Unlike
other organisms that derive their energy from consuming organic material, plants
obtain their energy from sunlight and atmospheric carbon dioxide. Sunlight
provides a natural energy source which plants are able to harness through
special cells in their leaves. The energy of sunlight is used to break down
carbon dioxide and water, which can then be used to create glucose, a form of
sugar, or storable energy. This process is called photosynthesis, and without
it, complex life could have never evolved on Earth. Providing light in the
aquarium is relatively simple but plants will do best under certain types of
light. The spectrum, intensity and duration of light all affect plant growth in
the aquarium.
The photosynthetic process
As mentioned above, light is vital for plants to carry out photosynthesis, the
production of energy, or food, using sunlight, water, and carbon dioxide. The
rate of photosynthesis is closely connected to a plants growth rate and ability
to repair and reproduce. Providing other factors such as nutrient supply and
environmental conditions are good, a plants rate of photosynthesis is directly
proportional to the amount of light the plant receives. At low light levels, a
plant may be unable to photosynthesis at a suitable rate, which results in slow
growth and sometimes ill health. Light can be considered as a source of 'food'
for the plants. Temperature also affects photosynthesis; a higher temperature
increases photosynthesis whilst a cooler temperature will cause a decrease.
However, an increase in the aquariums lighting and temperature will only benefit
plants if they are able to obtain a suitable amount of nutrients and carbon
dioxide. Therefore, if plants are not doing well in the aquarium, an increase in
lighting may not always be the answer.
A single fluorescent tube lights most aquariums, in shallow (30cm or less)
aquariums this may be sufficient for some plants. In most cases however, more
lighting may be needed. The type of light is equally important as the amount of
light.
Light spectrum
White light is made up of a number of different colours, each colour represents
a 'wavelength' of light. The wavelengths, or colours of light are collectively
known as the light spectrum. The spectrum can be seen in a rainbow, which is
produced when raindrops 'spread' sunlight into its individual wavelengths. A
light that appears slightly yellow, such as a normal household light bulb, can
be considered as peaking in the yellow area of the spectrum, whilst a traffic
light may peak in the red, yellow or green areas.
The light spectrum should be considered when providing light for aquatic plants.
Each colour, or wavelength of light has a specific 'energy' and only certain
wavelengths or 'energies' can be utilised by the special photosynthetic cells in
plants. Green light for instance, is not used by plants and is reflected, this
is why plants appear green. Red light has a 'long' wavelength and looses its
intensity as it travels through water. Although plants can efficiently utilise
red light, they receive a lower quantity of this light in nature. Blue light has
a short wavelength and passes through water easily; both plants and algae use
this type of light heavily. A good spectrum for aquatic plants would therefore
peak in the easily utilised red areas and penetrating blue areas.
Duration of lighting
Simply increasing the duration of light in the aquarium may at first glance
appear to be a good method of increasing a plants supply of light. Plants
however, require a 'rest' period of darkness to function properly. If a plant
were left in well-lit conditions 24 hours a day, it would most likely 'wear
itself out'. Increasing the duration of light also encourages algae to grow,
which is undesirable. For most plants, a period of 12-14 hours of light is
sufficient.
Types of light
Most aquariums come with fluorescent tubes, which provide a low-cost, and easy
maintenance form of lighting. For the majority of aquariums, fluorescent tubes
are sufficient, although there are a number of different types to choose from.
The different types of fluorescent tube all emit a different light spectrum.
Some tubes are specifically designed for plants and produce a slightly yellow
light whilst others increase the appearance of fish's colours (purple-blue
colour), or provide a more natural white colour (full-spectrum). To create a
good overall light for both the plants and the appearance of the aquarium, a
combination can be used. A mix of one or two tubes designed for plants and a
full spectrum tube usually produces an aesthetically pleasing light, which is
also useful to plants.
Other types of lighting are also available for aquariums. The two most common of
these are mercury vapour and metal halide lamps. These lamps are suspended above
the aquarium (in these cases the aquarium does not have a hood) and provide a
much higher intensity of light than fluorescent tubes. The lamps are expensive
but have an intensity that matches several fluorescent tubes. The running costs
are also far cheaper for lamps than for the equivalent number of fluorescent
tubes. Lamps should only be considered for deep aquaria with many plants, or
where the aquarium is specifically designed for plants.
Light loss in the aquarium
A great deal of light can be lost between the light source and the plants leaves
in the aquarium. The effect of light loss is significant and may be as much as
50% or more of the initial output. Light loss can be reduced however by a number
of methods.
Fluorescent tubes emit light in all directions and much of the light is absorbed
by the aquarium hood or released through the glass. To recover much of this
light, a reflector can be fitted which will direct all of the light downward,
into the aquarium.
Once the light is heading in the right direction, it may have to pass through a
cover glass or condensation tray. These are used in many aquariums to prevent
water loss or condensation on electrical equipment such as the lights. The
covers can quickly become dirty with water and algae deposits and will then
absorb a great deal of light. Regular cleaning will minimise any light loss
although in certain circumstances the cover can be removed altogether.
Fluorescent lights run at a relatively cool temperature so are not affected by
the odd splash of water or by condensation. If they are fitted with waterproof
connectors, they can be used inside a hood without any protection (cover glass
or tray).
Floating debris in the aquarium can also absorb some light so a good mechanical
filter and regular maintenance will help to keep the water clear and reduce
light loss through the water.
