Lobelia dortmanna, Dortmann's cardinalflower or water lobelia, is a species of flowering plant in the bellflower family Campanulaceae. This stoloniferous herbaceous perennial aquatic plant with basal leaf-rosettes and flower stalks grows to 0.7–2 m (2.3–6.6 ft) tall. The flowers are 1–2 cm long, with a five-lobed white to pale pink or pale blue corolla, produced in groups of one to ten on an erect raceme held above the water surface. The fruit is a capsule 5–10 mm long and 3–5 mm wide, containing numerous small seeds.

The leaves are almost cylindrical, blunt, 2.5–7.5 cm long and evergreen. They have no functional stomata. It is one of several unrelated species of plants from low nutrient lakes known as isoetids, owing to their superficial similarity to Isoetes. The leaves of Lobelia dortmanna are, however, easily distinguishable from those of other isoetids in having only two air-canals (Isoetes having four and most others several) and in the presence of milky sap. The plant has the unusual ability of removing carbon dioxide from the rooting zone rather than from the atmosphere.

The plant is featured on the coat of arms of Ādaži Municipality in Latvia, adopted in 2022. The endangered plant can be found in lakes within the municipality.

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