Marsh vegetation of the steps rivers of the Mykolaiv region and its state in modern ecological and hydrological environment conditions

Abstract

Over the last decades, the natural marsh vegetation of the steppe rivers in the investigated area has periodically been a subject of researches, mainly botanically oriented, whereas systematic environmental studies have rarely been conducted. Accordingly, the purpose of this research was to study and evaluate the status of marsh vegetation in the existing ecological and hydrological conditions of the steppe rivers in the Buh left bank area within the borders of Mykolaiv region.

The results of original studies of the Lower Pobuzhia left bank steppe watercourses, their floodplains and phytogroups made the basic material for investigation. These studies were carried out during expeditionary-route observations of the small rivers, performed in 2017-2019.

Standard methods of field hydrological, landscape and botanical surveys were chosen as the main research methods. Standard formulae of the Simpson index were used to estimate the species richness of the floristic complexes under study.

In the course of geological and hydrological studies, it was found that the main feature of the Buh-Inhul interfluve is the absence of powerful reservoir strata. This causes the limitation of groundwater reserves, which play a key role in maintaining the summer-autumn flow of the small rivers. Their drying directly points to the scanty water reserves of surface horizons. These negative processes are exacerbated by the impact of anthropogenic pressure on river valleys (plowing, grazing) and aridity of the climate.

In the investigated area, the main part of marsh vegetation is concentrated in the lower reaches of the rivers, flatter southern regions of the interfluve. The spatial and species structure of the latter is generally typical for short-flow watercourses. Thus, the factor of the hydrological regime of the ecotope of the floodplains contributes to the differentiation of biotopes and their vegetation cover. According to the latter, we distinguish: biotopes of constant water flowage; biotopes of partial watering and excess humidification; biotopes of sufficient and insufficient humidification.

It is established that, due to the instability of the trophic modes and the flowability of the hydrotope, the marsh biotopes of constant flow rate have low levels of phytodiversity (Simpson index – 1.6 (average index)) and are typical for the valley bottoms of the South Buh, the Inhul and only the estuarine sections of medium and small steppe streams. Within this group of biotopes, we distinguish biotopes of low water flowage/flowing freshwater and brackish water reservoirs inhabited by Confervaceae, Lemnoideae, Hydrocharitaceae, and Potamogetonaceae coenoses. The latter have adapted to the constant hydrogen fluctuations and salt regime of the hydrotope.

Ecotonic, coastal areas of periodic flooding are captured by thickets of typical multi-species wetland Cyperaceae and Gramineae coenoses which have the highest level of phytodiversity (Simpson index – 3.2) and are adapted to drastic changes in water level fluctuations.

Wetland and meadow biotopes of sufficient humidification are characterized by a high level of floristic richness (Simpson index – 2.68) and populated by polydominant hygromesophytes, mesophytes, meadow-halophytic and halophytic coenoses. At the same time, the salinity of the edaphotope cases a significant decrease in species richness (Simpson index – 1.8) and simplification of the species structure of phytogroups. The latter are formed by species of broad ecological amplitude, which are able to exist both in conditions of excess moisture (flood period) and periodic drought of the edaphotope (boundary period).

Up to 25% of the floodplain area of the steppe rivers is represented by zones of insufficient wetting, significant anthropogenic land degradation, which is atypical of the floodplain landscape. These conditions contribute to the unification of phyto-diversity (Simpson index – 2.23) and transformation of native floodplain vegetation into steppe meadows on the basis of mesoxerophyte dwarf herbage with a considerable proportion of ruderals (up to 50% of projective coverage).

Thus, the marsh vegetation of the steppe rivers of the the Buh-Inhul interfluve is represented, to a greater extent, by herbaceous phytocoenoses formed by aquatic, marsh, meadow, psamophytic, halophytic and weed species. Among the ecological types, mesophytes predominate, but among the latter, the proportion of mesoxerophytes and halophytes has increased each year, transforming the floodplain eco-complexes into cenotically and floristically poor steppe and saline meadows.

Accordingly, the typical natural vegetation of floodplain biotopes of wetland type in the current adverse ecological and hydrological conditions of the steppe rivers of Mykolaiv region is extremely depressed. These processes, initiated by the powerful anthropogenic pressure along with aridization of the region's climate, contribute not only to the disappearance of the marsh vegetation, but also to the threat of the total disappearance of the marshes, as natural areas with the unique level of biodiversity of natural complexes.

The prospects for further investigations are seen in more detailed ecological-hydrological and ecological-botanical studies of the steppe rivers’ marsh vegetation in Mykolaiv region left-bank areas.