top of page

The components of the plankton collected in Dale depend on several factors:

1. Tidal currents bringing material in from the open sea, e.g.fish eggs.

2. Life living on the seashores, both rocky and sediments. Invertebrates release eggs and larvae into the seawater, e.g. barnacles.

3. Water turbulence that can dislodge both creatures and diatoms living on the shore, e.g. the diatom Striatella

4. Rivers and streams flowing into Milford Haven, e.g. the desmid/diatom Micrasterias living in bogs.

Microscopic Life and Plankton
on the Gann Saltmarsh area

The Gann at high tide. Dale and the plankton sampling sites are off the photo to the left. A: the Pickleridge and Gann Lagoon. B: some of the many salt pans on the saltmarsh. C: The River Gann Estuary

The Gann area has a particular influence upon the plankton samples collected in Dale covering the above points 2 - 4.

Here I will briefly look at the 3 habitats shown on the map: A, B and C.  

A - The Gann Lagoon 

The lagoon is open to the sea by a limited channel, partially blocked by large boulders to reduce speed of flow. Primarily on spring tides the flow in and out exchanges seawater and even the occasional seal (that has become trapped on a neap tide). The shallow lagoon has a high population of Spionid polychaete worms called Polydora (photo). In the spring the adults release huge numbers of larvae. Sampling the lagoon water these larvae are the most abundant creature along with the rotifer Synchaeta. Each receding spring tide discharges these to the sea and they begin to appear in the Dale samples. Almost certainly the incoming tide brings plankton from Dale into the lagoon but as most will be unable to regulate osmosis they will die due to the brackish conditions.

The salinity of the lagoon varies according to the tidal state as some freshwater streams flow into it. Most significantly is the temperature variation that in late spring can become significantly higher than the external seawater. This probably contributes to higher breeding rates of both the worm and rotifer species. The density of these in the Dale plankton samples seem to correlate to the density in the lagoon. 

The sediment in the bottom of the lagoon also harbours bivalves, including the Lagoon Cockle Cerastoderma glaucum, that contribute to the bivalve veligers (photo) released during the year.

B - The Gann Saltmarsh 

Saltmarsh saltpan Gann.JPG

A saltpan in the middle saltmarsh. Note cracks where it has dried out but later filled with water (now drying on the right) showing the extremes of the habitat

During high spring tides the saltmarsh floods but can dry out during neap tides. Both the creeks that dissect the marsh and the saltpans (like muddy rockpools) have interesting microscopic life. Diatoms and protists dominate and although they are associated with the sediment, will at times appear in the water. Tidal movements can then flush them off the marsh to become a temporary part of Dale's plankton. Drying out is a constant problem living at the mud surface and many diatoms produce mucilage. As well as reducing water loss it helps to stick them to the sediment preventing them being flushed away. This also helps to stabilise the mud surface. The pans are especially vulnerable to extreme conditions and yet life has adapted to survive. A few examples are given below.  

Saltmarsh creek Gann.JPG

The edge of a creek showing water at the bottom left by the tide. Note the brown material - diatoms (photos below)

 Navicula mucilage tubes showing the diatoms inside
Low magnification of water sample showing extensive Melosira chains and the darker Navicula mucilage tubes

Much of the wet areas of sediment are dominated by chains of Melosira and mucilage tubes of Navicula. Within the tubes the diatoms can be seen under the microscope moving back and forth. On occasions they escape and some can be seen here outside of the protective mucilage.

Low magnification of water sample showing extensive Melosira chains and the darker Navicula mucilage tubes

 Navicula mucilage tubes showing the diatoms inside

Entomoneis diatom from Saltpan
Entomoneis diatom from Saltpan

Entomoneis diatom from Saltpan

Entomoneis diatom from Saltpan, side view

Both Entomoneis and Surirella gemma (I think that it is that species) are abundant in the saltpans along with many of the smaller Nitzschia and Navicula diatoms. All diatoms in the sediment live in the top millimetre or so. They need light for photosynthesis and migrate to the surface by day descending in the dark including when the turbid seawater covers them. They tend to be very active and under the microscope they can be difficult to photograph as they move relatively quickly.

A mix of moving Nitzschia and Navicula diatoms

Pleurosigma 

Surirella gemma diatom

Surirella gemma (?) diatom from Saltpan

Protists common in the saltpan sediment and creeks

These two long ciliates are enormous and just visible to the naked eye. They are common moving quickly through the surface sediment. Possible name for the genus is Trachelocerca (still checking)

The common ciliate Euplotes with mega and micronuclei. The simple cilia are beating at the top while the bottom ones are called membranelles that are packed tight with cilia

A common ciliate feeding

These are euglenoids and are prolific in the pans. Some like the typical Euglena are bright green and actively searching for light (note the red eyespot). They have a clear migration to the light by day returning deeper into the sediment by night. These are approximately 40 microns long, so very small. The photo of the non-pigmented species has only part of the extended flagellum showing. Keeping the bulk of the cell hidden in the organic matter it actively extended itself back and forth. The latter specimen was filmed as it was so active and this is just one frame.

Other microscopic fauna

Tiny flatworms, about 200 microns in length, inhabit the spaces between the sediment particles and decaying organic matter along with a plentiful supply of nematodes.

Flatworm (200 microns); note the halo of cilia around the body

Nematode from a saltpan

Keratella is a common rotifer in the brackish water

Harpacticoid copepods are common in the sediment and the nauplius larva of Stenhelia palustris, is particularly interesting. For one thing it is wider than it is long and incapable of swimming. Abundant in spring on the Gann it is very active as it moves sideways rotating on its axis through the substrate. The only chance of a photo is when it cannot grab hold of anything, flounders and just spins slowly around. Well adapted for living in the sediment and very different to other nauplii. To see planktonic nauplii click here to open a new window.

Nauplius of Stenhelia palustris, approx 100 microns across. Melosira diatom chain to its right

C - The Gann River Estuary 

Mysid late larva x4 stack1-2_2.jpg

A larval mysid

Shed skeleton (exuvia) of a Stenhalia nauplius

Mysids are the most significant species living in the River Gann. The estuary needs more sampling (an aim for this year 2024). This section is based on just a few months sampling.

The estuarine area around Neyland is significantly larger and you may wish to check this page: Neyland.   

An hour or two after high spring tide I held the plankton net in the flowing water near the small "foot bridge" crossing the river. It is surprising how much drifting material is collected from the current in just a few minutes. Some freshwater algal cells occur but most specimens are organisms from the saltmarsh mud surface. Spring tides flush the organisms off the sediment washing them into the river water. Notable were the exuvia of the copepod nauplii Stenhalia. The copepods were all harpacticoids that live between the mud particles.

Diatoms were common, primarily saltmarsh species like Entomoneis, Navicula, Pleurosigma and Gyrosigma.

Harpacticoid Copepod from the Gann River

Diatom Gyrosigma possibly G. balticum

Saltmarsh diatom Entomoneis from the Gann River

During the tide cycle the river will be flushed with seawater on an incoming tide and so plenty of marine plankton will be brought on to the Gann area, including into the lagoon. The sliding diatom group Bacillaria paxillifer and Odontella species are found in the samples. I have only collected on spring tides at the moment and I would expect the components to change on neap tides.  

bottom of page