Microplastics alter feeding selectivity and faecal density in the copepod, Calanus helgolandicus
Rachel L. Coppock, Tamara S. Galloway, Matthew Cole, Elaine S. Fileman, Ana M. Queirós, & Penelope K. Lindeque
Summarized by Adriane Lam
The Problem: There is a growing body of research that shows that microplastics, tiny (1um-5 mm) pieces of plastics, have made their way into the deepest reaches of our oceans and are being ingested by marine life. Microplastics ingested by animals have been shown to cause adverse health effects to them, but as consumers of marine animals, these same microplastics are making their way into our diets. As yet, we do not know the exact ways in which microplastics can affect human health on longer time scales.
Zooplankton, which are small animals and protists that float in the water column and feed on primary-producing phytoplankton, are an important link between phytoplankton and other, larger animals. Zooplankton make up the base of the food chain, and are the main food source of marine mammals such as blue whales.
One type of zooplankton is especially common in our oceans today. Copepods are marine crustaceans that are found in nearly every freshwater and saltwater habitat. In addition to being an important food source, copepod poop is an important part of the biological pump. In other words, these animals’ poop transports atmospheric carbon dioxide (which is trapped in organic matter, or fixed carbon) to the seafloor, where it is stored in seafloor sediments. The poop also provides important nutrients to other animals that live within or on top of these sediments. Copepods have been shown to ingest microplastics in the wild. The ingestion of microplastics by copepods may alter the way in which these animals select their food. And of course, if microplastics are being ingested, they are also being exported to the seafloor in fecal pellets. This study was designed to look at how microplastics alter how copepods choose their food and how the ingested plastic materials affect the sinking rate of copepod poop.
Methods: In this study, the scientists grew three species of microalgae (all that copepods like to feast on) in the lab and spiked it with different types of microplastics. The microplastics included things such as nylon, which is commonly found in clothing, especially active wear, and polyethylene, which is the most commonly-used plastic in the world (it is used to make shopping bags, shampoo bottles, and toys, to name a few uses).
The microalgae with microplastics was then fed to the copepods back in the lab, where the amount of microplastics ingested was measured. The fecal pellets from the copepods were then collected and rinsed over a screen. To determine if microplastics contained in the poop affected the sinking rate of the pellets, the scientists dropped the pellets into cylinders filled with filtered seawater. They marked where the pellet was in the cylinder every 40 mm. To determine how different each pellet sank with microplastics, the scientists also measured the rate at which copepod poop without microplastics fell through the water column. When the poop reached the bottom of the cylinders, they were taken out and examined under a microscope. This way, the scientists could count the number of plastic pieces in each pellet.
Results: The scientists found that copepods preferentially liked to eat microplastics in a smaller size range (10-20 um), with a preference for the polyethylene over nylon fibers. When the copepods were exposed to microplastics, they preferentially did not eat as much algae. In addition, the copepods shifted their preference for one species of algae over others. Nylon fibers impeded ingestion of algae that was a similar size and shape to the microplastics. The scientists think the copepods associated algae of similar size and shape with microplastics, and thus avoided eating that algae species in an attempt to avoid plastic consumption.
The study confirmed that fecal pellets that contained both polyethylene and nylon particles were slower to sink through the water column. There was a difference in sinking rates between poop that contained more polyethylene, a denser microplastic, compared to nylon, a less dense material.
Why is this study important? This study is one of several that highlight the ways in which plastics are negatively affecting the food web in the marine realm. The reduced sinking rate of fecal pellets may also affect the rate at which carbon dioxide, a major greenhouse gas, can be removed from the atmosphere through photosynthesizing algae who are then eaten by zooplankton. If fecal pellets are left to float for longer, there is also a higher potential of the microplastics being re-ingested by other zooplankton through coprophagy (ingestion of fecal pellets). On long and short timescales, the decreased export of poop and fixed carbon dioxide to the seafloor may have large consequences, as plastic within poop could keep more carbon from being exported and stored on the seafloor.
Citation: Coppock, R. L., Galloway, T. S., Cole, M., Fileman, E. S., Queirós, A. M., and Lindeque, P. K., 2019. Microplastics alter feeding selectivity and faecal density in the copepod, Calanus helgolandicus. Science of the Total Environment 687, 780-789. Online.