April 2025
Huge Diatom Bloom - Explosion of Polychaetes
At the end of March there was a huge increase in phytoplankton. This is almost entirely diatoms Although there were some Protoperidinium (dinoflagellates) present. With the good weather NASA acquired some brilliant, almost unique, images of the British Isles clear of cloud and displaying the phytoplankton bloom around the coast, especially around Pembrokeshire and Irish Sea. The very pale off-shore patches are coccoliths (haptophyte algae like those found in chalk). I never see them near the shore but a close relative is Phaeocystis globosa and that is common at the moment in shore plankton.
Plankton Bloom 7th April.
Photo credit: NASA Earth Observatory
The short chain diatom Lauderia annulata with external tubules
Long chains of the tiny diatom Skeletonema costatum x400
The diatom bloom consisted of just a few main species including the tiny diatom Skeletonema costatum. This has not been that abundant in recent years but Lauderia annulata is and often blooms in spring within the Haven. Thalassiosira gravida (it was rotula) also very common suddenly exploded in numbers this month. The genus Chaetoceros have "hairy" cells chained together (reduces sinking) and five species were common including C. socialis.
The polychaete trochophore larvae mentioned last month have developed into nectochaetes where the body is elongating as segments are added with more and longer bristles (chaetae). Most are Spionids or Polydora types with the prominent palps on the head. The larvae can be found throughout the year but are particularly abundant at this time of year.
Chaetoceros socialis chains
The dinoflagellate Protoperidinium with Thalassiosira gravida
Phytoplankton Sample dominated by Thalassiosira and Lauderia
One of many Polydora nectochaete larvae common this month.
Lovely to see the appearance of a number of pluteus larvae, in this case one of a sea urchin. A trochophore is caught up between two arms.
Synchaeta, marine rotifer female with egg
Barnacle Cypris larva
The abundant barnacle nauplii larvae last month have resulted in a mass of cypris larvae. Consisting of two valves it does not feed but contains large amounts of stored lipid (visible as round drops). The eye is the red spot. Protruding from between the valves (left) are the legs while the antennae at the front are sensory for detecting a good space to settle on the shore.
Additional crustacean larvae this month consist of zoea of both crabs and Galathea (Squat Lobster).
The rotifer population (Synchaeta) is developing in Dale. These are parthenogenic females and a large egg can be seen inside. Plenty of eggs in the water
Harpacticoid copepod Porcellidium, common this month
The rising numbers of the planktonic tunicate Oikopleura have stalled. This often happens when rapid diatom blooms occur. I think this is a female (gonad on top of head). I find them most abundant June - August after the spring diatom blooms.
March 2025
The Recovery continues now with Dinoflagellates
By mid-March there were dinoflagellates appearing, especially Protoperidinium species. Diversity of zooplankton larvae continues to expand. Nauplii of both barnacles and copepods had increased from last month. I have never seen so many crustacean zoea larvae in a haul and as well as crab zoea I had a very nice squat lobster Galathea zoea. In fact it was going to be my photo of the month until 15th March when I found a first for me: an antipathula larva. They are an exclusive larval form of the tube anemones. It is most likely Cerianthus lloydi that lives in a mucus tube in the mud. Looking like a toy rocket the four ciliated "pods" of the antipathula were slowly propelling it through the water when it stopped and gradually expelled what I believe to be old larval tissue through the slit mouth. The new slimline form then, over several hours, extended and increased the number of pods that will become tentacles in the juvenile anemone. The antipathula initially develops from a planula larva (typical of anemones) and I reported their sudden appearance last month. Planula numbers are currently higher than I have ever seen! See last months photo.
Squat lobster Galathea sp. zoea
Early antipathula larva of a tube anemone, mouth below
Later antipathula larva of a tube anemone, mouth above
Bryozoan coronate larva, 80 microns
Coronate larvae of certain species of Bryozoa last month but now prolific in numbers. Probably Bugulina sp. They only spend a matter of hours in the plankton before settling on the shore.
The dinoflagellate Protoperidinium blooming this month
The polychaetes have suddenly been releasing huge numbers of eggs and trochophore larvae. Probably the most prolific species is a sedentary spionid worm that lives in the middle of the seashore in mucus tubes within crevices and sediment, Malacoceros fuliginosa. The occasional egg is found but this month I was amazed at the number of them and plenty undergoing development into trochophores. The egg is distinctive in having a honeycomb envelope with specialised vesicles around the edge. Even the early trochophore larva has this honeycomb edge to it.
Egg case with developing embryo of Malacoceros, a polychaete worm
The phytoplankton is gradually increasing in diversity although abundance is still low. The sliding diatom Bacillaria paxillifer, which I always consider to be a winter diatom, is on decline while Odontella sinesis is about stable. Chaetoceros and Thalassiosira rotula are appearing in reasonable numbers. Corethron hystrix has always been a favourite species and is turning up in some numbers.
As expected, after several months of absence the dinoflagellates are back - resting stages from late autumn will be decysting. Protoperidinium is a spring/summer genus and is dominant. Tripos (formerly Ceratium) is abundant later in the year although a few T. furca and T. fusus have made an appearance.
The armoured dinoflagellate Tripos furca
The trochophore of Malacoceros. The arrows show where the ciliary band (prototroch) has formed.
February 2025
Rapid Recovery but still no Dinoflagellates
After a period of almost two months with the lowest diversity I can remember the first sample of February was amazing with more than double the number of species and a bloom of the large diatoms. By mid-February even smaller diatoms like Thalassionema and Chaetoceros were appearing. Most exciting is the presence of seashore larvae. Among the increasing numbers of barnacle and copepod nauplii larvae were those of sea anemones. One of my favourites to find these planulae are super to watch as the cilia are manic along the body but they just slowly glide around. Long sensory tufts at the front wave around. However, perhaps the most interesting to see was the sudden appearance of coronate larvae of a bryozoan species (sea mats or moss animals). These tiny hollow balls of cilia, 50 micron diameter, do not feed but spend just a matter of hours in the plankton before settling on the shore. Another bryozoan larva type called a cyphonautes was also abundant. This was probably the sea mat Membranipora. As they remain in the plankton, feeding and dispersing over a week or so they are commonly found unlike the coronates.
Planula larva of a sea anemone.
Note the tiny dots in the water, nanoplankton
Perhaps one of the most striking zooplankton species this month were the large number of Rhabditophoran flatworms that manically moved around the sample. They are so fast. The photo here shows some of the beautiful waves of cilia that moved across the body, just showing up along the edge. Approximately 450 microns when extended. They typically have 4 eyes but I am intrigued by what appears to be another two at the back! As carnivores they are feeding on the small zooplankton larvae. I am always amazed to see them swimming in the plankton as you might imagine that they need to be on a substrate like those living in freshwater or on the rocky shore.
Related to the Bryozoa is the phylum Phoronida. Very occasionally I find a young larva. On 13th February two were in the net. They must have just been released by the parents because in the first 24 hours they develop many lobes, mine barely had two. They grow into an amazing and large larva but by the time they get to this they have already been swept away by the tide. Interesting that this actinotrocha larva as it is called, along with coronates, are released at dawn. The low light on the water is the trigger. Too much light and the parents stop the release. In the case of the coronate larva it is to make sure the larva has chance to settle by the afternoon.
Bryozoan coronate larva, 80 microns
Rhabditophoran flatworm
Bryozoan cyphonautes larva
Other common larvae include the veligers of the Edible Periwinkle Littorina littorea. During January eggs of the small periwinkle Melaraphe neritoides were common but they have tailed off replaced by the larger periwinkle species. Melaraphe has so much yolk in the cell the larva spends only hours in the plankton while the L. littorea is around for a week or so. Greater chance of dispersal as it feeds in the plankton.
The complex actinotrocha Phoronis larva
Veliger larva of an Edible Periwinkle, common this month.
Larva of the polychaete Sabellaria with serrated bristles. Also common this month.
Several zoea larva of the Common Shore Crab
January 2025
Very Low Numbers So Far This Year
Although there has been a week of calm weather prior to the last plankton sample as well as resting stages of phytoplankton there was an amazing array of foraminiferans (a kind of amoeba living in a shell). They are benthic, living in the surface of the sediment but after a bit of disturbance can live in the plankton for some time until they settle again. I do not think I have ever seen so many as there were this time.
Happy New Year! Sampling for the last four weeks has been difficult due to poor weather conditions at suitable collection times. Generally, though, the plankton has been very low in diversity and density. A plankton collection in a sheltered area of the Haven near Milford at the start of the year was almost non-existent. Storm Darragh back in December decimated the plankton and recovery is going to be slow. The resting stages of dinoflagellates are still present and these will remain dormant until early spring. During the last 4 weeks I found just one living dinoflagellate. Diatom numbers are also still very low with the sliding diatom Bacillaria paxillifer being the dominant species although one could hardly say it is a bloom. This is unusual as I believe in the Haven there is always plenty of plankton to see at any time of the year with distinct winter species. A quick look at the 2024 blog at this time shows a reasonable biodiversity, then. On my scoring system this is the lowest value I have recorded in 4 years. Both common Odontella diatom species are multiplying at the moment but there are more dead diatoms than living ones, presently.
Two different dinoflagellate resting cysts
Forams (foraminiferans) found this month in the plankton
The zooplankton was the lowest density and diversity I have come across. A few dozen or so barnacle nauplii (photo of the month) occurred along with some flatworms (about 2 mm long). The occasional harpacticoid copepod scurried about, again these are benthic not planktonic. The small periwinkle Melaraphe neritoides is abundant on the jetty where I take samples and they are still releasing plenty of eggs, lots in the samples. They exist in the plankton for less than a day. A few bivalve veligers, probably larvae of cockles, were present.
A sample taken off Skomer a few days before my last sample showed a similar trend of low density and diversity. Usually with plenty of crab larvae, there was one, a few barnacle nauplii and radiolarians. Interestingly, the latter species, Acanthometra, was present and more abundant in the Dale samples. All were small specimens. Most spectacular in the Skomer sample were a couple of Tomopteris helgolandica. A real favourite, a centimetre or so long they are a polychaete that live permanently in the plankton.
It is going to be interesting to watch the plankton recovery over the next few months.
Tomopteris helgolandica, taken off Skomer in January
Acanthometra pellucida
A small Coscinodiscus- a Dale winter diatom