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.

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