Springtime in the North Sea

by René van Wezel, The Netherlands


Who thinks the ocean is such a big and wide space without big changes is wrong. Every season shows its own biodiversity, species come and species go. And every year we’re eagerly awaiting the springtime, in our garden and in our water samples.

Coastal areas show the seasonal influence rapidly. Upwelling of nutrients and a little heat from the sun make these areas nice hunting grounds for the microscopist. And though the water might still be cold, spring is definitely on its way. You can even see things happening from the air where satellites are monitoring the ‘greenness’ of the sea continually (www.marcoast.waterinsight.nl). We’ve watched a large bloom passing along the Dutch coast these last couple of weeks, eagerly awaiting the first live samples that will show us what exactly is happening. And yes, last weeks sample from the Wadden Sea (north of The Netherlands), showed us finally what we already expected: springtime has started early this year!

Not surprisingly, the tiny algal flagellate Phaeocystis shows up massively. As a swarmer, it has two flagella and a short little hair in between. This hair is quite different in structure from the flagella, and is called a haptonema. It is thought to be involved in food gathering, and all algae that have this structure belong to the class of haptophytes. Nevertheless, it is quite rudimentary in Phaeocystis and I doubt whether it is functioning as such in this species.

Phaeocystis sheds its flagella easily and then starts secreting a jelly like substance. Massive colonies of cells in mucilage are floating through the water, often washing ashore in huge foaming waves, hence the Dutch name of Schuimalg (Foam algae) for this nuisance organism.


Eventually, all other life risks a complete covering by these sticky colonies of Phaeocystis, like this strand of centric diatoms (probably Thalassiosira nordenskioeldii), and the spiny chainforming Chaetoceros decipiens in the next picture. You can actually see a couple of nice examples of small Phaeocystis colonies forming, consisting of a couple of cells in an envelope of mucilage.

Another chainforming diatom that is pretty much there the whole year round is Cerataulina pelagica that typically shows rings of droplets around the top and bottom of the cylindrical cells.

Also typical for this species are the two connection points between the cells that link them together. This is actually a good determination characteristic that is best seen at the end of the chains.

One of the nicest occurring centric diatoms in our coastal waters is Actinocyclus octonarius, a very beautiful solitary species which shows a dotted line pattern around the edge when focussing on the middle of the diatom. Also the pore (areolae) pattern on the valvar surface is characteristic for this species.

Not surprisingly, with plentiful food around, predators are on the hunt as well! One of the largest predatory dinoflagellates that I found was Polykrikos schwartzii. In fact, I first came across the cyst of this species, a thick walled and darkly brown coloured resting spore.

Dinoflagellates are a bit of a strange group, as the name sounds, from a long past. Some of them are heavily armored with cellulose plates, some appear naked. Generally, one flagel is running through a deep groove along the body, the second one is trailing behind. In fact Polykrikos has several of those grooves around its flexible body, indicating a multicellular origin, a pseudo-colony in which several nuclei share the same protoplasma. These large nuclei are visible in the next picture, where the genetic material shows as two threaded masses of permanently condensed chromosomes.

As a colourless cell, this species needs to gather its own food, by dragging the prey between the plates of the armour into its cell lumen. The prey can actually be as big as they are themselves. There it will be digested in food vesicles like the brown blobs in the upper picture.

To actually get hold of the prey, it has some remarkably specialized structures called nematocysts. These tiny harpoon-like structures are generally lying along the surface of the cell. After they are fired off, they are hauled in like whalers’ harpoons, dragging the prey within the dinoflagellate.


Of course a cell doesn’t want to loose these nematocysts too easily, so conditions have to be exactly right before these kind of specialized organelles are triggered and fired off. But what these conditions are, and how this organism senses its prey, is still a bit of a mystery. In fact, the organism itself is quite fragile, and after some examination in a crowded micro-aquarium under the microscope it quickly started to round off, and finally burst. Some of those nematocysts landed on the floor of the micro-aquarium, so I got a good look at the remarkable structure of this single organelle, which is only 13 µm long.


Author is working at an ecological research and consultancy company (www.koemanenbijkerk.nl) in the north of The Netherlands. Preserved seawater samples for the Dutch phytoplankton monitoring program are taken by the Directorate-General for Public Works and Water Management, and analysed in our laboratory. Also live samples are taken on a regular basis, which gives an indication of the biodiversity, and we’re closely watching for any toxic or nuisance algae that show up. As live samples can be in transport for several days before analysis, this can only considered to be semi-quantitative, and an example of the report for this sample can be found here. Analysis is carried out on inverted microscopes at 200x with the sample in so-called sedimentation cuvettes with a volume of around 1 ml. Images were taken with 20/0.7 and 60/1.4 lenses in brightfield.

Thanks to Bert Wetsteyn (DG for Public Works and Water Management) for friendly permission of the use of analysis results and helpful comments.

All comments to the author René van Wezel are welcomed.



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