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On the Up ...

Odontella mobiliensis diatom splitting to form two with another about to split

Odontella mobiliensis continues to multiply. 2 have split and another is about to. x400

Copepod egg with embryo.jpg

A stacked photo (20 images combined) of a developing nauplius in a copepod egg.

Cyphonautes larva of sea mat Bryozoa x200

Cyphonautes larva x200

Nauplius larva x200

Nauplius larva soon after hatching

A freshwater green alga, Pediastrum sp, I think, in the marine plankton. 60 microns in diameter, x400

A freshwater green alga, Pediastrum sp, in the marine plankton. 60 microns in diameter, x400

The decline in the plankton has taken a turn with a rise in the species count from 40 last month to 50. Tintinnids continue to be common although a new (this autumn), elongated species has appeared. The diatoms are increasing in density and abundance, both Odontella species, and a bloom of Bacillaria paxillifera, the sliding diatom, that disrupts the specimens under the microscope. Perhaps most immediately obvious is the rather sudden increase in copepod eggs. The density of copepod adults seems quite low but eggs with developing embryos and empty hatched eggs are common. There are also a fair number of newly hatched nauplii larvae although at the moment around half seem to be barnacle rather than copepods. I will keep an eye on that. The genus Ceratium (dinoflagellate) is common in autumn and winter and these have really increased in density as well as having four different species now present. In November there were a few Small Periwinkle Melaraphe neritoides eggs but this sample had much more. They tend to reach a peak in January and February if the last two years are anything to go by. They do not spend long in the plankton, a day or two, and probably occur in my samples as the sampling site is right next to a large population on the Jetty. Another trend at this time of year is a sudden increase in cyphonautes larvae of sea mats (Bryozoa). Always one organism appears that baffles me, in this case a tiny green alga-like cell barely 60 microns across. It struck me as like a freshwater alga. 24 hours on with searching through my books it appears to be a species of Pediastrum, planktonic in ponds. Fascinatingly, it is a colony called a coenobium, meaning there is a genetically fixed number of cells. With the amount of rain and flooding recently maybe it was washed into the Haven. An interloper!

At this time of year the Haven is quiet and invariably I find a cormorant fishing close to where I collect the sample on the Dale Fort jetty. Very fitting; the top carnivore, for the food web that has the plankton at the bottom. Merry Christmas and hopefully a good plankton New Year!

Melaraphe neritoides developing egg and developing embryo

Developing egg of the small periwinkle Melaraphe neritoides x100

Ceratium tripos  50 microns x400

All photographs taken on 16th December 2023

A Continuing Decline in density and diversity

The two samples this month shows a continuing decline in the plankton. The total species count was 49 and 40 respectively (October was 57). Tintinnids that had been in unusually high density was now beginning to reduce as were copepods and nauplii. Of the diatom species both Odontella were on a bloom particularly O. mobiliensis showing large numbers in splitting/pairs. Also high were the parasitic flatworm fluke cercaria larvae, probably the most I have seen in one sample (17th Nov). The zuphea stage 3 larva of a Gnathia was good to see (photo of the month). In the last 3-4 weeks there have been several storms and periods of rough weather. This may have brought up nutrients causing the increase in the large diatoms. The 1st Nov sample was taken during a strong north westerly and there was a good deal of detritus as expected. Unusually there was also a huge number of forams, probably from the sandy shore beneath Dale. The one organism that was common at the start of the month was something I am still a little unsure of. Many spheres with cilia moved around the sample, 100 microns in diameter: Prototrochophores? That is early stage larvae of polychaetes. Tunicates that had been so prolific all summer and early autumn virtually disappeared by the middle of the month.

The summer, probably due to the high water temperature, had been remarkably different to previous years. This month, however, the results of species and density was very similar to November 2022. Back to "normal".

Odontella mobiliensis pair of diatoms

A pair of Odontella mobiliensis. x400

marine fluke cercaria larva

One of many similar cercaria larvae (450 microns long), rapidly wriggling and moving around the sample. Spines produce drag and slows sinking.

Tintinnid ciliate

One of the tintinnids found in 17th November sample. 60 microns long

Prototrochophore polychaete larva

Is this a prototrochophore, an early stage larva of a polychaete? 100 microns diameter.

Biodiversity Decline

After a period of relatively calm weather there has been a slow decline in the biodiversity and density of plankton in the Haven. The diatom exceptions have been the two species of Odontella which have notably bloomed along with the tiny Podosira. Of the single-celled Protists, Tintinnids remain dominant and there was a sudden appearance, in reasonable numbers, of the beautiful and large radiolarian Acanthometra. While I have not previously recorded this species in the Haven it was abundant off Skomer through the summer. Likewise, the polychaete, Tomopteris, appeared in the Dale sample this month having found it in September off Skomer (photo of the month in September). It is interesting that reasonable densities of certain species off Skomer are reflected in the Dale plankton around a month later. Note that the water flea, Podon, that became common last month in both locations has now disappeared completely. Although copepods densities have diminished egg masses and the remains of hatched egg membranes were common. For the last three years September and October have been good months for seeing Arrow Worms Sagitta. This year followed the trend. A single specimen of Gnathia maxilliaris was good to see as normally they occur in the spring. A Skomer sample, provided by the reserve team a day after my Dale samples, was surprisingly low in biodiversity although two interesting polychaetes were present: the syllid Myrianida (Autolytus) complete with epitoke and a trochophore larva of a scale worm, probably Harmothoe.

Acanthometra radiolarian

The radiolarian Acanthometra photographed in the Dale sample

Autolytus / Myrianida polychaete and epitoke.jpg

Myrianida (Autolytus) with what looks like an epitoke at the back. For more on this click here.

Polychaete trochophore larva scale worm Harmothoe

The trochophore larva of a scale worm, probably Harmothoe

Lots of Jelly About

Initially I didn't see it as I was focusing at the bottom of the sample. I was focusing through layers of jelly so transparent it was almost impossible to see. Muggiaea atlantica is a Siphonophore and has caused me to rewrite that section under Jellies. The reason I was looking at the bottom was the huge density of light brown grains (see photo, right). These are the loricas/cases of single-celled tintinnids and I have never seen such numbers dominating the samples. Noctiluca is still common and the water flea, Podon, had the highest density I have ever seen along with a bloom of the diatom Coscinodiscus. That diatom is common and often blooms in the autumn. It was accompanied (quite high densities) by the warm water Neocalyptrella that appeared for the first time last autumn at this time. Water temperatures are around 17-18 degrees at the moment - really rather high - and could be the reason. Possibly following the high numbers of crustacean larvae in St Brides Bay last month there was a sudden increase in crab zoea and megalopas, plus barnacle cyprids. My favourite  copepod Ditrichocoryaeus with its huge front eye lenses seems to have disappeared. But without doubt the amount of jelly about was particularly exciting, both with large numbers of Muggiaea and also medusae from Obelia and others. 

Low magnification tintinnid sample
The double gelatinous bells of Muggiaea with colony bottom left with caught copepod. NB oil drop (top)

Muggiaea: approx 2mm tall it consists of 2 jelly bells inside each other. The animal is actually a colony of hydroids and is very small (bottom left) that has caught a copepod. More here.

NewsNote: I have added and updated Arrow Worms/Acorn Worms and Siphonophores. I have also added a Skomer & St Brides page

30th August 2023


Reports of bioluminescence came today from night swimmers at Dale. As they swam so the water sparkled. This is the dinoflagellate Noctiluca, a bioluminescent organism. For the last few years this surprisingly large creature, up to several millimetres across, appeared in samples during October and November. In late July this year I recorded it in the open sea in St Bride's Bay. By mid-August examples, around 200-400 micron across, arrived in Dale. Maybe the south easterly winds were bringing them into the Haven?

Noctiluca scintillans dinoflagellate

Late August

Highest Ever Biodiversity

Biodiversity reached the highest I have ever recorded. With my data collection I create a relative Biodiversity Index (BI) as an indication of change. The poor result in June was 79. BI in samples in August went 165, 200 and 177. 200 was on the 18th August. Compared with last year samples gave BI of 93 and 124 across August.

Many new species appeared this August including water fleas, probably from the open sea due to the wind. Most notable was the huge number of bivalve eggs in various stages of development that must have swept in from the Gann Flats nearby. Every drop sampled under the microscope had every stage in the development of the tunicate Oikopleura. Amazing!

Tunicate Oikopleura

Late July and August 2023

Increasing Biodiversity

Amazing change in 2 weeks. Many dead copepods and largest number of copepod cysts I have seen. Bloom of some small diatoms, especially Delphineis that attaches to solid substrates including exoskeletons. Probably because of the amount of dead material around huge increase in ciliates, particularly members of the Suctoria group like Tokophrya. Also bacteria massively increased. This was probably brought about because of the very windy conditions that occurred and continue to happen.  Several named storms occurred and by the middle of August strong easterlies were sweeping considerable debris up the Haven towards Dale. Striatella, a shore diatom, bloomed. 

The stalked ciliate Tokophrya

One of many Tokophrya

A group of Delphineis diatoms attached to a foram


Late June 2023


So far this year the samples taken at the end of June and 3rd July have shown very low biodiversity. Unusually, copepods, both Calanoids and Harpacticoids, are completely dominant. When I lifted the net from the water I initially thought a diatom bloom as the sample looked so dense. In fact other than veligers the variety of life was low. I say unusual because the previous two years May- August have been dominated by the rotifer Synchaeta. Copepods had been low to medium in numbers previous years.

copepods nauplii red pigment

Copepods of all ages including nauplii. The red patches is the pigment they can develop.

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