Population explosion of the Afro-Asian introduced species Pistia stratiotes on the Siversky Donets

I had to see firsthand what a population explosion is. On June 28, I caught a single bush of a plant in the Siverskyi Donets near the V.N. Karazin Kharkiv National University biological station that I had previously only seen in aquariums. In three weeks, this plant covered most of the water surface in some areas of the Donets. What will happen next? A single bush of Pistia stratiotes floats down the Donets... In Russian, this happiness is called pistia stratiotes or water lettuce. A rosette of leaves that repels water, between which inconspicuous flowers hide. Powerful stolons emerge from the leaf axils, on which daughter bushes develop. A trail of roots, covered with dense hairs, hangs in the water. Over time, there is more pistia... The stolon and roots are clearly visible (as well as my shadow: I am photographing the invader with my "lopatofon"). Near the base of the leaves, it gets wet. Duckweed collects there, and tadpoles sit... Over time, the amount of pistia carried by the Donets increases. Something like a continuous flow moves along the channel, carrying an ever-increasing biomass. Crossing the Donets, one has to clear a path in this flow. Where obstacles hold back the pistia, it forms accumulations, intertwining into a single mass. In these accumulations, it continues to grow, pushing new portions of greenery into the channel. In some sections of the river, a significant part of the water surface is covered. I wonder how long it will take before the Siverskyi Donets is completely covered by this growth? The first piles have appeared on the bank. The pistia dries out on top and rots at the bottom. I think this is just the beginning... Swimmers are trying to clear a clean water space. For now, it's possible (not for long). And now, the most interesting part. An attempt at a forecast. In short – the end of the Siverskyi Donets. The forecast is preliminary, largely based on my own understanding, as well as the opinions expressed by a botanist of higher plants (Associate Professor Yu.G. Gamuleya) and a botanist of lower plants (Associate Professor O.S. Gorbulin). Short-term forecast. Pistia will cover practically the entire expanse of the Donets over a significant length. The water will become clean for a short time (which is probably good) and practically lifeless (which is definitely bad). Under the cover of pistia, water evaporates much faster (it seems pistia transpiration is four times more intense than evaporation from the water surface), and much less light reaches it (which leads to the suppression of all aquatic vegetation, from planktonic to attached). Additionally, the binding of biogens by the powerful root system of pistia will hit all aquatic vegetation. The amount of oxygen in the water will sharply decrease, and the oxidation of decaying organic matter will worsen. Those components of the biocenosis that find a place in the microecosystem created by pistia will experience an increase in numbers (for how long?). Many other components will suffer seriously. And then the pistia will begin to die. A significant amount of organic matter will sink to the bottom. It will not have time to oxidize, and anoxic conditions will be created at the bottom. Hydrogen sulfide and other reducing gases will accumulate in the sediments and enter the water column. This will mean a serious change in living conditions for the entire bottom community. It's not just about the increase in the amount of organic matter reaching the bottom. Under normal conditions, all this organic matter manages to oxidize, forming relatively small amounts of light silt. It is yellowish-brown and has a pleasant odor. But if the supply of organic matter exceeds a certain limit, its lower layers will begin to decompose under anoxic conditions. This will form significant amounts of black silt. This silt is foul-smelling (due to hydrogen sulfide and other toxic gases). The accumulation of such silt leads to waterlogging. Therefore, the Siverskyi Donets awaits serious upheavals in the fall. The long-term forecast depends on whether pistia can overwinter and spread further. The fact that birds can spread it, and its seeds can withstand cooling, is very alarming information. It is interesting what the degree of genetic diversity of the huge biomass that has flooded the Siverskyi Donets is. The chances that further evolution of this plant will lead to the formation of forms adapted to our conditions depend on this. And finally, reference information. Most websites (including Wikipedia) retell the text from "The Life of Plants," a fundamental Soviet-era reference book. Here's what it says about pistia. Grudzinskaya I. A. Araceae Family // The Life of Plants. In 6 vols. Vol. 6. Flowering Plants / Ed. by Takhtajan A. L. – M.: Prosveshcheniye, 1982. – pp. 492–493. In the last, most specialized subfamily Pistioideae, there is only one species – Pistia stratiotes, or water lettuce. This is a pantropical floating plant with a short stem bearing delicate gray-green leaves and numerous feathery floating roots (Fig. 279). The leaves of pistia are spatulate, with the widest part at the end and slightly narrowed at the base, and have a number of special adaptations for life in a floating state. Almost parallel lateral veins are pressed in from above, making the leaf appear corrugated, but on the lower surface, they protrude as ribs – powerful at the base of the leaf and disappearing towards its end. This structure gives the leaf stability, and the well-developed aerenchyma increases its buoyancy. Short grayish hairs protect the leaf from wetting, acting as a water-repellent tissue. Water falling on such a leaf rolls off in droplets without wetting the leaf surface. Among the leaves clustered on the stem are barely noticeable greenish hairy inflorescences, usually not exceeding 2 cm in length. The spadix, slightly more than a centimeter long, is enclosed within a spathe that incompletely separates a single female flower, whose ovary contains numerous ovules, from 2-8 male flowers with stamens fused into a synandrium. This specialized inflorescence, in its structure, resembles the inflorescence of Ambrosia bassi, but differs significantly in that after the anthers open, the pollen falls directly onto the stigma, and self-pollination occurs. Despite the absence or limitation of cross-pollination, pistia fruits abundantly, develops viable seeds, and successfully reproduces by seed. However, it reproduces much faster vegetatively, using stolons that develop in the axils of the lower leaves. New individuals arise at the ends of the stolons, which, in turn, form new stolons, and so on. This small ornamental plant behaves extremely aggressively in natural conditions, and the name "vicious weed" does not fully reflect the extent of the damage caused to both nature and humans by this plant, which reproduces uncontrollably. In a short period, pistia can completely cover the water surface of a small reservoir and practically condemns it to extinction. The pistia cover significantly increases water loss from the reservoir, as its transpiration consumes incomparably more water than evaporation from the open water surface. It changes the gas exchange in the reservoir and creates all conditions for its complete overgrowth and waterlogging. In some areas, tens of kilometers of water surface are covered by pistia, hundreds of hectares are occupied by this plant. Pistia accumulations complicate navigation, it heavily pollutes rice and taro crops, often nullifying the expended labor. It creates favorable conditions for the life of mosquitoes and promotes their reproduction. Pistia can spread rapidly over long distances due to birds carrying not only seeds but also vegetative parts of the plant. In Africa, it successfully spreads along river currents, moving long distances along with accumulations of other plant debris. Similarly, vegetative parts of pistia are transported by the rivers of India and South America. Very recently, this true tropical plant began to colonize the water bodies of Denmark, rapidly developing there during the extremely hot summer of 1976. However, the Danish climate is not the best option for seed reproduction of pistia. As Dutch scientists have recently established, pistia seeds germinate after a short dormancy period (a few weeks), but at temperatures not lower than 20° C. With good lighting, seeds can germinate in water. They retain viability when the temperature drops to 4° C, as well as when exposed to sub-zero temperatures for a short time (within -5° C). But even if the seed reproduction of pistia is difficult in the water bodies of Denmark, this is not the only way it reproduces. Humans also contributed to the spread of pistia; it was cultivated as a medicinal plant and used in folk medicine to treat many diseases. Pistia is cultivated as an aquarium ornamental plant. It can also be used as feed and for fertilizer production, but a technology that ensures the economic profitability of this production has not yet been developed.