Clustering as a mechanism for enhanced reaction of buoyant species

Buoyant material has a tendency to form dense clusters at the ocean surface. This has been observed in distributions of marine life and floating plastic contaminants. The main mechanism behind this is that particles with positive/neutral buoyancy do not behave as passive tracers in stratified flows. It could be expected that coextensive clustering between plankton and toxic ocean contaminants could lead to enhanced ecological risk. However, such interactions cannot be sufficiently modelled in a standard passive tracer approximation. Given the large uncertainty in the form of converging currents and how to model interactions of buoyant tracers, we opt for an idealised modelling approach. The simplicity of our model allows easy interpretation of the novel physical considerations. We demonstrate that the global dynamics of our biogeochemical model are significantly altered by clustering forces. Most notably, a new balance in the ecosystem exists in which reactions are dominated entirely by those within the dense clusters. This greatly enhances the impact of destructive pollutants through efficient mixing. There is evidence that this equilibrium will be robust moving to more complex and realistic models.