Seasonal influences, growth cycles, and water body types.
Tropical and subtropical aquatic and marsh plants have colonized very diverse habitats throughout evolution. Their natural biotopes consist of marshy floodplains, swamps, ponds, lakes, streams, and rivers with varying currents, as well as man-made environments such as rice fields, ponds, and reservoirs. These ecosystems are subject to the tropical periodicity of rainy and dry seasons, along with associated fluctuations in water level and seasonally changing light and climate conditions.
Water bodies are divided into permanent and periodically drying (temporary) biotopes. The latter, in turn, can have standing or flowing water. Colonization of a specific water body is only achieved by plants capable of adapting to its ecological conditions. For example, temporary biotopes have long been inhabited only by species that show the ability to survive drought periods using special vegetative organs or drought-resistant seeds. Thus, the duration of the dry period is a decisive factor for the distribution of individual species, just as the presence of a specific plant in a certain region allows conclusions to be drawn about its life cycle. However, the presence of certain species in temporary water bodies does not mean they can also be found in permanent ones.
In contrast to plants from temporary biotopes, species from permanent water bodies—which are mostly rivers, large lakes, or very extensive wetland regions—have a completely different life cycle. The determining factor for colonization in such waters is the difference between high and low water levels caused by the alternation of rainy and dry seasons. Only a few aquatic and marsh plants are able to adapt to water levels rising by more than one and a half meters during the rainy season and the associated decrease in light. They are hardly able to live underwater for several months. Added to this is the strong current prevailing in many permanent water bodies, where only a few aquatic plants can exist. Therefore, this ecological niche is conquered by a small number of species.
Vegetation in permanent water bodies is found only on low river banks or at the water’s surface, and is usually absent at great depths. Accordingly, in rivers with drastically fluctuating water levels of several meters or extremely fast currents, rooted aquatic and marsh plants are rarely found, except for those that float on or beneath the water surface or live predominantly in calmer bays and inlets.
Climate cyclicity. Temperature changes and light intensity have a significant impact on the conditions in which tropical and subtropical plants develop; these are observed both during the day and due to seasonal periodicity. In the tropics, day and night length vary very little throughout the year, so overall the daytime warming and nighttime cooling phases last about 12 hours each. The stably humid equatorial zone is characterized by abundant, more or less evenly distributed precipitation throughout the year, as well as a balanced temperature, with the average annual temperature fluctuating between 25–27 °C and dropping no lower than 18 °C in the cold months. There are no distinct seasons in this climate zone, and plant growth is mainly determined by daily fluctuations in day and night temperatures.
As geographical latitude increases, the ratio between day and night changes more significantly throughout the year. Temperature also varies depending on these seasonal fluctuations, resulting in distinct seasons in the subtropics to which vegetation adapts. At the same time, the cold seasons with their short days are periods of lowest light intensity.
Plants from subtropical regions are best equipped to withstand extreme temperature fluctuations. For instance, in the subtropics of the Southern Hemisphere (from southern Brazil to northern Argentina), where numerous aquarium plants are found, the winter nighttime temperature sometimes drops to soil-freezing points. On the other hand, daytime temperatures can rise above 35 °C. A clear distinction should be made between species that inhabit small, standing water bodies and are therefore subject to large daily temperature fluctuations depending on geographical location, and species found in vast, standing or flowing waters. In the aforementioned tropical habitats, daily temperature variations are very small; conversely, the temperature regime of such water bodies outside the equatorial zone can undergo significant changes during seasonal shifts.
Environmental factors.
Temperature has a significant impact not only on plant metabolism but also on many other life processes, such as flower formation, fruit ripening, germination, etc. Plant photosynthesis depends to a huge extent on ambient temperature, because every plant requires a specific temperature range for growth. The lower limit of this range represents the minimum, and the upper limit the maximum at which the plant can still grow. Below or above these cardinal points, plant development stops. The value at which a plant achieves the highest growth intensity constitutes the temperature optimum.
Plants from warm regions have higher temperature requirements than plants from regions with pronounced seasonal temperature fluctuations. It has been established that plants in their natural habitat have adapted to temperature variations between day and night. For the successful cultivation of aquatic and marsh plants, it is necessary to determine the maximum permissible limits as well as the temperature optima for each individual species.
Light. Light is of great importance for many plant growth and development processes, as it is an irreplaceable source of energy. It has an important influence on the morphology, anatomy, and photoperiodism of plants. Light intensity and duration in nature are not constant as in an aquarium; they change throughout the day, depending on the season and geographical latitude.
Radiation is weaker in water than in air. Long-wave thermal rays are absorbed within the first millimeters of the upper layer, and most of the infrared radiation in the upper centimeters of water. No more than half of the radiation reaches a depth of about one meter. For the necessary photosynthesis of aquatic plants, the required range lies between 380 and 780 nm, and even more is absorbed with increasing depth. Light absorption and its spectrum are significantly affected by colored and turbidity-causing substances contained in the water (humus particles, plankton, algae, suspended matter). For example, in water tinted yellow or brown by humus, not only is there strong light absorption, but its spectral composition shifts, meaning that yellow rather than blue light penetrates to greater depths. Only a few higher plants are able to penetrate depths of 3–10 m in clear freshwater.
At a depth of about 30 m, only algae communities are found. However, most aquarium plants in their natural conditions grow either as marsh plants at the water’s surface or occur at depths up to 30 cm, submerging deeper only during floods. Most aquarium plants inhabit unshaded or semi-shaded areas in their natural biotopes. Nevertheless, the light intensity for aquarium plants in natural conditions is still significantly higher than in an aquarium.
A distinction is made between high-light (sun-loving) plants and low-light (shade-loving) plants. Shade-loving plants have the advantage of being able to better utilize reduced light intensity. They achieve maximum assimilation even under weak light. A characteristic sign is large and broad leaves, such as those found in broad-leaved Cryptocoryne species and Barclaya motleyi. Shade-loving plants die if exposed to intense irradiation. Conversely, sun-loving plants require intense irradiation and utilize it better thanks to increased photosynthesis. They perish if insufficiently lit.
Cultivation experience and research in natural biotopes teach us that most aquarium plants develop significantly better under intense lighting than under weak light. Therefore, there is hardly any need to fear that overly bright lighting in the aquarium might harm most cultivated species. On the contrary, there is a much higher risk that under weak lighting, the assimilation of some plants will drop so much that it will not reach the compensation point—where CO2 uptake (photosynthesis) and CO2 release (respiration) are equal—after which the plant will eventually begin to die off. Many fast-growing plants react to light deficiency with characteristic changes, manifested, for example, in long shoots and elongated leaf petioles, as well as small, flat leaves.
Adaptability of aquatic and marsh plants.
Geographical origin of aquarium plants. 
