Some species of vascular plants and other macrophytes (the plants, water mosses and algae that we can see) are adapted to marine water. This means, that the lower salinity level will limit their distribution. They also have a maximum salinity level where the environmental stress from the salty water will get too high and they can’t survive in the high salinity conditions any longer. These species will also get stressed, if the salinity is too low for them.
Freshwater species are adapted to low salinity or fresh water (the stuff that comes out of your tap when you turn the knob, your well when you take out a bucket of water or where you swim when you go to a lake or a river). If the salinity gets too high, the spices will get stressed, don’t cope well and their distribution can be limited by the water salinity.
Most of the species that we find in the Baltic Sea are somehow tolerating the low salinity conditions of the brackish water (mixture of fresh and marine water). In the Baltic Proper the salinity may be as high as 9 ‰ when the northern Bothnian Bay salinity is as low as 1-3 ‰. Quite many of the Baltic Sea species are marine species that just tolerate low salinity brackish water. Most of these species find it difficult to survive north of the Kvarken area and east of, say, Kotka. A few examples of these species would be the blue mussel Mytilus trossulus, barnacle Amphibalanus improvisus and a red algae Furcellaria lubmricalis. Only a very few species are truly brackish water species. One of them is a relict crustacean species Saduria entomon, which was trapped in the Baltic Sea less than 10 000 years ago after the last ice age.
Many of the vascular plants that can be found in the SEAmBOTH area would ordinarily be labelled as fresh water species but they can still be found in the Bothnian Bay. Of course, there are also people who think that the whole Baltic Sea is nothing but a huge river estuary and not a sea at all, but most of us still think it’s a proper sea, although it freezes in the winter and the water is very low in salinity, and there is no tide.
Salinity is especially low in the huge river estuaries of Tornio, Kemi, Kalix and Råneå rivers which discharge huge amounts of fresh water every second. Many of the fresh water vascular plants (see the photos) can be found in the river estuaries. Marine species are not found in these areas and the extent of fresh water can be seen from the species composition – where the fresh water species occur and none of the brackish water species can be found, the area can still be considered a river estuary and not yet the sea.
Many of the freshwater vascular plants can be found in shallow and sheltered bays and lagoons by the continental coast. Great meadows of broad-leaved pondweed Potamogeton natans, pond water crow foot Ranunculus peltatus ssp peltatus and an alien species pondweed Elodea canadensis can be found in low salinity fresh water. As the English name suggests, all of these species like pond water (pond is a small lake with fresh water). These species don’t venture to the outer islands and one of the reasons is the limiting factor of higher salinity. Another example of a freshwater species that can be found a bit further out in the Bothnian Bay is the large duck mussel Anodonta anatina. It can be found in the national parks and around Hailuoto island, as well as in many large and shallow bays.
Although brackish water is a harsh environment to thrive, it also makes it interesting for marine biologist to sort through the species list where marine, brackish and freshwater species exist side by side. There are not many places like this in the world.
Written by, Essi Keskinen