Eutrophication Dead Zone: Dissolved Oxygen Collapse

Bloom Drivers

Nutrient loading (N/P runoff) 45%

Thermal stratification 60%

Wind mixing / reaeration 35%

Simulation speed 3.0 h/s

Water Column Readout

Day 1, 08:00DAY

Surface DO

9.0 mg/L

Bottom DO

9.0 mg/L

Hypoxic zone (<2 mg/L)

Algae biomass

0.3 mg/L

How it works

The water column is split into 18 depth layers. Each layer tracks algal biomass A and dissolved oxygen DO, coupled by light, nutrients, and vertical mixing:

Light (Beer-Lambert):  I(z) = I0 * exp(-(k_w*z + k_a*ΣA))
Algae growth:          dA/dt = r*A*(1-A/K)*I/(I+I_half) - m*A
DO balance:            dDO/dt = P(A,I) - R(A) - D_decomp
                               + ∂/∂z[ D_z(z) * ∂DO/∂z ] + reaeration

Nutrient loading raises the carrying capacity K, so more algae grows per unit light. During the day, photosynthesis P can push surface DO above saturation; at night only respiration R runs, so DO falls. Dead algae sink and decompose, consuming oxygen at depth — this is the same biochemical-oxygen-demand mechanism behind the Streeter-Phelps sag curve, just distributed over depth instead of downstream distance.

Nutrient loading — sets algal carrying capacity K; heavier fertilizer/sewage runoff means a denser bloom.
Thermal stratification — a strong thermocline sharply cuts the vertical diffusion coefficient D_z at mid-depth, isolating the bottom hypolimnion from surface reaeration — the classic setup for a summer dead zone.
Wind mixing — scales surface reaeration and near-surface diffusion; strong wind resists stratification and reoxygenates the top layers.
Storm mixing — a five-second burst of full-column turbulent mixing, showing how a single storm can temporarily break down stratification and flush oxygen to depth.

Real-world relevance: this feedback loop — nutrients → algal bloom → decomposition → hypoxia — drives real seasonal dead zones such as the Chesapeake Bay, Lake Erie's central basin, and the Gulf of Mexico's Mississippi plume.

Oxygen-rich water Hypoxic / dead zone Algal biomass

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