The ocean water column is a four-level food chain, each level feeding the next (a simplified Lotka-Volterra cascade):
Phyto: dP/dt = r·P·(1−P/K) − a1·P·Z
Zooplankton: dZ/dt = a1·e1·P·Z − a2·Z·F
Fish: dF/dt = a2·e2·Z·F − a3·F·Pr − h·F
Predators: dPr/dt = a3·e3·F·Pr − m·Pr
Nutrient-rich upwelling raises phytoplankton's carrying capacity K, fueling every level above it. Warmer water speeds metabolic and growth rates but also raises phytoplankton's respiration losses (a simplified stand-in for real ocean thermal-stress effects like coral bleaching and reduced productivity above ~26°C). Fishing pressure removes fish and predator biomass directly (harvest term h·F), and because effects propagate up and down the chain, overfishing predators can trigger a trophic cascade — collapsing biomass at levels far from the one actually fished.
- Nutrient upwelling — raises phytoplankton carrying capacity, the base of the whole food web.
- Fishing pressure — direct harvest mortality on fish and predator levels.
- Sea surface temperature — speeds metabolic rates below about 24°C, but above that increasingly stresses phytoplankton productivity.
This four-level cascade mirrors real marine management questions — e.g. how removing sharks or tuna from a reef system can trigger booms and busts several trophic levels away, which is why ecosystem-based fisheries management looks at the whole food web, not just the target species.