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.

Species of the deep seafloor vary in particular according to the salinity and the openness of the habitat

Lightless seafloor areas are found along the entire Finnish coastline with a wide variety of species, both regionally and seasonally.

In the saline waters of the Archipelago Sea, the most abundant species found burrowing into the muddy sediments on dark seabeds include mollusc species, such as the Baltic tellin (Limecola balthica), as well as the amphipod crustacean (Monoporeia affinis). Hard surfaces are covered by colonies of blue mussels, i.e. Mytilus trossulus. In the eastern Gulf of Finland, the invasive zebra mussel (Dreissena polymorpha) is predominantly a species of hard bottoms.

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.

The largest invertebrate of deep seafloor 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.

An isopod, i.e. Saduria entomon, (Fin. kilkki) crawling on top of red algae and blue mussels.
The large isopod, i.e. Saduria entomon, is a carrion eater living on the deep seafloor.

In the deepest parts of the outer archipelago, the most common benthic species include two species of amphipod crustacean, as well as scale worms and priapulid worms. By contrast, species such as blue mussels, i.e. Mytilus trossulus, as well as polyps, attach themselves to hard-bottom substrates

In the middle archipelago zones, clams like the Baltic tellin (Limecola balthica), as well as tube-dwelling polychaete mud worms thrive in the soft sediments. Moreover, the larvae of non-biting midges are also found in the shallowest areas. These insect species (Chironomidae) are typically found in the inner reaches of the archipelago.

In the inner archipelago, commonly occurring species include the polychaete ragworm (Hediste diversicolor), midge larvae, oligochaete worms, and water snails. A recent newcomer, i.e. the Zuiderzee mud crab (Rhithropanopeus harrisii), has conquered the living spaces in the soft and stony bottoms of the inner archipelago.

Internal loading, anoxia, and the build-up of sediments makes 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. The spread and settlement of organisms from hard-bottom communities are also affected by the accumulation of fine sediment on rock surfaces.

Solid particles of sinking matter can be distinguished below the water surface.

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 (Electra crustulenta)
  • Zuiderzee mud crab (Rhithropanopeus harrisii)