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FAQ Proposed Section, and Krib news
Two points. First, I finally updated the Krib with some new articles and
photos for the first time since October. Some highlights: new plant
species photos, and every CO2 explosion disaster story posted on this list
in the last year. By the way, it's our 2-year anniversary, and the
1-year anniversary running on the net-sponsored caltech drive!
http://www.cco.caltech.edu/~aquaria/Krib
Other point. "Dr. Dave" has written an article for possible submission as
part of the plant FAQ. The FAQ group has made comments on this first
draft's appropriateness (generally we think in its current form it might
not be right in the posted FAQ, but it might be revised...) The group also
suggested we post it here for more comments & suggestions as well.
So have at it! (It's also archived on the Krib in the Plants directory).
- Erik
WATER PLANTS 101
A basic Introduction to the physiology and ecology of aquatic plants
From eworobe at cc_UManitoba.CA
Date: Wed, 31 Jan 1996 17:40:01 -0600 (CST)
From: Dave Huebert <eworobe at cc_UManitoba.CA>
To: eriko at elmer_wrq.com
Subject: water plants
Carbon Dioxide
Dissolved Inorganic Carbon (DIC) in freshwater occurs as four
different species in equilibrium with one another. The four species of
DIC are; carbon dioxide (CO2), carbonic acid (H2CO3), bicarbonate
(HCO3-), and carbonate (CO3=). The total amount of DIC largely
determines the buffering capacity of freshwater, and the ratio of
these species with one another largely determines the pH.
Carbon dioxide dissolves readily in water. At air equilibrium, the
concentration of CO2 in air and water is approximately equal at about
0.5 mg/L. Unfortunately, CO2 diffuses about ten thousand times slower
in water than in air. This problem is compounded by the relatively
thick unstirred layer (or Prandtl boundary) that surrounds aquatic
plant leaves. The unstirred layer in aquatic plants is a layer of
still water through which gases and nutrients must diffuse to reach
the plant leaf. It is about 0.5 mm thick, which is ten times thicker
than in terrestrial plants. The result is that approximately 30 mg/L
free CO2 is required to saturate photosynthesis in submerged aquatic
plants.
The low diffusivity of CO2 in water, the relatively thick unstirred
layer and the high CO2 concentration needed to saturate photosynthesis
have prompted one scientist to state, "For freshwater submerged
aquatic macrophyte plants, the naturally occurring DIC levels impose a
major limitation on photosynthesis ... The DIC limitations on aquatic
macrophytes and its corollary, the need to conserve carbon, are
becoming increasingly apparent as important ecological features of
aquatic environments (George Bowes, 'Inorganic Carbon Uptake by
Aquatic Photosynthetic Organisms, 1985)."
Aquatic plants have adapted to CO2 limitation in several ways. They
have thin, often dissected leaves. This increases the surface to
volume ratio and decreases the thickness of the unstirred layer. They
have extensive air channels, called aerenchyma, that allow gases to
move freely throughout the plants. This allows respired CO2 to be
trapped inside the plant and in some species even allows CO2 from the
sediment to diffuse into the leaves. Finally, many species of aquatic
plants are able to photosynthesize using bicarbonate as well as CO2.
This is important, since at pH values between 6.4 and 10.4 the
majority of DIC in freshwater exists in the form of bicarbonate.
For the aquarist, the supply of CO2 can be augmented in two ways. Both
methods work by increasing the rate of diffusion of CO2 into the
plants. First, the rate of water movement in the aquarium can be
increased. This will decrease the thickness of the boundary layer and
ensure that CO2 levels are at air equilibrium. This method is
inexpensive, easy to implement and will produce excellent growth of
aquatic plants under most conditions. Secondly, CO2 can be injected
into the aquarium. This method can be expensive, and if done
improperly, can be lethal to fish. This latter method is only
essential, however, if there is a significant daily pH fluctuation in
the aquarium, or if the species of plants being cultured are
completely unable to use bicarbonate (such as Cabomba sp.).
Light
Plant chlorophyll absorbs light at wavelengths of 400 to 700 nm. This
is termed Photosynthetically Active Radiation (PAR). The intensity of
full, natural sunlight is approximately 2,000 umoles/m2/s, or 100
klux, of PAR. Light is attenuated rapidly in freshwater, however, so
that submerged aquatic plants receive far less than this amount.
Submerged aquatic plants are adapted to the low light levels found in
freshwater, and are classified as shade plants on the basis of these
adaptations. For instance, aquatic plant chloroplasts, which are the
organelles that contain chlorophyll, are often located in the top cell
layer of leaves to ensure that as much light as possible is absorbed.
Additionally, photosynthesis is saturated at only 15 to 50% full
sunlight intensity. Aquatic plants also have a low light compensation
point (LCP). The LCP is the point at which the rate of photosynthesis
equals the rate of respiration and growth stops. This allows them to
grow to depths that receive only 1 to 4% full sunlight (20 to 80
umoles/m2/s PAR).
For the aquarist, high light intensities are those which saturate
photosynthesis. Only metal halide bulbs can provide this level of
intensity. Medium intensities can be provided by 2 to 4 Watts per
gallon of fluorescent lights. At this level of intensity,
photosynthesis will not be at its highest level but will still be
greater than respiration. Anything less than 2 Watts per gallon is low
light. At this level of lighting, light compensation points will be
approached for many aquatic plants and only the most light tolerant
species will flourish.
The attenuation of light in water is wavelength specific. Water
absorbs light in the infrared and ultraviolet bands of the spectrum,
organic solutes absorb blue, violet and ultraviolet light,
phytoplankton absorb blue and orange-red light, and suspended silt
absorbs light fairly uniformly at all wavelengths. Aquatic plants are
therefore exposed to light that is vastly different in quality than
incident radiation. Moreover, light quality underwater can change
rapidly depending on water depth, turbidity, algal blooms and the
level and type of organic solutes present. These data suggest that
aquatic plants are flexible as to their light requirements and that
the pursuit of 'full spectrum' light is unnecessary in the freshwater
aquarium.
There is in fact clear evidence in the scientific literature that
freshwater plants can sustain high growth rates under simple
cool-white fluorescent light. Full spectrum lighting may perhaps be
useful, however, for true color rendition, and for attempts by the
hobbyist to achieve flowering in 'difficult' aquatic plants.
Plants are sensitive to daylength. The pigment that senses light in
plants is called phytochrome, and it absorbs light in the red/far-red
end of the spectrum. Research has shown that some aquatic plants are
short-day plants, some are long-day plants, and some are indifferent
to daylength. When exposed to the ' wrong' daylength, plants will
continue to photosynthesize in the presence of light, and grow
vegetatively, but will not complete their lifecycle and flower. This
is true of both terrestrial and aquatic plants.
Generally, it is safest to assume that tropical aquarium plants are
short-day plants, which means they are more likely to flower with a
duration of 10 to 12 hours of light per day. Plants which grow in
temperate zones are generally long-day plants and are most likely to
flower with 14 to 16 hours of light per day.
Mineral Nutrients
Essential mineral nutrients are conveniently separated into two
categories. Nutrients used by plants in relatively large amounts are
termed macronutrients. They are nitrogen (N), phosphorus (P), sulfur
(S), calcium (Ca), magnesium (Mg) and potassium (K). Nutrients used by
plants in small amounts are termed micronutrients. They are iron (Fe),
manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), cobalt (Co),
and boron (B). Other mineral elements, such as sodium (Na), are also
present in plants, but there are at present no definite roles for them
and so they are not classified as essential nutrients.
Aquatic plants, unlike their terrestrial counterparts, can absorb
mineral nutrients both from the water through their leaves and from
the sediment through their roots. Unfortunately, it is often assumed
that rooted aquatic plants can obtain all their mineral nutrient
requirements through their leaves. This is, however, incorrect. As
early as 1905 a researcher by the name of Raymond H. Pond stated that,
" ... a soil substratum is requisite for normal growth." and that, "
[rooted aquatic plants] make a better growth on a good loam soil, just
as many land plants do." Since then, the dramatic and consistently
superior growth of plants rooted in soil compared to plants rooted in
sand has been shown repeatedly for many different aquatic plant
species from many different types of habitat.
While the reasons for this superior growth are not completely
understood, certain facts are clear. First, submerged soils are
generally lacking in oxygen. This is of benefit to rooted aquatic
plants since under anoxic conditions Fe, P and N are more readily
available than under aerobic conditions. Second, nutrient
concentrations are higher in a fertile soil than in the overlying
water. Third, there is no competition with phytoplankton for available
nutrients. This latter point is important because with water based
nutrition, too much fertilizer and the algae bloom, and too little and
the plants stop growing.
Rooted aquatic plants are well adapted to growing in an anaerobic
substrate. They are able to 'pump' enough oxygen to the roots so that
in many cases the oxygen actually diffuses into the surrounding
sediment. They can also respire anaerobically if necessary and produce
lactic acid or ethanol instead of CO2 as a byproduct. The root
meristems (growing tips) of some species are even inhibited in the
presence of oxygen.
Aquatic plants also have requirements for certain nutrients in the
overlying water. Most rooted aquatic plants need Ca, Mg, K and a
carbon source in the water if they are to thrive. I say most, since
some aquatic species such as Isoetes sp. and Lobelia dortmanna
actually obtain even their carbon dioxide from the sediment. These
plants are adapted to growing in acidic softwater lakes that have
extremely low levels of DIC in the water and so absorb CO2 from the
sediment through their roots.
Aquatic plants grow in an environment that is often poor in mineral
nutrients. Perhaps for this reason, these plants can absorb and store
large quatities of nutrients for later use. Concentrations of some
mineral nutrients in plants, most notably micronutrients such as Fe
and Cu, can exceed the level in the water by 1,000 to 1,000,000 times.
Regular additions of mineral nutrients, particularly Fe, are therefore
essential for the sustained growth of aquatic plants in the aquarium.
---
Erik D. Olson amazingly, at home
eriko at wrq_com