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Convergence of the trajectory of buoyant microplastics with algal biofouling towards underwater equilibrium.
Summary
Tiny plastic bits floating in the ocean pick up algae growth, which makes them heavier and causes them to sink—but as the algae eventually dies off, the plastic settles at a stable depth rather than continuing to sink to the ocean floor. In this study's example, that resting point was about 62 meters (roughly 200 feet) underwater, suggesting microplastics may cluster at predictable depths instead of scattering randomly. Knowing where microplastics tend to accumulate could help scientists better track how they enter marine food chains—and eventually, the seafood we eat.
Exposure to microplastics adversely affects ecosystems. Empirical studies show that microplastics are lost in deep regions of water. It is important to understand the trajectories of the microplastics in the ocean and to attempt to find their locations. We present a 3-dimensional dynamical system's model describing trajectory of microplastics with variable amount of algal biofouling. We study phase portrait: depth vs. number of algal cells. We present an example of specific water environment, microplastics and algae, and show that clean microplastics (starting its trajectory from oceanic surface) sink accumulating the algae, but later, due to death of algae, stabilize at equilibrium depth z with a small number of algae. For our choice of parameters, z is approximately 62m. This is the depth at which microplastics are likely to be found in this specific environment.