Non-indigenous species

Non-indigenous species are organisms that are expanding to new areas through human actions, intentionally or accidentally. The increase in sea traffic and the speed of ships have heavily affected the spread to new areas. The term “ecological roulette” describes the unpredictability and irreversibility of the changes these species can cause in the ecosystem. The changes are often permanent, because it is impossible to remove the species from the aquatic ecosystem after a permanent population has been established.

Many researchers believe that a well-functioning ecosystem can prevent newcomers from taking over new areas. The Gulf of Finland is, however, polluted and loaded with nutrients, and therefore especially vulnerable. Non-indigenous species are usually true survivors and the most successful are species with resting stages or pelagic larvae that spread via water currents. Non-indigenous species are considered to be one of the major threats to biodiversity in the world’s oceans, in addition to eutrophication, the overuse of natural resources and the disappearance of habitats.

Cercopagis pengoi, Fishhook waterflea

Cercopagis pengoi is a cladoceran originally from the Ponto-Caspian region from where it spread to the Gulf of Finland in the 1990s via ballast waters. It reproduces effectively and feeds on other zooplankton, using the same food sources as pelagic fish such as herring and sprat. With its long tail, C. pengoi may get stuck in fishing nets and clog them.

Amphibalanus improvisus , Bay barnacle

Amphibalanus improvisus is one of the earliest alien species known to occur in the Gulf of Finland. It most likely came from North America attached to the hulls of sailing boats as it can attach to almost any surface.

Marenzelleria spp. (Marenzelleria polychaete)

The polychaetes Marenzelleria spp. (three known species) are brackish water animals, which makes the Baltic Sea a perfect environment for them. Marenzelleria spp. were found for the first time in the Gulf of Finland in the summer of 1990. These bristle worms burrow deep into the sea bottom sediment, thus improving the oxygen conditions for other benthic animals.


Planktonic algae, i.e. phytoplankton, are found on the surface of the sea to a depth of several meters as single cells, chains or colonies. These form a diverse community of tiny planktonic plant-like organisms, such as cyanobacteria or blue-green algae, dinoflagellates and others. Eventually the phytoplankton community sinks from the upper water column and ends up as a food source for zooplankton as well as bottom-dwelling animals and bacteria.

Nitrogen and phosphorus are both vital nutrients for phytoplankton growth. However, the excessive nitrogen input from the land and from atmospheric deposition fosters an excessive production of planktonic algae, such as dinoflagellates. Whereas phosphorus load especially favours the growth of cyanobacteria, i.e. blue-green algae. This is called eutrophication and it can lead to many ecosystem threats such as decreasing biodiversity or a lack of oxygen.


Cyanobacteria are often called blue-green algae. Their structure is similar to bacteria, but functionally they resemble other producers, photosynthesising with the help of solar energy and producing oxygen. Nodularin, produced by the cyanobacterium Nodularia spumigena, is a hepatotoxin and water containing nodularin may cause allergic and toxic symptoms.

Alexandrium ostenfeldii

Alexandrium ostenfeldii occurs mainly in shallow and enclosed bays during the warm water period. It produces toxic compounds that can accumulate in filter feeding organisms such as mussels and is possibly transferred to higher trophic level organisms such as waterfowl. Water containing a lot of A. ostenfeldii can cause skin irritation in humans. In late summer evenings the presence of A. ostenfeldii can be revealed by bioluminence, “sea fire”, which makes the water glow bright blue.