☀ surface — high UV, low biofilm growth
▬ dashed line — photic zone limit (~10% light)
● deep water — dark, defouling favoured
Plastic debris in the ocean does not just drift — it lives through a slow biological and physical cycle. This 2D side-on cross-section of the water column follows a population of floating PE, PP and PS fragments as microbial biofilms grow on their surface, shifting their effective density until buoyancy and quadratic drag settle them into a sinking or rising terminal velocity. Sunlight drives photodegradation of the exposed plastic core through a Beer–Lambert light-attenuation profile, while biofilm growth is logistic and photo-inhibited near the bright surface, favouring the dim mid-column — and genuinely decays once a fragment sinks below the photic zone and its fouling organisms lose their light. Storms and grazers periodically strip a fragment's biofilm shell too, kicking off the fouling/defouling "elevator effect" measured in real ocean microplastic studies, and fragments that photodegrade past a size threshold are logged as newly created microplastic. Adjust sunlight, biofilm growth, sloughing frequency and simulation speed to see how each lever reshapes the vertical distribution of ocean plastic.