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Diatoms - the Phytoplankton

Diatoms are the main producers of energy in the phytoplankton but others include the dinoflagellates. Diatoms are present in every sample taken but their form is very diverse and beautiful when seen under the microscope. Although just a single cell they vary in size and some of the largest in the Haven are over 0.5mm in length. They are all in a clear silica box (called a frustule), made up of two halves, one slightly larger as it over hangs the other. Many diatoms are seen as individuals while others form chains, like Thalassiosira. There is a wide range of species in Milford Haven and the dominant species change from month to month. Silica is crucial for the growth of the frustule and so water disturbance brings silica nutrients from the bottom sediments up into the water column. This can produce blooms, high densities of the diatoms. Most books show these to peak in the spring and again in the autumn. The last 4 years of Milford sampling shows peaks can occur at anytime. In 2022 the greatest bloom happened around Christmas time.  Phytoplankton is responsible for more than 50% of the oxygen produced on earth although most of this is consumed by ocean life.

The photos of diatoms commonly found in the Haven are shown below in alphabetic order, around 40 species.

Mixed diatoms, mainly Rhizosolenia species
Coscinodiscus, a large diatom, girdle view
"sliding" diatom Bacillaria

Very abundant "sliding" diatom Bacillaria. When blooming the sample never stays still

Coscinodiscus, a large and common diatom. Left, valve view, right, girdle view

The diatom Coscinodiscus surface view
Girdle view of large diatom Coscinodiscus
Corethron criophilum diatom

Corethron criophilum

diatom Asterionellopsis glacialis

Asterionellopsis glacialis

diatom Chaetoceros socialis

Chaetoceros socialis

diatom Delphineis surirella
Diatoms of Delphineis surirella attached to mineral substrate

Delphineis surirella

Chain of diatoms Chaetoceros curvisetus

Chaetoceros curvisetus

Diatom Ditylum brightwellii

Ditylum brightwellii

diatom Ditylum brightwellii
Eucampia zodiacus diatom

Eucampia zodiacus

The diatom Helicotheca tamesis

Helicotheca tamesis

Guinardia flaccida with possible axiospores
Guinardia striata diatom

Guinardia flaccida with possible axiospores

Guinardia striata

Isthmia enervis, dead diatom frustule

Isthmia enervis, dead diatom frustule

Licmophora benthic diatom

Licmophora sp

Licmophora sp diatom colony

Licmophora sp colony; note the small attached ciliated

Short diatom chain Meuniera sp

Meuniera sp

diatom Neocalyptrella robusta

Neocalyptrella robusta

Neocalyptrella robusta diatom

Neocalyptrella robusta

Odontella mobiliensis diatom

Odontella mobiliensis

Odontella sinensis diatom dividing

Odontella sinensis

diatom Parallia

Parallia sp

Pseudo-nitzschia diatom

Pseudo-nitzschia sp

Pleurosigma diatom


Rhizosolenia sp diatom

Rhizosolenia sp

Stephanopyxis turris diatom

Stephanopyxis turris

The diatom Rhizosolenia sp

Rhizosolenia sp

Rhizosolenia sp diatom

Rhizosolenia sp

Striatella. A benthic species common after strong wind

Striatella. A benthic species common after strong wind

Thalassionema nitzschioides diatom group

Thalassionema nitzschioides

Thallasiosira anguste-lineata diatom chain

Thallasiosira anguste-lineata


Triceratium diatom

This is a colony of Phaeocystis, common in the phytoplankton and classified as algae but are not diatoms. They are part of a group called the Haptophyta and important in global weather and climate change. Click to find out more.

Colony of the flagellate Phaeocystis globosa

This is Entomoneis a diatom from the Gann saltmarsh, not plankton. If you would like to see examples found here click the photo

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