Marine food webs stack trophic levels: sunlight-fixing phytoplankton, grazing zooplankton, and predatory fish. Each level's growth depends on the resource below it and losses to the level above — a coupled set of logistic/Lotka–Volterra style equations.
dP/dt = r·L·P·(1 − P/K) − g·Z·P
dZ/dt = a·g·Z·P − m·F·Z − dZ·Z
dF/dt = b·m·F·Z − dF·F
L = light/nutrient factor, g = grazing rate, m = predation rate
- Sunlight / nutrients — scales phytoplankton's growth rate, the base of the whole pyramid.
- Zooplankton grazing rate — how fast zooplankton consume phytoplankton.
- Fish predation rate — how fast fish consume zooplankton.
- Water temperature — raises everyone's metabolic (growth and death) rates, speeding the whole cycle.
Real oceans show exactly this cascade: nutrient-rich upwelling zones support huge phytoplankton blooms, which in turn support the fisheries that feed roughly a billion people worldwide.