Microplastics, particles of waste polymers a lot less than 1 millimeter across, are plaguing our oceans. A new study has shown that they could be detrimental to marine ecosystems in a far more insidious way than anyone could have imagined by causing plankton poop to linger longer in the water and become plastic-bearing food for coprophagous organisms that are then eating by predators so potentially taking the microplastics further up the food chain.
We have all heard that there are vast island of plastic waste in the Pacific Ocean, although the problem is far more subtle. There are regions where ocean currents and winds causing plastics to accumulate, but far more sinister are the tiny fragments and beads of plastics, such as expanded polystyrene (Styrofoam) and others that are almost ubiquitous across the planet.
Zooplankton can readily ingest tiny beads of plastic, according to Matthew Cole of the University of Exeter and Plymouth Marine Laboratory, United Kingdom, and colleagues Penelope Lindeque, Elaine Fileman, James Clark, Ceri Lewis, Claudia Halsband and Tamara Galloway. Writing in the journal Environmental Science & Technology [Cole et al. Environ. Sci. Technol. (2016) DOI: 10.1021/acs.est.5b05905] the team explains that these tiny plastic particles eaten by zooplankton are later egested in the fecal pellets of the tiny creatures; the feces are subsequently consumed by other creatures.
"Microplastics include consumer items manufactured to be of a microscopic size (for example, exfoliating materials in personal care products), or derive from the biological-, photo-, and/or mechanical degradation and subsequent fragmentation of larger pieces of plastic," the team reports. They point out that plastic debris in the oceans from many different sources is not only environmentally persistent but buoyant and can be transported vast distances on ocean currents and as such reaches remote ecosystems including polar waters, deep-sea habitats, and mid-oceanic gyres.
Until now, little was known about how microplastics might affect the properties of the feces or marine organisms. Obviously, their feces are an important nutritional component of the marine ecosystem providing one component of the vertical flux of particulate organic matter as part of the biological pump. As such, the team has now tested the hypotheses that (i) fecal pellets are a vector for the transport of microplastics, (ii) polystyrene microplastics can alter the properties and sinking rates of zooplankton feces and (iii) fecal pellets can facilitate the transfer of plastics to poop-scooping organisms, the coprophagous biota.
They exposed the copepods Centropages typicus and Calanus helgolandicus, microscopic crustacea, to 20.6 micrometer polystyrene beads, 1000 per milliliter of water and included natural prey (about 1650 algae per milliliter) in the system. The density of the fecal pellets was significantly lower leading to a fall in sinking rate of 2.25 times and increasing the amount of fragmentation. "Our results support the proposal that sinking fecal matter represents a mechanism by which floating plastics can be vertically transported away from surface waters," the team reports.
"The vertical flux of fecal material can facilitate the movement of anthropogenic pollutants, including polycyclic aromatic hydrocarbons (PAHs) and hydrocarbon petroleum residues, to deeper waters," the team reports. The new results confirm that copepod fecal pellets can also facilitate the vertical transport of microplastics. Fish, whales, dolphins and other marine mammals, turtles, seabirds, invertebrates, and zooplankton are all known to be inadvertently ingesting plastic debris. Plastic has spread widely across the surface of the oceans, this new work on plastic-laden fecal pellets demonstrates how plastic could be drawn down to affect the deeper oceanic ecosystems detrimentally.
"We know from our laboratory experiments that microplastics can be consumed and later egested by copepods and other marine animals," Cole told me. "We also know animals in the wild consume plastics. What we need to work out now is how commonly these animals consume plastic, and are these plastics passing through the animals into their faeces. If so, is the amount of plastic in their poop sufficient to change the properties or sinking rate of this material in the water column?"
One might imagine that the "verticalization" of plastic waste in this way could be a good thing, drawing down plastic particles to greater depths where there is less life. But, one has to remember there are lots of bottom feeders out there and deep-sea creatures like sharks, whales and squid and those glowing fish, so they may all now be exposed to the problem whereas they were not before. Given that the "supply" of waste plastic is seemingly endless, we are perhaps now approaching a time when we are not only coating the oceans in plastic but we are filling their depths too with assistance from planktonic poop.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".