Skomer Island and St Bride's Bay
St Bride's Bay forms much of the west coast of Pembrokeshire. Bordered to the north and south by long headlands projecting into the Atlantic. At their tips are islands important for their wildlife. The city of St David's is in the north with Ramsey Island. The southern, Marloes Peninsula, ends with Skomer Island, of international importance for the seabirds like Puffins and Manx Shearwaters. More than 50% of the world's population of the latter live here. The water around the island is the Skomer Marine Conservation Zone and managed by Natural Resources Wales with a base at Martin's Haven.
Off-shore Plankton Sampling
The staff at Martin's Haven are regularly out on the sea around Skomer and as part of their monitoring work collect zooplankton samples. Since 2022 the team have regularly provided me with some of these, where possible, at least once a month. These are usually taken at 30 to 40 metres depth as during the day zooplankton migrates down away from the light and potential predation. They move back up at night.
These samples have been useful to see a comparison between what is happening off-shore and the data I collect in Milford Haven. The enclosed waterway of the latter can be several degrees warmer, have a wider range of larval stages from the more varied shores and have organisms arriving from the rivers and estuaries. Additionally, on every tide plankton arrives from off-shore, outside the Haven. If the density of a population around Skomer, e.g. water fleas like Podon, reach high numbers within a few weeks an influx into the Haven occurs and temporary populations develop there.
The zoea larva of crabs like the Common Shore Crab Carcinus maenas are long-lived (several months) and can be found in almost any month of the year reaching high densities from late spring into the autumn.
Acanthometra pellucida
The "radiolarian"-type protist Acanthometra pellucida is found throughout the year in St Bride's Bay. The highest density is found in late spring and through summer months. Examples appear in the Haven by June and may form a bloom in July before declining. Although a single-celled "animal" it is quite large and sticking together with spines, etc, they easy are picked up in a zooplankton net.
The marine water flea Podon
Crab zoea
Seasonal Changes
Spring
This is a rough indication of what I have seen between 2022 and 2025. Samples are literally a drop in the ocean and vary as sea temperatures rise causing gelatinous plankton like hydromedusae and ctenophores (sea gooseberries) to increase.
By March the large Coscinodiscus diatoms bloom along with the gelatinous colonies of Phaeocystis. Coscinodiscus is common throughout the winter but peaks in the spring. Phaeocystis colonies vary in quantity but are typical of April and May. Hydromedusae (tiny jellyfish-like stage of hydroids) begin to appear, steadily increasing in numbers through to late summer.
Group of Pleurobranchia ctenophores
The sea gooseberry or ctenophore Pleurobranchia appears in spring and by May can be very common with many arriving in the Haven. Odd individuals are present through the summer. See more.
The marine water flea Evadne
The two water flea genera of Podon and Evadne are abundant in spring. Some years they continue into the autumn. Parthenogenetic females appear first producing large numbers of offspring asexually before later males appear in the population with females mating to create resting eggs to survive poor environmental conditions.
Fish eggs and larvae can be common in spring but may also turn up in early autumn. Larvae still with a yolk sac are difficult to identify, developing their characteristics after this stage. Probably the most common fish larvae are those of the Sand Eel Ammodytes, a species of huge importance as prey for the seabirds living on Skomer.
Summer - Autumn
Poecilochaetus larva
Tomopteris
Unlike the plankton within Milford Haven polychaete larvae do not form high densities in the Bay during late spring and summer. Poecilochaetus is the most common species found between June and August. Growing through this period they become longer, gaining more segments and finally drop to the bottom in the autumn. Usually there is a peak of Sand Mason Lanice conchilega larvae in July /August. Adults are very common on the sandy shores along the coast. The larva secrets a tube, initially covered with debris. See more. Tomopteris lives permanently in the plankton and can occur at anytime but can peak in the autumn.
Helicotheca tamesis
By using a zooplankton net (mesh size approx. 220 microns) most phytoplankton passes through and only traps the larger diatoms like Coscinodiscus. These are invariably present but by summer will be in very low numbers to be replaced by others such as Helicotheca tamesis which produce chains looking like sweet wrappers.
Broad-clawed Porcelain Crab Porcelana platycheles
During July/August zoea larvae of the Broad-clawed Porcelain Crab Porcelana platycheles reach high densities.
Brittlestar ophiopluteus
Brittlestar juvenile
Echinoderm larvae are most abundant at this time. Those of brittlestars are most abundant probably because adults can form huge carpets of individuals on the sea bed. July/August is the peak for the ophiopluteus stage with maturing juveniles common in September.
Muggiaea with trapped copepods
The dinoflagellate Noctiluca blooming
Both the dinoflagellate Noctiluca and the siphonophore Muggiaea may appear in July/August. In the warm shallow water they multiply reaching a peak in the autumn. See Muggiaea.
Arrow Worm with porcelain crab zoea
Arrow Worm Parasagitta elegans is a permanent plankton member, possible to find at anytime, and are important predators on other zooplankton. They become very common during August through to November. This species is typical of the Celtic Sea area.
Winter
In November there is still plenty of plankton present although a steady decline in density occurs. Much of the above will have disappeared leaving a relatively high number of copepods present. The large calanoids at this time are filling the oil sac that runs along the back of the animal, primarily as a food store for the winter. The posterior tip of the sac is red with pigment, probably to reduce light being channelled down to the posterior end where predators would detect a sparkle of light. The oil sac acts like a light tunnel. Winter storms are dangerous for copepods near the surface and so they descend to deeper water where they can pass the rest of the winter in a state of diapause (an active form of hibernation).
Much more debris is present in winter samples along with some Acanthometra, crab zoea and Tomopteris. Also the cyphonautes larvae of Bryozoa (sea mat) are abundant at this time.
The copepod Calanus photographed in early winter. NB the red tipped oil sac.
Cyphonautes