The Baltic water-plantain, Alisma wahlenbergii

It may be the most iconic plant species for the northern Bothnian Bay, but only few people have ever seen it. The Baltic water-plantain only exists in areas within the Baltic sea, it is listed as vulnerable (VU) on the international IUCN red list of threatened species and is listed as a priority species on Annex II of the EU Habitats Directive. In both Sweden and Finland it is also protected by law.

But who is this Baltic water-plantain, and what does it look like? The plant is perennial and grows to around 10–45 cm in height. Its leaves are extremely long and slender, extending out from the center point of the plant. The flowers are found on top of stalks, which are shorter than the leaves. It grows underwater at depths from 0,1 meter to approximately 2 meters, but is most commonly found at the shallowest areas of the beach. It favors relatively calm waters with a sandy bottom with some clay and mud mixed in it. If it’s only sand or the bottom is too muddy, you don’t seem to find it there. These very specific environmental conditions may be one of the reasons why it is so rare and only found at certain places around the Baltic Sea.

Alisma wahlenbergii under the water growing from sandy bottom. Plant has bud-like flowers.
The flowers of the Baltic water-plantain extend from a stalk from the center of the plant. The flowers are kleistogamous, which means they are always closed, like a bud. Photo by Anna Engdahl, County Administrative Board of Norrbotten.

One of the intriguing things about the water-plantain is that it is definitely not a brackish water species which would tolerate a high dose of salinity, but at the same time, it is not a freshwater species either. It does not tolerate estuaries well, either because of too fresh water, or nutrient discharge, or water movement, or who knows what. You can never find Baltic water-plantain in the river estuary itself, but just outside it.

A snorkeler standing in a very shallow water with a note pad.
The plant seems to prefer habitats like these shallow bottoms. That also means that if you’re studying it by snorkeling, you’ll have to swim – and sometimes walk – in very shallow waters. Photo by Aimi Hamberg, County Administrative Board of Norrbotten.

In Sweden it is known from some localities within the lake Mälaren (which was part of the Baltic Sea long back in history) and from places within Rånefjärden and the archipelago of Haparanda, which are part of the pilot study areas of the SEAmBOTH project. Almost 80 % of the world population, however, can be found on the northern Finnish side of the Bothnian Bay, right in the project area.

The plant has been of interest for botanists for a while. In Norrbotten, Sweden, the populations have been monitored since at least around the 1990’s and in Finland, even longer. However, there are still many things we don’t know about the plant, and many places where it might be growing which we still haven’t been able to identify. For example, it was found in a completely new area in the Bothnian Bay national park in Finland, on the main island, where underwater inventories have been carried out since 2007.

Alisma wahlenbergii growing with some other vascular plants under water.
Alisma wahlenbergii growing with potamogeton perfoliatus and some other vascular plants under water.
Sometimes the Baltic water-plantain can be found forming underwater meadows with many other vascular plant species. Baltic water-plantain is not a strong competitor so if the other vegetation is too thick or tall, water-plantain will perish. Two photos above by Niina Kurikka, Metsähallitus.

We need to hurry up and learn more about them as well, since they are currently thought to be decreasing in numbers. Increased load of nutrients in the water (so called eutrophication) decreases the water quality and may make other plants, e.g. reed, on the beaches grow faster and outcompete the water-plantain. The fact that humans are extending their housing areas and other activities onto the beaches and into the water, causes living environments for the water-plantain to disappear.

If you want to know more about the species of the SEAmBOTH area, see species guides on the Results page!

Written by Linnea Bergdahl & Essi Keskinen


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