Microplastics: no effect on the productivity of the marine environment?!

Microplastics: no effect on the productivity of the marine environment?!by Amaelle Bisch and Anaelle Bouloy

Published by Charlotte Recapet the July 6, 2020 on 2:45 PM

According to the paper: “Do microplastics affect marine ecosystem productivity?” by Troost et al. 2018, microplastics would have almost no effect on primary and secondary production, or at least this effect could not be demonstrated! 

But what are microplastics?

Plastics appeared at the beginning of the 20th century and quickly became indispensable in everyday life. Plastics are composed of polymers (long carbon chain) of synthetic or natural origin. Microplastics are plastic particles smaller than 5 mm (. There are two types of microplastics: primary (directly manufactured at this size) and secondary (resulting from the degradation of macroplastics). The major disadvantage of microplastics is that they are easily ingested by marine biota (2,3).

Primary production, the basic link in a long chain 

Primary producers are at the base of the trophic chain. They are autotrophic organisms that produce their organic matter from a light (photosynthesis) or mineral source. In the marine environment, these organisms correspond to algae, phytoplankton and cyanobacteria and are the basis for zooplankton feeding. Zooplankton are secondary producers and heterotrophic organisms unable to synthesize their organic matter.

Phytoplankton
Zooplankton

How can microplastics affect these organisms?

In most of laboratory experiments, the impacts showed on plankton, they were for primary producers an inhibition on the algae growth, chlorophyll content and photosynthesis (4,5,6). For zooplankton, it was observed a reduced food consumption and an increasing in energy consumption with a low allocation of this energy for growth (7,8). But these effects depend to the species of plankton and the nature of microplastics (9). However, are these observations noted in the laboratory really transposable to the ecosystem scale?

The models spoke...

The study by Troost et al. (1) shows, by means of modelling, that at the level of primary production (algal biomass) there is no significant impact of microplastics. Indeed, this can be explained by different theories: (i) environmental conditions (availability of nutrients and light) already strongly impact the growth of algae, (ii) a transport technique (advection) would provide some protection.

Concerning zooplankton, or secondary production, the impact of microplastics is considered low over the entire North Sea because the observed changes are both positive and negative and therefore compensate each other. Exposure to microplastics leads to changes in spatial patterns and strangely enough the impact is not greatest in areas with the highest concentration of microplastics. Surprising but not so much because this can be explained simply by the small concentrations of algae found in off-shore areas (areas with the least concentration of microplastics) making zooplankton more sensitive to any change.

And how they have managed to demonstrate that?

The difficulty lies in a successfully integration of the data observed in the laboratory into an ecosystem-scale model. They modelled biogeochemical transport, hydrodynamics, nutrient inputs from rivers, primary production, zooplankton biomass and also microplastic concentrations in the North Sea. In the end, the results obtained are only based on modelling and could not be verified in the field, so the conclusions should be "swallowed" with caution.

Cited articles

1. Troost, A., Desclaux T., Leslie, A., Van Der Meulen, M., Dick Vethaak, A., 2018. Do microplastics affect marine ecosystem productivity? Marine Pollution Bulletin 135 (2018) 17–29
2. Ivar do Sul, J., Costa, M.F., 2014. The present and future of microplastic pollution in the marine environment. Environ. Pollut. 185, 352–364. http://dx.doi.org/10.1016/j. envpol.2013.10.036.
3. GESAMP, 2016. Sources, fate and effects of microplastics in the marine environment: part two of a global assessment. In: Kershaw, P.J., Rochmann, C.M. (Eds.), IMO/FAO/ UNESCO-IOC/UNIDO/WMO/IAEA/UN/ UNEP/UNDP Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection. Rep. Stud. 2016pp. 220 GESAMP No. 93.
4. Zhang, C., et al., 2017. Toxic effects of microplastic on marine microalgae Skeletonema costatum: interactions between microplastic and algae. Environ. Pollut. 220, 1282–1288.
5. Sjollema, S.B., et al., 2016. Do plastic particles affect microalgal photosynthesis and growth? Aquat. Toxicol. 170, 259–261.
6. Casado, M.P., et al., 2013. Ecotoxicological assessment of silica and polystyrene nano-particles assessed by a multitrophic test battery. Environ. Int. 51, 97–105.
7. Watts, A.J., et al., 2015. Ingestion of plastic microfibers by the crab carcinus maenas and its effect on food consumption and energy balance. Environ. Sci. Technol. 49, 14597–14604.
8. Van Cauwenberghe, L., et al., 2015. Microplastics are taken up by mussels (Mytilus edulis) and lugworms (Arenicola marina) living in natural habitats. Environ. Pollut. 199, 10–17.
9. Wenfeng Wang, Hui Gao, Shuaichen Jin, Ruijing Li, Guangshui Na, 2019. The ecotoxicological effects of microplastics on aquatic food web, from primary producer to human: A review. Ecotoxicology and Environmental Safety 173, 110–117

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