Abstract
Every year, about four percent of the plastic waste generated worldwide ends up in the ocean. What happens to the plastic there is poorly understood, though a growing body of evidence suggests it is rapidly spreading throughout the global ocean. The mechanisms of this spread are straightforward for buoyant larger plastics that can be accurately modelled using Lagrangian particle models. But the fate of the smallest size fractions (the microplastics) are less straightforward, in part because they can aggregate in sinking marine snow and faecal pellets. This biologically-mediated pathway is suspected to be a primary surface microplastic removal mechanism, but exactly how it might work in the real ocean is unknown. We search the parameter space of a new microplastic model embedded in an earth system model to show that biological uptake can significantly shape global microplastic inventory and distributions and even account for the budgetary “missing” fraction of surface microplastic, despite being an inefficient removal mechanism. While a lack of observational data hampers our ability to choose a set of “best” model parameters, our effort represents a first tool for quantitatively assessing hypotheses for microplastic interaction with ocean biology at the global scale.
Highlights
Every year, about four percent of the plastic waste generated worldwide ends up in the ocean
The three simulations presented in the main text (Table 1) were selected to represent the range of “free” MP surface concentrations produced within a 14-member subset of the third 300-member Hypercube. This subset was produced by removing all samples of the Hypercube that were numerically unstable or using plastic input rates that greatly exceed the estimated annual global plastic release to the ocean[30] and marine snow particle aggregation rates far outside the estimated average range measured in the open ocean[31]
Local surface or sub-surface particle minima in the unattached MP compartment are present in 12 of the simulations, which is broadly consistent with the findings of widespread “missing” microplastic in surface s ampling[1,3]
Summary
About four percent of the plastic waste generated worldwide ends up in the ocean. Accounting for subsurface transport in a Lagrangian model changes MP distributions in the ocean, moving a significant fraction to the high latitude sub-surface[26] It does not simulate the surprisingly high accumulation rates reported in trenches[15], where concentrations of MP are reported to exceed 2000 particles per cubic meter, roughly 500. Eulerian ocean modelling was recently applied to plastics transport to examine the effects of buoyancy and idealized removal on plastics a ccumulation[18] They predicted potentially large quantities accumulating in the deep sea (negatively buoyant particles) and throughout the water column (neutrally buoyant particles). That both particle buoyancy and subsurface particle release[26] can strongly affect the development of accumulation zones suggests that biological transport might be influential in shaping global plastic distributions. Consideration of the spatial co-occurrence of biologically-active plastic and biology would be important in predicting plastic accumulation zones
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