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Phytoplankton Image Gallery

Phytoplankton can be observed under a microscope

Phytoplankton consists of microscopic algae that float or drift in water. To the human eye, algae often appear as brown, red, or green discoloration in the water or even as a mass, but when viewed under a microscope, an incredibly diverse world opens up. In the phytoplankton image gallery, you can see what different plankton species look like!

The images in the gallery have been produced as part of the Finnish Environment Institute’s phytoplankton monitoring. When using the images, a credit to the photographer must be included. The photographer’s name and additional information about the species are visible when clicking on the image.

Blue-Green Algae

Blue-green algae, or cyanobacteria (Cyanophyceae), are among the oldest organisms on Earth, having existed for 2.7 billion years. Although blue-green algae are classified as phytoplankton, they are structurally bacteria. However, they photosynthesize like plants, making them important producers in the food web. The characteristic bluish color of blue-green algae is due to a-chlorophyll and phycocyanin pigments. About 80 planktonic blue-green algae species live in the Baltic Sea, and some of them are toxic. Harmful blue-green algae blooms are most often caused by the species Aphanizomenon flos-aquae and Nodularia spumigena.

Read more about blue-green algae on Järvi-meriwiki website (in Finnish).(siirryt toiseen palveluun)

Aphanizomenon flosaquae is a common filamentous blue-green algae that causes blooms in the Baltic Sea. The filaments of this species are green or yellowish when alive, but turn bluish when they die, causing the shores to appear turquoise in late summer. No toxic strains of this species have been found in the Baltic Sea, although they do produce toxins in freshwater. All blue-green algae blooms should be approached with appropriate caution.
Aphanizomenon flosaquae
Blue-green algae of the Oscillatoriales group appear as filaments. Some species are particularly abundant during the summer season.
Oscillatoriales
Woronichinia elorantae
Blue-green algae of the Dolichospermum genus form blooms in the summer. Some species produce liver and neurotoxins.
Dolichospermum lemmermannii
Nodularia spumigena is a brackish water species with a width of about 7-15 micrometers. It is one of the dominant species in blue-green algae blooms in the Baltic Sea and forms yellowish-green algal mats. It produces a liver toxin called nodularin.
Nodularia spumigena
It occurs in plate-like colonies and is a common planktonic species in both freshwater and brackish water.
Merismopedia punctata
Free-floating blue-green algae cells that form parallel rows. A marine species found, for example, in the central Baltic Sea.
Aphanothece paralleliformis

Dinoflagellates

Dinoflagellates are primarily unicellular and biflagellate algae. There are over 200 species of dinoflagellates in the Baltic Sea, and they form an important part of the Baltic Sea’s phytoplankton. This group of algae gets its name from the cellulose plates that cover some species. Some dinoflagellates are toxic. Dinoflagellates occasionally form mass occurrences that color the water brown or reddish. Some species also produce light through bioluminescence.

Dinoflagellates are present in the Baltic Sea year-round and make up a large part of the Baltic Sea’s spring bloom, which is the increase in algae that occurs in the spring. Some species also occur in ice cavities.

An example of a cosmopolitan phytoplankton species, found in various seas around the world. It is common in the Baltic Sea. Under suitable conditions, such as after upwelling in late summer, it can proliferate to the extent that it colors the water reddish-brown. It is not toxic, but in large quantities, it can cause oxygen depletion in enclosed bays.
Heterocapsa triquetra
A small and common brackish water species found throughout the Baltic Sea. In rare cases, it can form reddish algae blooms. It is not toxic.
Heterocapsa rotundata
Prorocentrum cordatum (commonly known as Sydänkärkipiikkilevä) is an invasive species in the Baltic Sea and it is a rare phytoplankton species because it has a Finnish name. It was first observed in the Baltic Sea in the early 1980s. Today, it is found almost everywhere in the Baltic Sea except for the Gulf of Bothnia. It is a species that occasionally appears in large numbers in late summer and early autumn phytoplankton communities, benefiting from high nitrogen levels in the water. The species may be toxic.
Prorocentrum cordatum
Dinophysis acuminata is a very common dinoflagellate that occurs throughout the growing season and can form blooms. Like many other species in the genus Dinophysis, D. acuminata produces the toxic okadaic acid.
Dinophysis acuminata
A typical spring bloom dinoflagellate. It occurs either individually or in chains of multiple cells.
Peridiniella catenata
There are many similar-looking dinoflagellates in the Baltic Sea that cannot be identified to the species level using phytoplankton monitoring methods, such as light microscopy of preserved water samples. These dinoflagellates are identified to the order or genus level.
Gymnodiniales
A rarely occurring warm-water dinoflagellate, found in summer and autumn.
Gonyaulax verior
It usually occurs during the plankton minimum following the spring bloom in early summer.
Amylax triacantha

Diatoms

Diatoms are unicellular yellow-brown algae. There are about 700-800 planktonic diatom species in the Baltic Sea, and they are an important part of the Baltic Sea’s phytoplankton. Diatoms get their name from their silica-based cell wall, called a frustule. Diatoms are present year-round, but they are most abundant in cold water. Some species also occur in ice cavities. In spring, diatoms can appear in large numbers and color the water brown, but they are not toxic.

Read more about diatoms on Järvi-meriwiki website (in Finnish).(siirryt toiseen palveluun)

The diatom genus Skeletonema is recognized by the silicon strips that connect the cells. Skeletonema marinoi is one of the most common phytoplankton species in the Baltic Sea during the spring bloom, but it can also be abundant at other times of the year.
Skeletonema marinoi
A small diatom of the Baltic Sea spring bloom.
Thalassiosira levanderi
Chaetoceros wighamii
Chaetoceros danicus
The disc-shaped Thalassiosira baltica is a common and abundant diatom during the spring bloom.
Thalassiosira baltica
A cold-water diatom found in belt-like colonies. A common spring bloom species.
Pauliella taeniata
A small-sized warm-water diatom favored by eutrophication.
Cyclotella choctawhatcheeana
The species is one of the largest centric diatoms in the northern Baltic Sea.
Coscinodiscus granii
It grows as an epiphyte, or surface algae, on the homes of ciliates and cyanobacterial filaments.
Nitzschia paleacea
A species that occurs during the summer months and may occasionally form blooms.
Cylindrotheca closterium
A cold-water species of the sea and brackish water, which also lives on and within sea ice.
Melosira arctica
Chaetoceros castracanei
Dactyliosolen fragilissimus is a marine diatom species that occurs only in the southern parts of the Baltic Sea, where the salinity is higher than in the northern Baltic Sea. It occasionally appears in very large numbers.
Dactyliosolen fragilissimus

Cryptomonads

Cryptomonads are unicellular flagellates, with about twenty species living in the Baltic Sea. Most of them are present year-round and can occasionally be very abundant. Their name comes from the cavity on the ventral side of the cell, called the gullet, to which the algae’s two different-length swimming flagella are attached. The cells are flat and oval or teardrop-shaped, and they do not have a rigid cell wall. No toxic species of euglenoids are known.

Read more about cryptomonads on Järvi-meriwiki website (in Finnish).(siirryt toiseen palveluun)

Plagioselmis prolonga

Euglenoids

Euglenoids are unicellular algae with one or two (sometimes even four) swimming flagella. They are named after the red eyespot located at the front of the cell, which they use to sense light. Most euglenoids are freshwater species, with about 40 species living in the Baltic Sea. Occasionally, especially the species Eutreptiella gymnastica forms blooms, coloring the water green.

Read more about euglenoids on Järvi-meriwiki website (in Finnish).(siirryt toiseen palveluun)

The euglenoid Eutreptiella gymnastica is occasionally quite abundant and may cause the water to turn green.
Eutreptiella gymnastica

Chrysophytes

Chrysophytes are primarily yellowish-brown algae that thrive mainly in freshwater.

Read more about chrysophytes on Järvi-meriwiki website (in Finnish).(siirryt toiseen palveluun)

Dinobryon balticum, which forms bush-like colonies or communities, is one of the largest chrysophytes in the Baltic Sea.
Dinobryon balticum
Pseudopedinella sp.

Green Algae

Green algae are a large and diverse group of algae, most of which live in freshwater. They are the algae most closely related to true plants. The group of green algae includes both macroalgae, which are large algae visible to the naked eye, and microscopic phytoplankton algae.

Read more about green algae on Järvi-meriwiki website (in Finnish).(siirryt toiseen palveluun)

Pyramimonas sp.
Oocystis sp.
The species Monoraphidium contortum, which occurs as curved individual cells, is often one of the dominant species in the Gulf of Finland in June.
Monoraphidium contortum
Cymbomonas tetramitiformis
Binuclearia lauterbornii

Haptophytes

Haptophytes are small flagellated algae. Some species produce algal toxins and cause fish deaths.

Read more about haptophytes on Järvi-meriwiki website (in Finnish).(siirryt toiseen palveluun)

Chrysochromulina spp. (sensu lato)

Related topics

Järvi-meriwiki: phytoplankton algae (in Finnish)(siirryt toiseen palveluun)

Lue seuraavaksi

Algae bring colour to the underwater environment