The taxonomy of the genus Trapa appears to be complex, with only this species recognised throughout much of Europe, but up to 15 species recognised in the east. The distinguishing character for the different subordinate taxa appear to be useful, but no information on the degree to which they have been tested or any details of molecular studies could be found. T. natans is treated here as one of 15 taxa of the genus occurring in Europe.

The distribution of T. natans sensu stricto is obscured by variation in the degree to which other taxa are recognised. It is likely that the information employed here will include information on other taxa and will therefore present a more positive idea of the status of the species than is actually the case.

In typical populations of this species (var. natans), the crown of the fruit is 4-5 mm wide while the distance between the base of the fruit and the base of the lower cornicles is 6.5-10.0 mm. In var. borysthenica (V. Vassil.) Tzvel., the crown of the fruit is 5-6(7) mm wide and the distance between the base of the fruit and the base of the lower cornicles is 4.0-6.5 mm (which could be the result of ancient hybridisation with T. sibirica). The variety with almost cylindrical tubercles between the bases of the apical and lower cornicles (such tubercles in all four), which may also be considered a result of abnormal development of fruit, is so far known only from the Berezine River and from the Para River in the Oka Basin (Tzvelev 2006).

European regional assessment: Near Threatened (NT)
EU 27 regional assessment: Near Threatened (NT)
 
This species is widespread but rare and declining everywhere throughout its range. The rate of decline within the last three generations is not known neither are the causes of the decline. It is suspected that the decline has been ongoing since 900 BC due to a constant cooling of the climate, but in this case the current increase in mean temperatures could benefit this species. It is difficult to assess it under Criterion A but the species is clearly under threat and is therefore precautionary assessed as Near Threatened.

Trapa natans has been reported from most of Europe north into southern Sweden and east into European Russia. Outside Europe, it occurs east throughout most of Asia into China, the Philippines and Malaysia. It has apparently been introduced into Australia and North America where it is now widespread and may be invasive.

It is difficult to interpret information on the distribution of this species with any confidence due to taxonomic confusion. Thus, for example, according to (Stevanović 1999) the only Trapa species occurring in Serbia was T. annosa, but T. natans is widely reported in the literature as occurring in Serbia - the different statements almost certainly refer to the same taxon but only in a few cases is it possible to establish which taxa are intended.

With reference to the historic distribution of T. natans and its relationship with man, there is wide speculation that its range was strongly influenced by its value as a crop. Arber (1920) states that T. natans has been used from early times for food, medicine and magic, and is supposed to have been introduced into Switzerland as long ago as the period of the lake dwellings, while Vankina (1970) goes further to say that "During the Atlantic and sub-boreal periods the area of distribution of T. natans stretched as far north as Middle Sweden and southern Finland. In Latvia, it appeared in the warm Atlantic period and has been found here at a total of 13 sites, including in the cultural layers of six settlement sites. At Sārnate, it has been found in dense layers of nutshells up to 40 cm thick around the hearths, occupying areas of several square metres. It has been suggested that the people of the Stone Age deliberately cultivated the water chestnut near their settlements. There is evidence that in the 16^th century roasted water chestnuts or water chestnut flour were sold in the markets of Mantua and Ferrara. At the beginning of the same century, in southern France the water chestnut was consumed raw, boiled or roasted, and also used in the form of flour. It is known that during the time of the First World War the water chestnut was also sold at the market in the Latvian town of Jēkabpils. At around 900 BC, evidently as a result of climatic cooling, the water chestnut began to die out."

Trapa natans appears to be very rare in the western European part of its range. It is extinct in Spain, there is only a single known population in Switzerland (Welten and Sutter 1982), two populations known in Greece.

There are two populations currently known in Latvia (Lakes Pokrote near Balvi and Lakes Klaucāni and Priekulāni in Jēkabpils District). In Lake Klaucāni, the water chestnut grows in the western and southwestern bays of the lake, where it is protected from wind by forest, very near the lakeshore, where, at a depth of 2 m, the water on the lakebed during the vegetative period of the water chestnut retains a temperature of 21 degrees C. Nowadays, these sites have been placed under state protection (Vankina 1970).

Trapa natans is an annual (up to 3 m height) floating-leaved plant, growing in stagnant waters, lakes, channels with weak currents, ponds and marshes.

It primarily occurs in unpolluted lowland nutrient-rich, but not strongly calcareous waters that have a muddy bottom and plenty of light; it is important as food source for birds and provides fish spawning grounds (Morgan and Leon 1992).

The causes of the decline in populations of T. natans are not clear, although they are usually attributed to loss and degradation of habitats. Vankina (1970) suggests that the current evidence of decline is simply part of a process that has been ongoing since 900 BC, due to cooling climate. If this is the case, then the current trends toward temperature increase may actually favour this species.

The fruit of T. natans has apparently been a staple food source since the stone age, at least in Latvia where middens have been found with a depth of up to 40 cm of shells of this species (Vankina 1970), while it has been associated with the stone age lake dwellings in Switzerland (Keller 1866). The fruits were apparently sold in markets in Europe into the 16^th century (Vankina 1970).

The fruits are still important in continental Asia, Malaysia and India, especially in China. Several countries in southeast Asia, and also Chinese communities in Europe and North America, import the fruit directly from China. Fruits are used in the preparation of liniments to treat elephantiasis, pestilent fevers, rheumatism, sores, sunburn and skin complaints. Used also as food for pigs and other livestock in southeast Asia.

Trapa natans is included under Appendix I of the Convention on the Conservation of European Wildlife and Natural Habitats (Bern Convention) and in the following national and regional red lists:

• Black Sea Region (Vulnerable)
• Bulgaria (Endangered)
  
• Carpathian Mountains (Endangered)
• Czech Republic (Critically Endangered)
• Germany (Endangered - level 2)
  
• Russia (Near Threatened) (I. Illarionova pers. comm. 2010)
• Slovakia (Vulnerable)
• Spain (Extinct)
  
• Sweden (Extinct)
• Switzerland (Critically Endangered)
• Turkey (Vulnerable)
• Ukraine (Endangered)

In France, Trapa natans is protected at regional level (Alsace, Basse-Normandie, Limousin) and at departemental level (Ardennes). In Hungary, it is protected but not endangered.