The amount of plastic discharged by rivers to our oceans and seas has been overestimated by two to three orders of magnitude. This would explain why a large volume of microplastics seems to disappear in a mysterious ocean sink. Erroneous calculations on the fluxes and overall mass of plastics discharged into the ocean result from a lack of critical vision, and of common methodologies and guidelines in international research in this area, according to an article published in the journal Science, which features Professor Miquel Canals, from the Department of Earth and Ocean Dynamics, Faculty of Earth Sciences of UB, as one of the co-authors.
The paper invites the international scientific community to unify criteria and overcome methodological biases in the studies about plastic pollution ─specifically microplastics─ of marine ecosystems. Other authors of the article are Lisa Weiss, Wolfgang Ludwig, Serge Heussner, Mel Constant and Philippe Kerhervé, from Centre of Education and Research on Mediterranean Environments (CEFREM) of the University of Perpignan; Jean-François Ghiglione, from the French National Centre for Scientific Research (CNRS), and Claude Estournel, from the University Toulouse III.
The mysterious ocean plastic sink
Rivers are the main source for plastic discharge in the ocean. According to current assessments, the floating stock of microplastics at the ocean surface ─from tens to hundreds of metric tons per year─ would be just a small part of the millions of metric tons that are discharged by rivers. This unequal balance has led to the hypothesis of the existence of a large ocean plastic sink where the necessary amount of microplastics to make the budget fit would accumulate, so that the amount of those at the ocean surface plus those in the missing sink would equal those presumably discharged by rivers into the sea.
“The need for a sink where microplastics would massively end disappears if we consider that a key factor in the equation ─i.e., the river contributions─ is overestimated because of cumulative errors in the methodology and approach commonly applied by most research teams”, notes Professor Miquel Canals, head of the Consolidated Research Group on Marine Geosciences of the UB.
“Therefore, we can now firmly state that the missing ocean sink is no more needed as it has been taken away by the rivers, as shown after our critical review of methodologies, assumptions and calculations in previously published studies”, notes Canals.
The new study identifies the main methodological mistakes leading to erroneous assessments when quantifying the fluxes and overall mass of microplastics discharged by rivers into the sea at a global scale. In particular, mistakes result from the difficulty to obtain robust datasets for mass conversion after numbers of microplastics; from the integration of non-comparable scientific data that were obtained by means of different sampling techniques; and from assessments based on the relation between microplastic fluxes and the MPW index (mismanaged plastic waste). Regarding the latter, estimations become more consistent when adding the population density and the drainage intensity to the equation.
Thus, the time cycle of microplastics in the oceans known to date is distorted by erroneous calculations and overestimated values of the flux of plastics discharge by rivers into the oceans. Correcting methodological biases in the scientific literature “would involve changing the concept of residence time of microplastics on the ocean surface ─so far considered ultrafast─ for a more realistic and logical view that would involve periods of a few years”, states Canals.
A fight without frontiers to preserve the planet’s oceans
The North Atlantic is the marine area with the most different estimations, as stated in the study. In this area, which receives less than 6% of the global river discharge, the value of the flux of river microplastics was considered, so far, to be low when compared to those from Asia and Africa. However, if the density of population and the intensity of drainage are considered, we get higher values that adjust much better to accumulations observed at the surface of this ocean.
Marine waste does not care about frontiers and has reached the most remote areas of our oceans and seas. To fight the pollution that results from microplastics, “we need to act on the sources where plastic waste is generated. That is, we need to act where we human beings are living, and change consumption habits of our waste society, and do it at a large scale, in extensive territories, worldwide”, affirms Canals.
“Our study shows that marine microplastic pollution does not come only from Asian and African countries ─with poor or zero waste management─ as one might think, but also from countries with well-established waste management systems. Would the discharge of microplastics from rivers to the sea stop suddenly, the amount of floating particles and their harmful effects on marine ecosystems will still persist during many years at least”.
The study on the impact of plastics on the marine environment is a recent field of scientific research that has generated a high number of scientific publications in the last few years. For a while now, several research teams have started thinking on the strengths and weaknesses of the work conducted so far, including sampling and analytical protocols, the consistency of the results obtained to date and the future of marine litter research. The published article in Science is a call to the scientific community to overcome inertia from the past, correct mistakes and work on common protocols and guidelines for the progress of knowledge and the provision of sound information facilitating decision-making processes for the urgent environmental protection of our seas and oceans.
Weiss, L.; Ludwig, W.; Heussner, S.; Canals, M.; Ghiglione, J.F.; Estournel, C.; Constant, M.; Kerhervé, P. “The missing ocean plastic sink: Gone with the rivers“. Science, July 2021. Doi:10.1126/science.abe0290