From Surface to Sink: How Plastic Characteristics Dictate Biofilm Formation and Fate.

The “missing plastic” paradox highlights a critical gap in our understanding of plastic transport in the ocean. A leading hypothesis suggests a significant fraction is vertically distributed within the water column, modulated, among other effects, by biofouling—the colonization of plastic by algae. To investigate this process, we used a biofouling model by Kooi et al. (2017). Our study systematically investigated a wide range of polymer densities representative of the most prevalent plastics found in the marine environment while combining a parametric analysis of the biofouling process to clarify how plastic properties govern biofilm development and subsequent particle dynamics.

The model reveals that biofouling induces complex, oscillatory vertical migrations. Particles experience rapid biofilm growth, increasing their density and sinking. As they descend into colder waters, growth is suppressed, and metabolic losses reduce the biofilm, causing the particles to regain buoyancy and return to the surface to restart the cycle. Our analysis further demonstrates that particle size and density are critical drivers: smaller particles support a larger biofilm relative to their size, while density significantly influences the timescale of sinking onset for larger particles. Long riverine plastic emissions in the eastern Atlantic Ocean were tracked to study the accumulation areas in the ocean as a function of seasonal biofouling patterns and local currents patterns.

These results underscore that accurately modeling biofouling is essential for predicting the fate and distribution of marine plastic pollution in the ocean, moving beyond the simplistic assumption of particles as passive Lagrangian tracers.