This is a thermal-stress integrator, not a spatial diffusion field: it tracks one coral head over time using the same metric reef managers use in the field — NOAA's Degree Heating Weeks (DHW), the rolling 12-week sum of how far sea-surface temperature (SST) sits above the Maximum Monthly Mean (MMM) plus a 1 °C bleaching threshold:
HS(t) = max(0, SST(t) − MMM − 1°C)
DHW(t) = Σ HS(τ) dτ over the trailing 12 weeks [°C·weeks]
HSeff = HS(t) · (1 − 0.4·flow) (flow thins the thermal boundary layer)
ROS(t) = light · (1 + HSeff/2) (heat + light → excess photo-oxidative stress)
dN/dt = −kb·ROS(t)·N + kr·(Nmax−N)·recovery(flow, HSeff)
N is the zooxanthellae (Symbiodinium) density inside the coral tissue. Combined heat and light drive reactive-oxygen production (Lesser 1996; Jones et al. 1998) that the coral responds to by expelling its symbionts — the literal mechanism of "bleaching," since the algae are what give coral its colour. Water flow does double duty: it reduces the effective thermal stress by thinning the mass-transfer boundary layer (Nakamura & van Woesik 2001) and it accelerates recolonisation once temperatures fall.
- Coral panel — colour interpolates from healthy brown to bleached white as N/Nmax falls; drifting flecks are expelled symbionts.
- SST / DHW panel — the simulated heatwave against the MMM+1°C bleaching threshold, with the accumulating DHW curve.
- Symbiont density panel — N/Nmax over time; bleaching typically becomes visible once density drops below ~50%.