Bacteria are the dominant elements of the lake's ecosystem and are also related to its therapeutic effects. Several groups can be observed in the water. Characteristic cyanobacteria include filamentous blue-green algae (Oscillatoria princeps, O. tenuis, O. jasoruensis, O. chlorina, Spirulina major). A typical phenomenon is that half of the blue-green algae are warm-water, stenothermic species. Two warm-water cyanobacteria (Pseudanabaena papillaterminata, P. crassa, Anomoeoneis serians) occur only in this location in Hungary.
Bacterial coatings are characteristic formations in Lake Hévíz, covering the crater walls from 1.5 meters deep to the entrance of the spring cave, primarily made up of Thiothrix sp. and Beggiotoa sp. These, along with other sulfur bacteria, play an important role in the lake’s sulfur metabolism {Sulfur bacteria (Beggiatoa minima, B. alba), sulfur oxidizers (Thiobacillus sp.), sulfate reducers (Desulfovibrio sp.)}. Actinomycetes (Streptomyces sp., Micromonospora sp.) are significant, with Micromonospora heviziensis, a potent protein and cellulose degrader found only in Lake Hévíz, and another (Microbispora amethystogenes), a bacterium that accumulates iodine crystals in its cells, being found in very few lakes worldwide. This bacterium has not been detected in the lake since the late 1980s due to mining activities.
The significant environmental changes also affected other bacteria in the lake. Micromonospora carbonacea and M. chalcea were not detectable in the lake in the 1990s. The newly composed Actinomycetes community is now dominated by Micromonospora purpureochromogenes, while M. heviziensis has receded.
Among the Streptomyces bacteria, antibiotic producers are likely present.
Green algae, especially Chlorella species, are found throughout the lake, except for the area around the spring cave.
Among the diatoms, Navicula cryptocephala and N. capitatoradiata are commonly found in the lake.
The phytoplankton of Lake Hévíz is extremely species-poor and has low biomass, reflecting the lake's trophic state, which ranges from oligotrophic to mesotrophic.
Significant Higher Plants
Indian Red Water Lily (Nymphaea rubra var. longiflora)
In 1898, Sándor Lovassy successfully attempted to naturalize the Indian Red Water Lily in Lake Hévíz. Its native home is tropical East India.
In Hévíz, the red water lily propagates vegetatively. A single tuber lives for 6 years, during which it produces numerous daughter rhizomes (the plant’s vegetative reproductive organ). A single, undisturbed plant can develop 10-20 leaves during the summer. Its cyclamen-pink flowers, reminiscent of cedar wood scent, rise 5-7 cm above the water. They open late at night and close by 11 AM the next day. Flowering typically begins in June, peaks in August and September, and continues until the end of November. Like most tropical plants, the red water lily is sensitive. It thrives in undisturbed environments with stable climates. Each flower lasts for three days. The red water lily is a protected plant in Lake Hévíz, and it is forbidden to disturb it or cut its flowers. In recent years, a mutant with light (white-pink) flowers has been observed in the lake.
White Water Lily (Nymphaea candida minor)
This species has disappeared from the lake but still thrives in large numbers in the industrial water channel.
Coastal Bulrush (Schoenoplectus litoralis)
This tertiary relic plant, which forms dense stands along with reeds at the outlet edge, is a potential "red list" species.
Hungarian Ash (Fraxinus angustifolia ssp. pannonica)
A typical hardwood species, often planted in Hungarian regions.
Swamp Cypress (Taxodium distichum)
A decorative, fresh green tree with a loose crown, native to North America. Its significant population in Hévíz lines several roads.
Several planted ornamental trees and spontaneously established aquatic plants still live in the area.
Notable among these is the plane tree (Platanus x hispanica) avenue along the promenade, which can be seen in early 20th-century photographs, as well as the aged trees planted around the same time as the plane tree avenue (ash – Fraxinus angustifolia ssp. pannonica, swamp cypress – Taxodium distichum, Chinese dawn redwood – Metasequoia glyptostroboides, species of Abies and Picea).
Among the rarities are swamp cypress – Taxodium distichum, Chinese dawn redwood – Metasequoia glyptostroboides, ginkgo – Ginkgo biloba, California nutmeg – Torreya californica, California incense cedar – Calocedrus decurrens, Atlas cedar – Cedrus atlantica, Manchurian yew – Cephalotaxus harringtonii, coastal giant sequoia – Sequoia sempervirens, weeping hornbeam – Carpinus betulus "Pendula", weeping beech – Fagus sylvatica "Pendula".
Among the shrubs, there are also several remarkable species suitable for collection gardens, such as the ivy-bearing form of Hedera helix "Arborescens", eastern mahonia Mahonia bealei, late yew Calocedrus decurrens, magnolia with large flowers - Magnolia x soulangiana, and valuable individuals due to their age and development, such as Japanese spindle – Euonymus japonicus, evergreen viburnums – Viburnum rhytidophyllum, and yew species – Taxus baccata.
