So far 311 vascular plant species from 205 genuses and 63 families have been established in the Atanasovsko Lake lagoon (Grozeva, 2005). They include the following domineering species: the Marsh samphire (Salicornia herbacea, Salicornia europaea), Sea blite (Sueda maritima), Reed (Phragmites australis), Narrowleaf cattail (Typha angustifolia), vicia (Vicia campestris), sea wormwood (Artemisia maritima) etc. The first information about the flora of the lake dates from the late 19 century. The first targeted floristic study of the region was conducted during the 1990s during the preparation of the management plan for this wetland. Other studies during the recent years have added the most recent taxa in the rather full list of plant species established in the lagoon.
The most frequently occurring plants are the representatives of the families Legumes (Fabaceae) – 29 species, Cereals (Poaceae) – 24 species, Roses (Rosaceae) – 22 speccies, Goosefoot (Chenopodiaceae) – 16 species, Cruciferous (Brassicaceae) – 15 species, Umbelliferous (Apiaceae) – 11 species, Forget-me-nots (Boraginaceae) and Sedges (Cyperaceae) – 10 species each. The representatives of further 18 families have been described but are present only as singular individuals.
The gramineous plants occurring in the Atanasovsko Lake lagoon comprise 90% of all vascular plants described there, with only 10% left for the tree species. The annual plants amount to 37% or 116 species, and the biannual plants are most rare, with 17 species or 5% of the total number.
According to its attachment to water, the flora in the lagoon is divided into 8 types:
- hygrophytes - 35 species. These are plants growing well with constant or periodic excess of water, such as reed, narrow-leaf cattail and broad-leaf cattail (Typha latifolia), common club rush (Schoenoplectus lacustris) etc. Reeds cover the dykes encompassing the surrounding channel on the outside and on the inside. The outermost pools on the northern and southwestern parts of the lake, which have not been flooded for a long time, are almost entirely overgrown. There reeds have occupied the weakly saline wet soils. No reed belts are present in the immediate proximity of the dykes of the subsequent pools with relatively more saline water. The reed-covered areas include also broad-leaf cattail and, in relatively dryer areas, narrow-leaf cattal. Some very rare occurrences of hygrophilous tree vegetation occur along the banks of the fresh-water lake such as singular individuals of white willows, rosemary leaved willow and brittle willow (Salix alba, Salix eleagnus, Salix fragilis) and, less frequently, aspen (Populus tremula);
- hygromesophytes - 44 species, mesophytic species adapted to growing on high-humidity soils. The lower and substantially more humid locations are home to communities with significant participation of Ravenna grass (Erianthus ravenae), sea rush and black rush (Juncus maritimus, Juncus atratus).
- mesophytes - 57 species growing on soils of moderate humidity. The lagoon is located among mesophytic grass formations represented by meadow fescue (Festuceta pratensis) etc. These formations affect the vegetation in the reserve indirectly.
- xerophytes - 53 species. These plants grow in dry habitats and are capable of surviving prolonged atmospheric and soil draughts during which they remain biologically active. These plants occupy the middle and high areas of the old dykes in the Atanasovsko lake, and occur along the periphery of the reserve and in some pools which remain dry for long periods. They are represented by communities such as Agropyreta intermediae, Festuceta pseudovinae, mammoth wild rye (Leymeta racemosae), Elimeta elongatae, bulbous bluegrass (Poaeta bulbosae), Lolieta perennae etc. with couch grass (Cynodon dactilon), yellow bluestem (Dichantium ischaemum) and scented grass (Chrysopogon gryllus).
- halophytes - 17 plant species on humid, saline soils. These are represented by communities of individual halophytic plants where marsh samphire, sea blite, Siberian statice (Limonium gmelinii), opposite leaved saltwort (Salsola soda) etc. prevail. They occupy the areas with most saline soils during seasonal, incidental and prolonged draught of the pools with higher salt concentration in water, and at the toes of the dykes in the cases of significant terrain subsidence. In some areas, the marsh samphire covers entire areas and, mostly on repaired dykes, is a pioneer element. These plants are mixed with other grass species, mainly along the dykes, where other typical halophytes occur such as sea blite, false sheep's fescue (Festuca pseudovin), Jerusalem oak goosefoot (Chenopodium botris), field sedgewort (Artemisia campestris), Siberian statice, coast barbgrass (Parapholis incurva), etc.
- hydrophytes – 15 species, with submerged vegetative buds and with tillers covered by water. Significant areas overgrown by vascular pants such as Common Duckweed (Lemna minor), rigid hornwort (Ceratophyllum demersum), holly-leaved naiad (Najas marina), curly-leaf pondweed (Potamogeton crispus), greater duckweed (Spirodella polyrrhiza) etc. exist in the channel and in the fresh-water lake.
- psammophytes - 18 species of plants inhabiting sandy terrains (dunes etc.). The sand substrate, a mandatory element in parts of the lake, is the prerequisite for the thriving of these plats, some of which form small unmixed communities. Thus, although less frequently, communities of mammoth wild rye, marram grass (Amophylleta arenariae), Centaureeta arenariae etc.
- xero psammophytes - 7 psamophytic species thriving in dry areas.
The plant species established in the lagoon may, according to their origin, be divided into the following species:
- Eurasian geoelements prevalent in the lake flora - 55 species;
- Euro-Mediterranean - 39 species;
- Sub-Mediterranean - 29 species;
- Mediterranean - 26 species;
- Cosmopolitan - 26 species;
- Pontic - 32 species.
1 Bulgarian and 7 Balkan Peninsula endemic species have been established in the Atanasovsko Lake lagoon. These are:
- Ovate goatgrass Aegilops geniculata – a Bulgarian endemic species;
- Bupleurum apiculatum
- fumitory Funaria petteri
- Nonea atra
- Heptaptera triquetra
- Black Sea campion Silene euxina – vulnerable, low in number.
- mullein Verbascum purpureum – the most rare species and critically low in numbers, represented by 3 - 5 individuals only
- carnation Dianthus moesiacus – vulnerable, low in numbers.
40 of the species established in the Atanasovsko Lake are included in Bulgaria's Red Data Book. This number includes one species – sea purslane (Halimione portucaloides); 6 species are threatened with extinction – yellow centaury (Centaurium maritimum), saw-sedge (Claudium mariscus), Elymus picnanthus, European seaheath (Frankenia pulverulenta), German statice (Goniolimon tataricum), blue dogbae (Trachonitum venetum); 33 species are rare.