BOTANY WITH BASICS OF HYDROBOTANY (AQUATIC PLANTS OF UKRAINE) - B.Ye. Yakubenko - 2011

I. CONDITIONS FOR THE FORMATION AND GROWTH OF AQUATIC PLANTS

The impact of water level dynamics on the variability of aquatic plants

Water level fluctuations determine The Nature and structural Variability of species and communities (biocenoses) in both space and time. These fluctuations occur with a certain temporal regularity (ecophase) and throughout the growing season (ecoperiods). The alternation of potential ecoperiods—for instance, from one water drawdown in a water body to the next—spans a longer duration than a single growing season, which constitutes an ecocycle. An ecophase is a dynamic living environment that influences the morphogenesis of species populations and the synmorphology of plant communities.

Depending on the water level height, several ecophases are distinguished: the limnic ecophase (LE) with deep inundation; the littoral ecophase with shallow inundation (10–50 cm depth), designated as LitEP; the palustrine (marsh) ecophase with a shallow water depth up to bottom sediments protruding to the surface (PEP); and the terrestrial ecophase in the absence of surface flooding (TE). Additionally, The concepts of agro-littoral, agro-palustrine, and agro-terrestrial ecophases are utilized.

An ecoperiod encompasses all ecophases that succeed one another over the course of a year (the growing season).

An ecocycle is the sequential change of ecoperiods over the span of several years. The duration of an ecocycle is limited by the littoral-terrestrial period.

Depending on the amplitude of water level fluctuations and their nature of impact on aquatic macrophytes, the following potential sequential changes of ecophases within an ecocycle are distinguished:

- with a constant water level — a water-littoral sequence of ecophase changes;

- with a pronounced drop in the water level down to the exposure of the shore zone or the bottom of the water body — a littoral-terrestrial sequence;

- with a gradual (or sudden) rise in the water level following a preceding drop (exposure, drainage) — a terrestrial-littoral sequence.

In the temperate climatic zone, the ecoperiod practically coincides with the growing season. In the tropical climatic zone, it matches the calendar year.

The limiting factor for the selective action on The Development of species and certain macrophyte communities is the sequential temporal shift of ecoperiods from littoral-terrestrial to terrestrial-littoral, transitioning from water-submerged to unsubstituted (exposed, terrestrial) states and vice versa. In habitats, this corresponds to the alternation of all potentially possible life forms and growth forms of herbaceous species—ranging from hydrophytic to hygromesophytic.

Each habitat can be positioned within a series of ecophases over a specific time interval, and various aspects of the water regime can be analyzed both temporally and spatially based on the number and duration of ecophases. This is of considerable importance for assessing water level dynamics in specific water bodies and The Nature of macrophyte bed development. It provides an opportunity for an objective Assessment of the qualitative characteristics of habitats used for vegetation studies. However, water level fluctuations cannot directly drive the selective process of species development in either an ecophase or an ecoperiod. The determining factor is time, specifically the duration of individual ecophases within the ecocycle. The restrictive factor of selective action on species ontogeny and biocenosis diversity is the littoral-terrestrial and terrestrial-littoral sequence of ecophase changes [136]. From an ecological perspective, this requires the transition of ecotopes formed by submerged bottom sediments into exposed and open ones, followed by their subsequent re-flooding. The corresponding time span is termed the critical ecoperiod, which is characteristic of riverbank ecotopes [138]. In this case, the critical period essentially acts as the trigger for a new ecocycle and exerts constructive and destructive effects on the Development of the entire chain of ecosystems and association groups during the transgression and regression of shallow sea and large lake margins. The littoral zone, with its potential for prolonged development, exerts an ecogenetic influence on the differentiation of life forms and growth forms of species, as well as on biocenotic diversity [34]. Taking into account the frequency of its manifestation, the critical ecoperiod can serve as a criterion for the duration of an ecocycle and, consequently, as a criterion for the Classification of specific ecotopes subject to water level fluctuations (excluding pulsation effects at the land–sea/ocean boundary).

Ecotopes that dry out during the arid growing season, where ecocycles do not exceed one year in duration, are typical of tropical and subtropical regions characterized by alternating dry and rainy seasons. In the temperate zone, they can be observed persistently in small water bodies with flat bottom surfaces that fill with water annually and dry up during the growing season. In such cases, the ecoperiod may coincide with the ecocycle.

Ecotopes that dry out over a period spanning several years are typical of fishponds when fish are not farmed and water replenishment does not occur. Ecocycles last 3 years, less than 5 years, and more than 10 years. A three-year cycle occurs in ponds managed under a three-year fish-rearing system. An ecocycle lasting at least five years is characteristic of alluvial waters with a dry season. An ecocycle lasting more than 10 years represents a critical period involving the drainage of the entire water body.

Water level fluctuations affect the nature of littoral habitats, hydrological, hydro-edaphic, and ecological conditions, which is reflected in the diversity and articulation of ecosystems and their components. An annual equilibrium between open and closed ecosystems is observed in these areas: a rise in the water level results in the flooding of terrestrial ecosystems and restricts the germination of plant propagules; a drop in the water level exposes large areas, enabling the germination of the first seeds or diaspores. The more regular the rhythm of water level fluctuations, the more ecologically balanced the ecosystem is, and vice versa.

Consequently, the disruption of equilibrium in a hydrologically balanced ecosystem leads to the accumulation of organic matter and the infilling of the water body.

Ecosystem diversity unfolds from the water's edge, moving outward from the outer margin of the littoral belt toward the center of the water body. These areas are characterized by prolonged dynamic processes, the rates of which depend on climatic and biological factors that influence the pace of sediment accumulation. With respect to natural drivers, this process is irreversible, and siltation proceeds via the gradual replacement of the original water body's space by terrestrial communities. In the case of Siltation and Overgrowth, we are dealing with the Spatial Structure of ecosystems; in the absence of siltation and overgrowth, we observe The phenomenon of zonality with homogeneous, relatively narrow belts, where ecosystem diversity is driven by phase frequency, manifesting as a greater or lesser frequency of plant belts across a given space of the water body. In a hydrologically unbalanced system, it is necessary to account for the timing of ecophase fluctuations, which progressively drive the transformation of cenoses into a mosaic of reservoir marginal strips [19]. This gives rise to vegetation patchiness, the suppression of established phytocenoses, and—due to alterations in the water regime—the proliferation of adventive species within communities.

In the tropics, due to water level fluctuations in river channels and regions of large lakes and wetlands, regular annual changes occur. In the temperate zone, such fluctuations are tied exclusively to the growing season. In the tropical zone, the succession of species, synusiae, and phytocenoses unfolds over two ecologically distinct periods of the year.

Therefore, water level fluctuations are a decisive factor significantly influencing The formation of ecotopes in water-logged ecosystems, the variability of aquatic plants, and their communities, which in turn affect the ecotopes involved in the formation of unanchored floating mat soils.