Aquatic plant biodiversity at high latitudes : patterns of richness and rarity in Finnish freshwater macrophytes

We examined patterns, causes, and implications of freshwater macrophyte richness and rarity in Finland. The provincial richness of all macrophyte species and that of rare species showed strong declines with latitude, and this pattern did not vary between helophytes, hydrophytes, and shore plants. Aquatic mosses deviated from the patterns found for the other groups. Nationally rare species showed strong preferences for mesotrophic and eutrophic conditions, while nationally common species occurred more evenly across the whole gradient of trophic conditions. Although some species classified as rare in Finland were also rare in other regions in Europe, the majority of nationally rare species appeared to be inclined to eutrophic conditions and do well in the more southerly regions. By contrast, many species that are still common in Finland tended to be either endangered in or absent from regions further south in Europe. Such contrasting patterns of species' rarity among regions call for international evaluation in the conservation efforts for freshwater macrophyte biodiversity.