New insights into which microplastics pose the biggest risk to zooplankton

A new study led by a Essex PhD student Zara Botterell has found the shape and smell of microplastics may influence whether they are eaten by tiny but vital marine zooplankton in our oceans.

Zara Botterell, from the School of Life Sciences, has been working with researchers at Plymouth Marine Laboratory and the University of Plymouth. Their study shows the likelihood of different zooplankton eating microplastics is related to their different feeding strategies, and, in particular, suggests that species using chemosensory clues to find their food are at an increased risk of ingesting microplastics.

Marine zooplankton (drifting, mostly microscopically small, creatures) are an important part of the marine food web and one of the groups of organisms that are likely to be affected by the increasing amount of microplastics in the ocean. Despite this, the underlying mechanisms that influence how zooplankton can end up eating microplastics are still poorly understood.

The researchers investigated how the microplastic ingestion rate was affected by different shapes of plastic and whether it was affected by the presence of chemicals derived from marine algae that can colonise the surface of microplastics.

Three species of plankton were used in the study: copepod species Calanus helgolandicus and Acartia tonsa; and the larvae of the European lobster Homarus gammarus. Their feeding habits were studied using bead, fibre and fragment-shaped microplastics to understand whether there was a preference towards certain shapes.

Additionally, some of the microplastics were infused with chemical compounds (including dimethyl sulfide, or DMS) that are produced by marine algal species. Such small algal species are able to live on the microplastics as part of the biofilm that covers the surface over time. The algae are a food source for zooplankton, so the researchers suspected that these chemicals, which may mimic the smell of natural prey, could attract the zooplankton to consume the microplastics.

The results showed that each of the three zooplankton species had a preference for a different microplastic shape. Copepods C. helgolandicus and A. tonsa went for fragments and fibres, respectively, whilst the larval lobsters preferred the beads. This indicates that the different feeding strategies of the zooplankton may well be influencing which microplastics they end up consuming.

Importantly, the presence of the algal-derived chemicals had a clear effect on the plankton's feeding. The H. gammarus larvae and A. tonsa copepod both ate more fibres and fragments that were infused with DMS than not, whilst the C. helgolandicus’s microplastic ingestion increased across all shapes when infused with DMS.

Zara Botterell, lead author said: "When we're looking at the impacts of marine microplastics, it's important to remember the different ways that marine organisms can interact with them. As microplastics age in the sea, they build up biofilms which, as our study suggests, could be playing a significant part in the likelihood of zooplankton eating the microplastic.

"We need to understand the interaction between plastic and marine animals to predict the effects plastics are having on the marine environment, to support both efforts to reduce marine plastic pollution, and the wider work going on around conserving our fragile ocean ecosystems."

• Picture courtesy of Plymouth Marine Laboratory