Water nymph habitats are slowly decreasing

Spiny water nymph (Najas marina) grows commonly along the Finnish coast, as far north as the northern Kvarken. Spiny water nymph does not occur north of Kalajoki, as the species thrives best in water with a salinity of 3–4 per mill. Spiny water nymph also requires a lot of light, and it usually grows in places with fairly clear water, most often at depths of less than a meter.

Spiny water nymph can most commonly be found in very sheltered areas, such as the ends of flads or lagoons. The plant also thrives well in gaps in reed beds.

The dominant species of water nymph habitats

Spiny water nymph is the dominant species of the water nymph habitats. Its rare relative Najas tenuissima has only been found in a few locations in the eastern Gulf of Finland. Najas tenuissima requires very low salinity water and is more sensitive to competition than spiny water nymph, probably due to its smaller size. Najas tenuissima is globally endangered.

Other vascular plants, such as fennel pondweed (Stuckenia pectinata), can also grow in water nymph dominated habitats. Of stoneworts, the most common include rough stonewort (Chara aspera) and coral stonewort (C. tomentosa).

Reed encroachment and dredging reduce the populations

Spiny water nymph tolerates water turbidity and increased nutrient levels quite well compared to the stoneworts and some pondweeds (Potamogeton spp., Stuckenia spp.) growing in the same areas. Spiny water nymph is known to thrive even in very fertile water areas. However, reed encroachment can push the species into an increasingly narrow growth zone in the middle of the reed bed or right to the water’s edge.

In addition to reed encroachment, dredging and coastal construction are taxing water nymph habitats. It is estimated that the habitat has decreased by approximately 20–30 percent over the past 50 years due to these reasons, which is why the habitat is classified as near threatened.

Species of water nymph habitats

Spiny water nymph (Najas marina)

Rough stonewort (Chara aspera)

Coral stonewort (Chara tomentosa)

Sago or fennel pondweed (Stuckenia pectinata)