke said.