Bothnian Bay Action Plan!
Characteristics of the Bothnian Bay
The Bothnian Bay is the northernmost basin of the Gulf of Bothnia in the Baltic Sea. The area has experienced recurring ice ages. The melting of the ice sheet from the latest ice age began 18 000 years ago, and the whole of the area was clear of ice 9 300 years ago. As the ice melted, earth's crust began to rise. This uplift still continues in the Bothnian Bay area 7.5 - 9 mm per year. The low-lying and shallow coast is thus extending towards the sea, bays are separated from the sea, islands coalesce with the coast, and ship and boat channels have to be dredged constantly. Large areas of earlier sea bottom are now high and dry.
The Bothnian Bay inside a yellow circle at
above.
The Bothnian Bay area
The catchment area includes the northernmost regions of Sweden and Finland reaching the borders of Norway in the west, Russia in the east and the latitude of 68° - 69° in the north. The northern location brings along a long winter. Due to the Gulf Stream, the winter weathers are on the average temperate compared with other locations in the same latitude, with typical average annual temperatures of 1-3 ºC. Nearly the whole of the Bothnian Bay and its catchment belong to the midboreal vegetation zone, a narrow strip to the southboreal zone and the remotest areas in the north and west to the northboreal zone. The annual average rainfall is 450-550 mm with a relatively large share of rain coming in the form of snow. Snow covers most of the area for 150-200 days a year. The long snowy period together with the late occurring melting of the snow cause strong floods in the late spring and early summer.
The Bothnian Bay and the northern parts of the Quark, separating the Bothnian Bay from the Bothnian Sea, receive water from an area of 280 000 km2. Nearly three quarters of the catchment area is covered by forests. The share of treeless mountaintops and glaciers is 10 %, swamps and mires make another 10 %, while lakes, watercourses and agricultural lands form smaller areas. The proportion of settled areas is only 0.2 %. Agricultural lands are located mostly in the southern parts of the catchment area on the Finnish side. The Swedish coastline of the mainland from Haparanda in the north to Umeå in the south is 2 358 km long. Including islands the length of the coastline is 5 900 km. The Finnish coastline of the mainland from Tornio in the north to Vaasa in the south is 3 401 km long when islands with a bridge connecting them to mainland are included. Including also other islands, with a surface area of at least 1 hectare, there is 7 791 km of Finnish coastline.
Land use in the Bothnian Bay region (including
river Umeälven)
Brackish water environment
The Bothnian Bay is a shallow sea with an average depth of 40 m. The rivers, including the river Umeälven, bring 3 600 m3 water per second or 115 km3 per year. Nine rivers have a catchment area that is over 10 000 km2 and a mean discharge of over 100 m3/s. The high fresh water inflow causes the brackish water character of the sea. In surface water the salinity varies from 2 ‰ in the north to 4 ‰ in the south. The water level does not vary regularly due to lack of tide. The whole sea area is covered by an ice layer during winters. Main part remains ice-covered for at least 120 days a year, the northern parts more than half a year. The oxygen regime in the open sea bottom is good with an oxygen saturation of around 80-95 %.
Table 1. Catchment areas and mean discharges of the Bothnian Bay rivers, including river Umeälven. Named rivers have catchments > 10 000 km2 and discharges > 100 m3/s. Medium-sized rivers (19) have catchments 1 000-5 000 km2 and discharges 10-50 m3/s. Small rivers (26) have catchments 100-1 000 km2 and mean discharges < 10 m3/s. Source: Sveriges Statistiska Centralbyrå (Sweden) and the Hertta database (Finland).
| River | Country | Catchment area km2 | % of the Bothnian Bay catchment area* | Mean discharge m3/s |
|---|---|---|---|---|
| Kemijoki | Finland | 49 448 | 18 | 610 |
| Tornionjoki | Finland | 39 644 | 14 | 469 |
| Umeälven | Sweden | 26 654 | 9 | 440 |
| Luleälven | Sweden | 25 229 | 9 | 513 |
| Oulujoki | Finland | 22 514 | 8 | 269 |
| Kalixälven | Sweden | 18 130 | 6 | 225 |
| Iijoki | Finland | 14 189 | 5 | 176 |
| Skellefteälven | Sweden | 11 731 | 4 | 156 |
| Piteälven | Sweden | 11 285 | 4 | 169 |
| Big rivers | 218 825 | 78 | 3 028 | |
| Medium sized rivers | 51 051 | 18 | 502 | |
| Small rivers | 10 754 | 4 | 100 | |
| Total all rivers | 280 630* | 3 630* |
The Bothnian Bay is nutrient-poor. Unlike in the southern Baltic Sea, phosphorus is the limiting nutrient for the phytoplankton production in the open sea and also in the coastal regions. In the outer sea the concentration of phosphate phosphorus is mostly less than 3 µg /l throughout the year. The role of phosphorus is explained by the large amount of fresh water flowing into the sea. Since phytoplankton cannot utilize the nitrogen reserves totally, its concentrations are higher than in the southern basins of the Baltic Sea. In coastal waters within the range of rivers´ influence nitrogen can become the limiting nutrient of the production of phytoplankton during flood in late spring.
Biological features
One characteristic feature is the scarcity of species. Especially in the coastal waters and in the northern regions there are many fresh water species. Even some species originating from oceans have managed to settle into the Bothnian Bay. Some species, such as the glacial relics, have their origin in brackish water. The food web is unique compared to that of many other sea areas, as some 40 % of the energy of the food web come from organic substances, mainly from the humus transported by rivers.
The annual primary production in the open water is significantly lower in the Bothnian Bay than in the Bothnian Sea further south. Respectively, the phytoplankton biomass is around half of that in the Bothnian Sea and the community consists of less species. In the coastal zone the phytoplankton biomass and the primary production are higher than in the open sea and their annual variation is greater. The biomass is usually the highest in the early and the late summer. Although the amount of phytoplankton decreases to a low level during the winter, there is a lot of phytoplankton, especially diatoms, both right underneath and inside the ice particularly in April and May. There are also other organisms in the ice, for instance bacteria and small flagellates and ciliates. The zooplankton in the Bothian Bay is relatively scarce when it comes to the number of species but surprisingly productive considering the small production of the phytoplankton, probably partly maintained by the humus from the river waters.
The number of species belonging to macrofauna in the deep bottoms of the Bothnian Bay is extremely small. Practically, the only species in the deep bottoms until the present day have been the amphipod Monoporeia affinis and the aquatic sowbug Mesidotea entomon. A new species for the Baltic Sea, the polychaete Marenzelleria viridis has arrived in the area in recent years. Opossum shrimps, which belong partly in the free waters, can also be found on the bottom or immediately above them. For many species the border for permanent range in the north is in the northern Quark, and e.g. the clams are totally absent in the outer sea region of the Bothnian Bay. The number of species increases towards the coastal regions. The meiofauna does not see as clear a decrease in numbers as the macrofauna when moving northwards in the Baltic sea, and in the Bothnian Bay it is much more species-rich than the macrofauna. The most common groups are the nematods, segmented worms, benthic copepods and the ostracods.
The habitats of the shallow bottoms are more complex than those of the deep bottoms. However, there are typically few species on the hard gravel, stone, boulder and cliff bottoms of the Bothnian Bay. The thick and often moving ice during the winter makes most bottoms unfavourable for rooted vegetation. Also the water level variation increases the severity of the habitats in most shallow zones, which can run dry for long periods of time. Hard bottoms are important spawning habitats for grayling, Baltic herring and sea-spawning whitefish. Shallow, soft bottoms support more diverse plant and animal communities, especially in sheltered places, where the influence of fresh water is great. Sand bottoms can provide entirely different conditions as the bed material strongly moulded by wind and waves is mobile, and more or less bare. Shallow water areas abundant with vegetation are important habitats for many species of fish and birds.
The northern Quark forms the boundary for the regular occurrence of such marine species as flounder, sprat and cod. The warm water species, e.g. perch, roach, ruffe, bream, ide, pike and bleak live in relatively restricted areas in shallow coastal waters whereas the cold water species, e.g. Baltic herring, vendace, whitefish and fourhorned sculpin, live mostly in deeper waters further away. A number of rivers sustain migratory fish species. It is also common for some of the fresh water species to seek to the river mouths, fladas and glo-lakes to spawn.