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