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An Introduction to the Marine Plankton of Milford Haven

Milford Haven is intriguing for marine biologists, being at a junction between the Irish Sea, Bristol Channel and the vast Atlantic. The Gulf Stream brings amazing creatures from the tropics and sub-tropics. In August the Sun Fish is one such visitor. 

The fascination for the Haven is down to some particularly good seashores that have been used for decades by scientists and students. The Gann Flats by Dale village has huge populations of bivalves and polychaetes living in the sediment. Rocky shores like Black Rock and Musselwick have a stunning array of marine invertebrates. 99% of these produce eggs and larvae that drift up into the plankton. Some like the weird Cumaceans leave the shore on certain tides only to settle somewhere else in the Haven. Daily, new material enters the Haven from the open sea.

Plankton is the assemblage of organisms that drift in the water column, their destination ultimately determined by the currents and tidal movements. Within this huge watery space they may be able to move. For example, copepods migrate upwards at night to feed and descend as dawn approaches to try and avoid predators (e.g. fish) that would otherwise see them. However, currents within the Haven will move them around the waterway. Phytoplankton is the component that photosynthesise to trap light energy. Consumed by zooplankton complex food-webs form and change. Some of the plankton species live out their entire life cycle in the water column, called holoplankton, while larvae living there from invertebrate adults on the shore will be meroplankton. Tychoplankton is the name for organisms that are temporary like the Cumaceans that live in sediment but occasionally enter the plankton for a brief spell before returning back to the benthos. The planktonic organisms are best classified by size.

Phaeocystis globosa photosynthetic flagellate colony Polychaete larva Phyllodoce

Phaeocystis globosa is a very important flagellate group. They carry out photosynthesis and when conditions are right they quickly multiply in millions, secreting globules of mucilage that can produce huge sticky masses along the coast. In May 2016 much of the coast of south Pembrokeshire was slimed in this way. Planktonic animals would stick to it, here the larva of a marine worm. Magnification x5

Classification of Plankton by Size

Category              Size Range                 Examples

Femtoplankton        < 0.2 µm                 Marine Viruses

Picoplankton           0.2→ 2 µm              Bacteria

Nanoplankton         2→ 20 µm       Small diatoms and protists

Microplankton       20→ 200 µm     Most diatoms, protists, rotifers, many larvae

Mesoplankton       0.2→ 20 mm  Copepods, medusae, water fleas, arrowworms

Macroplankton      2→ 20 cm           Medusae, krill, ctenophores

Megaplankton           > 20 cm               Jellyfish, cephalopods

Climate change is making the plankton visitors and even residents change. Species that I have not recorded, like the diatom Neocalyptrella robusta, have appeared in the last few years and are now becoming regular in most summer samples. Hopefully a site like this will be useful to store some of the data as well as demonstrate biodiversity and change. In the summer of 2023 the seawater temperature here reached a record high of 18 degrees C and it was noticeable that medusae and siphonophores increased. The crustacean components of the zooplankton also changed to be different to anything else I had recorded before. Check previous Blog - News.

I have been collecting and analysing plankton in the western area of Milford Haven for several decades but only in 2020 did I do this in a systematic way: fortnightly and throughout the year. Prior to this I was primarily recording plankton changes over just spring and summer.

I have a considerable number of photographs and large amounts of data of the changing biodiversity. Early in 2023 I decided to create the web site to share the material. A small selection is available under The Species heading while some of the noticeable changes are given under Data. As more data arrives, generally twice a month, I have included a monthly Blog-News section. 

Cumacean in plankton

A cumacean from seawater above the Gann Flats. Length 3mm. Note the diatoms Coscinodiscus around it.

Spionid Polydora type polychaete larva stack_edited.jpg

Larva of a spionid polychaete worm, probably Polydora. The adult lives on the shore while the larva feeds and disperses in the plankton.

Identification of the life in the plankton is not straightforward. I have been reliant on several ID books given at the end of Methods. However, larvae can change enormously during development not to mention that under the microscope you are never quite sure which angle you are looking at the creature. For this reason I would say that my identification cannot be guaranteed, in some cases it is my best guess, e.g. phoronids. If you know better please email me.



Dale is the main sampling point but learn about other areas of Milford Haven especially saltmarsh and estuary

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