Watch a real coupled nutrient-phytoplankton-zooplankton (NPZ) model unfold: nutrients fuel a fast-growing bloom that greens the ocean surface, until grazing zooplankton and nutrient depletion crash it back down — the same cycle that drives satellite chlorophyll maps every spring.
Michaelis-Menten nutrient limitation, Holling type-II grazing, and quadratic zooplankton mortality combine into differential equations that genuinely reproduce a spring bloom, its crash, and — depending on your settings — repeating oscillations.
Raise the nutrient supply slider to feed a bloom, lower initial zooplankton to delay grazing pressure, and adjust light/temperature to speed up or slow down growth. Watch the ocean panel green and the graph trace N, P and Z over simulated days.
Sinking phytoplankton and faecal pellets from grazing zooplankton form the ocean's "biological pump," exporting carbon to the deep sea and making spring blooms a genuine player in the global carbon cycle.
This simulation solves a coupled NPZ (nutrient-phytoplankton-zooplankton) model, a standard tool in biological oceanography. Phytoplankton growth is limited by dissolved nutrients through Michaelis-Menten kinetics: uptake rises steeply while nutrients are scarce, then saturates once they are abundant. Zooplankton graze on phytoplankton with a similarly saturating response, so grazing pressure only really bites once phytoplankton biomass is high enough — a lag that is essential to reproducing a real bloom-crash cycle rather than a flat equilibrium.
Spring blooms happen because longer days and stratifying surface waters raise the effective growth rate right as winter has replenished deep-water nutrients through upwelling and mixing. Phytoplankton biomass explodes for one to a few weeks. As it grows, zooplankton populations respond with a delay — assimilating grazed phytoplankton into their own biomass — and eventually graze the bloom back down as nutrients are simultaneously drawn toward depletion. This pattern matters far beyond the classroom: it drives the seasonal satellite chlorophyll maps oceanographers use to track ocean health, underpins the productivity of many fisheries, and controls how much carbon the "biological pump" exports from the surface to the deep ocean each year.
Why do phytoplankton blooms happen in spring?
Winter mixing restocks the surface ocean with nutrients from depth, but low light and a well-mixed, unstratified water column keep phytoplankton from accumulating. As spring brings longer days and warmer surface water, the water column stratifies, trapping phytoplankton in the sunlit layer with plenty of nutrient still available, and growth outpaces losses for a few explosive weeks.
What actually limits phytoplankton growth in this model?
Two things: dissolved nutrient concentration, through a Michaelis-Menten saturation term, and a light/temperature multiplier representing season. When nutrients are scarce or light is weak, growth slows sharply; when both are abundant, growth approaches its maximum rate.
Why does the zooplankton population cause a crash?
Zooplankton graze on phytoplankton, and as the bloom grows there is more food to sustain and grow the grazer population. That grazer population responds with a lag, but once it catches up its consumption rate — combined with dwindling nutrients — pulls the phytoplankton population back down, sometimes triggering a full boom-and-bust oscillation.
It is a saturating relationship, N/(kN+N), where growth (or grazing) rises almost linearly when the resource is scarce but levels off toward a maximum as the resource becomes abundant. The half-saturation constant kN is the nutrient concentration at which growth reaches half its maximum rate — a smaller kN means phytoplankton can thrive even at very low nutrient levels.
Nutrient supply sets how fast deep-water upwelling replenishes surface nutrients toward their baseline; initial zooplankton sets the starting grazer population; light/temperature scales the maximum growth rate to mimic season; and simulation speed controls how many simulated days elapse per second of real time.
Yes — NPZ (and more complex NPZD, adding detritus) models are a standard tool in biological oceanography and are used, often coupled to ocean circulation models, to interpret satellite chlorophyll data, forecast fisheries productivity, and estimate how much carbon the ocean's biological pump exports to depth each year.
The green tint you see is a stylised stand-in for chlorophyll-a concentration, the pigment phytoplankton use for photosynthesis and the same signal satellites detect from space. As the phytoplankton state variable P rises, the panel shifts from dark blue toward mottled green, mimicking a real satellite chlorophyll map during a bloom.