Light Spectrum, PAR and Photosynthesis in Aquarium Plants

What PAR, PPFD and PUR measure, where chlorophyll absorbs light, why a Kelvin rating says little about plant growth, how intensity falls with depth and how long to light a tank.

Light as the energy source

In a planted aquarium, light is the main energy source for plant growth, and the light available limits which plants can be grown. The amount of light also has to stay in balance with carbon dioxide and nutrients: a common planted-tank strategy pairs adequate light and CO2 for fast growth of desired plants with control of nutrient levels, while excess nutrients and waste are associated with algae (Wikipedia, Aquascaping).

PAR, PPF, PPFD and PUR

PAR (photosynthetically active radiation) is the band of radiation from 400 to 700 nm that photosynthetic organisms can use; it roughly matches the range visible to the human eye.
PPF (photosynthetic photon flux) is the total number of PAR photons a lamp emits per second, in µmol·s⁻¹.
PPFD (photosynthetic photon flux density) is the number of PAR photons arriving on one square metre per second, in µmol·m⁻²·s⁻¹. This is the value a PAR meter reports at a given point, for example at the substrate.
PUR (photosynthetically usable radiation) weights the light spectrum by how strongly chlorophyll and accessory pigments actually absorb each wavelength, instead of counting every photon equally; it is expressed in the same photon units.

Plant scientists prefer photon-based units because photosynthesis is a quantum process: its chemistry depends more on how many photons arrive than on the energy each photon carries (Wikipedia, Photosynthetically active radiation). A related measure, yield photon flux (YPF), weights photons from 360 to 760 nm by measured photosynthetic response; on that curve orange-red photons near 610 nm give the most photosynthesis per photon. The daily light integral (DLI) combines PPFD with the number of hours of exposure to give the total photons received per day (Wikipedia, Grow light).

Lumen and lux, which are printed on many lamp packages, are weighted to human vision rather than to plant pigments: lumen describes the output of a source and lux the light falling on an area as the eye perceives it. The quantity relevant to plants is photon flux in µmol/m²/s over 400–700 nm.

Chlorophyll absorption peaks

Chlorophyll captures red and blue light most efficiently. Measured in diethyl ether, chlorophyll a has absorbance maxima near 430 nm and 662 nm, and chlorophyll b near 453 nm and 642 nm; in 90% acetone the peaks shift slightly to 430/664 nm and 460/647 nm (Wikipedia, Chlorophyll). Plants look green because green light is absorbed less and is reflected and scattered by cell walls. Accessory pigments such as carotenoids absorb wavelengths outside chlorophyll's narrow peaks, pass the energy to the photosystems and protect chlorophyll.

Absorption peaks of extracted pigments do not mean other colours are useless. In leaves of land plants, green light penetrates deeper into the leaf interior and can drive photosynthesis efficiently (Wikipedia, Photosynthetically active radiation), and plant growth is reported to be better when red and blue light are supplemented with green (Wikipedia, Grow light).

Kelvin is not a spectrum

Colour temperature, in kelvins, compares the colour of a lamp with that of an idealised black body. For fluorescent tubes and LEDs, which do not emit thermal radiation, the figure is a correlated colour temperature (CCT): the black-body temperature whose perceived colour most closely matches the lamp (Wikipedia, Color temperature). It is therefore a description of how the light looks to a human observer. Different lamp types with similar colour can have very different spectral power distributions; fluorescent lamps, for example, have a spiky distribution unlike the smooth curve of an incandescent bulb.

As a general tendency, cooler (higher-kelvin) white light contains relatively more blue photons and warmer light more red photons (Wikipedia, Grow light), but a kelvin value says nothing about how many photons reach the plants. Freshwater planted aquariums are commonly lit with neutral to daylight white lamps chosen for natural-looking colours, while strongly bluish light is associated mainly with marine tanks: in reef aquariums colour temperature matters because shorter wavelengths penetrate deeper into water and supply energy to the algae hosted by corals (Wikipedia, Color temperature). For freshwater plants these are largely appearance criteria; growth is governed by PPFD and photoperiod.

Intensity tiers and depth

Planted-tank guidance commonly groups plants into three demand levels, traditionally described by lamp power per volume of water rather than by PAR:

Undemanding plants: grow under low light, commonly without added CO2.
Medium-demand plants: need stronger light together with added CO2 and fertiliser.
Demanding plants: need strong light, extra CO2 and fertiliser, and frequent trimming.

Watt-per-volume figures, such as the 0.5–1 W per litre (2–4 W per US gallon) that Wikipedia gives as usual for planted aquariums, are approximations that depend on lamp efficiency; PPFD measured at the substrate describes what the lowest plants actually receive. Light weakens with distance from the lamp as it spreads over a larger area, so plants higher in the tank and near the centre receive more than those on the substrate or in the corners. Physiological studies show how little light some submerged species need. For Egeria najas, a 2003 study in the Brazilian Archives of Biology and Technology found a light compensation point of about 6–22 µmol·m⁻²·s⁻¹ PAR depending on the method, and maximum growth at about 50 µmol·m⁻²·s⁻¹, consistent with the species' ability to colonise turbid reservoir water.

Photoperiod

A timer keeps the light cycle constant so plants acclimate to a set rhythm (Wikipedia, Aquascaping). For a newly planted aquarium, nursery guidance commonly suggests a shorter photoperiod for the first weeks, while plants root and the system is not yet balanced, followed by a gradual increase, and warns that too much light promotes algae.