The deep seafloor – there is also life in the darkness
The light or photic zone of the Baltic Sea extends to 20 metres at best, after which darkness begins to become the permanent state. As the depth increases, the species composition changes and eventually the last plants and algae that require sunlight to photosynthesize disappear. They are replaced by various invertebrate animals.
The Bay Barnacle
The bay barnacle, i.e. Amphibalanus improvisus, entered the Baltic Sea as early as the beginning of the 19th century. It is the only cirripede crustacean living on the Finnish coast. The actual animal hides inside a calcareous shell made of multiple segments. Barnacles are not found in the Bay of Bothnia because the seawater there is not salty enough for them to survive.
Saduria entomon (Fin. kilkki)
Saduria entomon is a large bottom-dwelling isopod crustacean that can grow up to 10 cm in length. These large isopods eat carrion and prey on both smaller amphipod crustaceans and bivalve molluscs, such as the Baltic tellin (Limecola balthica). Saduria thrives in cool water, thus during summertime, they remain on the deep seabed. They also prefer a dim or dark habitat.
The Baltic Tellin
The Baltic tellin, i.e. Limecola balthica, is the most widespread species of bivalve mollusc in the Baltic Sea. It thrives in all sorts of soft bottoms. The larvae of this species are particularly suitable as food for both herring and benthic isopods.
Monoporeia affinis (Fin. valkokatka)
The benthic amphipod, i.e. Monoporeia affinis, occurs all along the Finnish coast. It eats phytoplankton that have sunk to the bottom of the sea. Up to 10,000 individuals can be found scurrying around in a single square metre of the seabed. However, this species suffers from the deoxygenation of the seafloor caused by eutrophication.
Benthic species are similar across the Finnish coast
The most common species of the deep bottoms of the open sea areas are amphipod crustaceans (Monoporeia affinis, Pontoporeia femorata), scale worms (Harmothoe sarsi) and priapulid worms (Halicryptus spinulosus). Blue mussels (Mytilus trossulus), bay barnacles (Amphibalanus improvisus), the encrusting bryozoan (Einhornia crustulenta) and polyps (Laomedea loveni, Cordylophora caspia) live attached on hard substrates.
In more sheltered areas, clams like the Baltic tellin (Limecola balthica), as well as tube-dwelling polychaete mud worms thrive in the soft sediments. In slightly shallower bottoms, common residents include the ragworm (Hediste diversicolor), midge larvae (Chironomidae), oligochaete worms (Oligochaeta) and aquatic gastropods. The Zuiderzee mud crab (Rhithropanopeus harrisii) is a fairly new arrival that has taken over habitats especially in the soft and rocky bottoms of the inner archipelago.
The largest invertebrate of deep seafloors is the isopod (Saduria entomon). This predator can grow up to ten centimetres in length. Although it creeps along the bottom looking for prey, it is also a carrion feeder and will readily devour any carcasses that sink to the seabed.
The large isopod, i.e. Saduria entomon, is a carrion eater living on the deep seafloor.
In the brackish waters of the Bothnian Bay, the most important filter feeders are the brackish water sponge and the only bryozoan in the Baltic Sea (Ephydatia fluviatilis, Einhornia crustulenta). In addition to these two, the bay barnacle also commonly occurs in the Gulf of Finland. In the easternmost waters, the alien species Zebra mussel (Dreissena polymorpha) has occasionally completely taken over hard substrates.
The alien tubeworm species, known as the red-gilled mud worm, i.e. Marenzelleria spp., has now spread along the entire Finnish coastline and burrows deep corridors in the soft bottom sediments.
Internal loading, anoxia and sedimentation make life difficult for benthic animals in the Baltic Sea
The internal nutrient loading and oxygen-depleted deep bottoms of the Baltic Sea pose a threat to benthic communities, especially in the Archipelago Sea and the Gulf of Finland.
Animals of soft sediments, such as amphipod crustaceans (Monoporeia affinis, Pontoporeia femorata) that require decent oxygen conditions and the less demanding ragworms (Hediste diversicolor) and the recently arrived red-gilled mud worms (Marenzelleria spp.) churn the sediment and bring oxygen to the top layers. As such, these animals increase phosphorus binding and reduce eutrophication. However, chronic long-term anoxia will kill even the hardiest of benthic animals.
Sedimentation also affects the establishment and spread of rocky bottom communities. In eutrophic waters, sinking organic detritus and sedimentation suffocate adult organisms and the sludge on the bottom prevents invertebrate larvae from attaching to the substrate.
One consequence of eutrophication is the local depletion of oxygen in the benthos. Phosphorus bound to the bottom sediments of the sea is slowly released back into the water when oxygen is depleted, resulting in a self-feeding spiral where eutrophic waters produce sinking organic matter which is decomposed and thus further consumes the bottom's declining oxygen reserves. This phenomenon is called internal loading.
Material sinking from the surface to the bottom can affect the communities of the deep seafloor.
Animal species of the deep seafloor
- The Baltic tellin (Macoma balthica)
- Amphipod crustacean (Monoporeia affinis)
- Zebra mussel (Dreissena polymorpha)
- Red-gilled mud worm (Marenzelleria spp.)
- Isopod crustacean (Saduria entomon)
- Amphipod crustacean (Pontoporeia femorata)
- Scale worm (Harmothoe sarsi)
- Priapulid worm (Halicryptus spinulosus)
- Blue mussel (Mytilus trossulus)
- Bay barnacle (Amphibalanus improvisus)
- Polyps (Laomeda loveni, Cordylopa caspia)
- Midge larvae (Chironomidae)
- Ragworm (Hediste diversicolor)
- Oligochaete worms (Oligochaeta)
- Brackish water sponge (Ephydatia fluviatilis)
- Encrusting bryozoan (Einhornia crustulenta)
- Zuiderzee mud crab (Rhithropanopeus harrisii)