Microplastic Biofouling & Sinking

Water Conditions

Water temperature 20 °C

Nutrient / turbidity level 5

Plastic Particles

Plastic density 930 kg/m³

Particle radius 0.8 mm

Water Column

Mean effective density

1025 kg/m³

Reached seafloor

Simulated time elapsed

0.0 days

How it works

Most everyday plastics (PE, PP) are less dense than seawater and float. But once in the water, microbes and algae colonize the surface as a biofilm. Because that living layer is itself denser than water, its growth steadily raises the particle's average density — the biofouling hypothesis for how floating plastic ends up on the seafloor (Kooi et al., 2017).

Biofilm coverage A (0-1), logistic growth with light-limited growth and grazing loss:
  dA/dt = k_g · L(depth) · A·(1-A) − k_loss · A
  L(depth) = exp(-depth / attenuation_depth)   (less light with depth)
  k_g = k_g0 · Q10^((T-20)/10)                 (warmer water → faster growth)

Effective density from a plastic core + biofilm shell of thickness A·δmax:
  ρ_eff = (ρ_plastic·V_core + ρ_biofilm·V_shell) / (V_core + V_shell),  ρ_biofilm ≈ 1388 kg/m³

Settling velocity from a force balance (weight − buoyancy = drag), solved
iteratively with the Schiller–Naumann drag correction (valid beyond the
low-Reynolds Stokes regime that plain Stokes' law assumes):
  v = sqrt( 8·g·r·|ρ_eff − ρ_water| / (3·Cd·ρ_water) ),  Cd = 24/Re·(1+0.15·Re^0.687)

Temperature — raises the microbial growth rate (a Q10 = 2.2 response), so biofilm builds up faster in warm water.
Nutrient / turbidity — richer, more productive water supports a thicker biofilm shell and a shallower light zone.
Plastic density / radius — sets how much extra biofilm mass is needed before the particle crosses neutral buoyancy (ρ_water ≈ 1025 kg/m³) and starts to sink.

Because a particle sinking out of sunlight grows less biofilm while grazers keep eating it, density can drop again and the particle can rise back up — this is the "yo-yo" vertical migration reported for lightly fouled microplastic in the real ocean. Vertical motion and the growth clock are sped up for watchability; the growth, mixing and drag equations themselves are the real ones used in the biofouling literature.

Bare plastic Biofilm-covered Settled on seafloor

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.