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The Dynamics of Buoyant Microplastic in the Ocean Forced by Unsteady Insolation
Summary
This mathematical modeling study examined how sunlight-driven biological growth (biofouling) on microplastics affects their up-and-down movement in the ocean over seasons and latitudes. In summer, algae grow on plastic surfaces and weigh them down toward deeper water; in winter at high latitudes, low light halts biofilm growth and plastics float back to the surface for longer periods. The model reveals that microplastics are not static in the water column — they cycle vertically with the seasons, complicating estimates of where they accumulate and who is exposed to them.
Building on the deterministic mathematical models of Kooi et al. (Environ. Sci. Technol. 51, 2017) and Kreczak et al. (Limnol. Oceanogr. 66, 2021), this study investigates the trajectories of biofouled microplastic particles forced by unsteady insolation. A new, non-dimensional system of governing equations is derived to predict the particle trajectory in a stratified, quiescent ocean subject to unsteady insolation. In the absence of stratification, unsteady insolation drives surface-to-depth oscillations with amplitude varying seasonally, attaining a maximum/minimum in the summer/winter, respectively. At high latitudes, a particle spends an increasing length of time floating on the sea surface in the winter when biofilm production is minimal or absent altogether. We demonstrate that, at 70N, the oscillations are modulated; in summer they are briefly subsurface, while in spring/fall they reach the sea surface and exhibit the largest amplitude throughout the year. In contrast, forcing the particle motion with constant, annually averaged insolation, at any given latitude, always generates persistent surface-to-depth periodic oscillations. In a stratified ocean, the previously reported persistence of subsurface particle oscillations forced by constant insolation is no longer exhibited for unsteady solar forcing. At lower latitudes, surface-to-depth oscillations with seasonally varying amplitude occur in a stratified ocean. In polar latitudes, the particle dynamics displays three regimes: (i) floating at the sea surface in winter, (ii) surface-to-depth oscillations in spring/fall with time-varying amplitude, (iii) subsurface oscillations around the compensation depth, where biofilm production and mortality rates balance. Decreasing the particle size leads to longer oscillation periods, and at high latitudes the particle either floats or performs subsurface oscillations with seasonally varying amplitude about the compensation depth.