Unique habitats: Mudflats and estuaries

Different natural habitats are defined in the Natura 2000 to create a network of special areas that are under conservation to maintain and restore the natural habitats and species. Natura 2000 is a network of both terrestrial and marine nature protection areas in the European Union. Many of the natural habitats defined in Natura 2000 can be found in the SEAmBOTH-project area. One of these unique marine habitats in Natura 2000 is called tidal mudflats (more correctly “Mudflats and sandflats not covered by seawater at low tide”). These tidal mudflats don’t exist in the Bothnian Bay area per se (because of the absence of tides), but we have other ecologically similar areas.

Shallow shore with a flad.
One Natura 2000 habitat is coastal lagoon, and in Baltic Sea flads belong to this habitat. Flads are greatly affected by the changes in water level. A blog about flads in particular is here! (Photo by Essi Keskinen, Metsähallitus)

One of the aims in SEAmBOTH-project is to harmonize the definitions of nature types between Finland and Sweden. This is not an easy task, because there is a lot of variation between different areas, and many times the definitions are either very broad, or very narrow. So, we aim to find similarities between the habitats and species in Finland and Sweden to make the harmonization possible.

All the people living on the shores of Bothnian Bay know the fact that water level can change dramatically in a short period of time. Many beaches on the mainland and on the islands have very shallow areas, which are sometimes under the water surface and sometimes above it. Usually these changes in water level are due to strong winds from either south or north. Even though the change is not due to tides, the ecological effect is quite similar. The plants growing on the shallow beaches must adapt to the reality, that sometimes they find themselves on a dry land. The difference is, that tides are predictable, but the changes in water level due to winds are unpredictable.

Sago pondweed (Stuckenia pectinata) on dry land due to low water level (Photo by Essi Keskinen, Metsähallitus).

The strong winds can have a dramatic change in the landscape of the Bothnian Bay. The beach can grow tens of meters longer when wind is from north, or the seawater can reach the steps of your summer cabin or sauna when wind is blowing from south. So, in reality, these shallow beach areas can form one kind of a mudflat -habitat, if the bottom sediments of the beach consist of fine materials.

Very low water, rocks are showing kilometers away.
Water level minus one meter in Ulkokrunni (Photo by Essi Keskinen, Metsähallitus).

Delta formed in an estuary is a good example of mudflats in Northern Bothnian Bay. An estuary is the area where the river meets the sea, where the fine sediments carried by the river are deposited creating vast, shallow banks of sand and clay. The current is usually strong right by the river outflow but it slows down as it enters the sea and here a delta can be formed. Characteristic for the estuaries is the large input of fresh water, which makes it a very special environment. Plants that otherwise only live in fresh water may also be found in here.

Satellite image of Torne river from the Finnish side (Image from the National Land survey of Finland).

Water lilies (e.g. Nuphar lutea and Nymphaea alba) and broad-leaved pondweed (Potamogeton natans) are examples of characteristic plants of the estuaries. Due to the shallow water, sheltered location and presence of nutrient rich sediments carried out by the river, estuaries are important for many species of fish and birds. Wading birds walk around in the shallow water foraging for food, e.g. small insects and molluscs, living in and on the bottom. In the delta area where sediments are deposited, mudflats can form. These mudflats can be from time to time above or below surface, and as the sediments keep depositing and land keeps uplifting, these areas can start forming small islands.

Torne river is a 522 km long river that runs along the border between Finland and Sweden. By the towns of Haparanda and Torneå it enters the Bothnian Bay. Here an estuary is formed with a delta of several small islands with narrow passages in between.

River estuary with reed beds on the shores.
The vegetation on the islands in the estuary are rich with common club-rush (Schoenoplectus lacustris) and common reed (Phragmites australis) extending out into the water. (Photo: Aimi Hamberg, County Administrative Board of Norrbotten)

In the sheltered areas of the Torne river estuary one can for example find the flowering plant arrowhead (Sagittaria sp.), grassy pondweed (Potamogeton gramineus) and water horsetail (Equisetum fluviatile). Here you can see how it looks like in the Torne river:

Written by Suvi Saarnio, Metsähallitus and Linnea Bergdahl, Länsstyrelsen.


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