Day 0Water 20 °CAlgae —Herons 0FPS 0

Drag to orbit (the cut-away stays in view), wheel or pinch to zoom. Glowing arrows are nutrient (nitrogen) flows between levels, thicker = faster. Organisms are drawn larger than life; their numbers follow the model.

Pond Food Web: Nutrients, Plankton, Fish and Heron Chain Lab

Follow nitrogen up a pond food chain and back down through the mud. Change light, warmth, nutrients, fish and herons and watch the cascade.

Pond conditions

Time

Live readouts (mg N per litre)

Nutrients, plankton, detritus

Fish (small fish and bass)

Experiment: enrich the pond

Runs the pond to a steady state (1500 days) for nutrient inflow 0 to 1.5 mg/L with the current settings and plots each group as a fraction of its own maximum.

Independent numerical checks

Controls

Drag / one finger: orbit; wheel / pinch: zoom. Time scale = simulated days per real second.
Presets set the sliders and restart the pond. Switching "Small fish" or "Bass" off removes that level, so you can watch what each level does to the ones below it (trophic cascade).
Herons are an external predator on fish (small fish and bass in proportion to their biomass); part of what they eat leaves the pond when they fly away.

The science

Currency. Everything is tracked as nitrogen, mg N per litre of pond water: dissolved nutrient N, phytoplankton P, zooplankton Z, small fish F, bass B and detritus D (dead material in the mud and water).

Uptake: up = v_max f_T f_L N/(k_N+N) P (Monod), f_T = 1.8^(T-20)/10, light f_L = 1.25 L/(0.25+L).
Grazing and predation use a type III functional response a X²/(h²+X²) Y, which gives prey a refuge at low density (plants, mud). A fraction e of what is eaten becomes new predator biomass; the rest (1 - e) is egested to detritus.
Every group also loses biomass by linear loss and, for Z, F and B, a quadratic crowding term. All losses go to detritus.
Decomposers (worms, snails, bacteria) return detritus to nutrient: rem = 2.0^(T-20)/10 r_D D.
Flushing: q (N_in - N) brings nutrient in and washes plankton and detritus out (fish stay).
Herons: H c_H X/(k_H+X), X = F + B; 30 % of the nitrogen leaves with the bird, 70 % returns as droppings.

Conservation. With no flushing and no herons the sum N+P+Z+F+B+D is constant (the page checks it). Integration is 4th-order Runge-Kutta, step 0.1 d.

Honest simplifications. One well-mixed water column (no depth, no stratification, no oxygen, no seasons, no sediment phosphorus); all rate constants are illustrative, in the right order of magnitude but not fitted to a particular pond; fish are lumped into two groups and bass eat only small fish; frogs, dragonflies, tadpoles and invertebrates in the picture are decoration driven by the model, not separate state variables. Numbers describe behaviour (cascades, enrichment, warming), not a forecast.

Models built with Blender (fish, heron, frog, plants, bank cut-away) and rendered with Three.js.