Three Rock Pools on Dale Fort Jetty Beach. Part the Second.
Readers of the previous edition of this blog (Dale Fort Blog Number 90, July 2021) might recall my surprise at the changes I found in three rock pools that I last surveyed in 1996. I’d expected some changes after 25 years but the magnitude of these changes was something of a shock.
You may also recall that the main reason for revisiting the pools had been to show local rock pool enthusiast Bill Dixon the methodology. Bill has set up a rock pool monitoring project and you can find out more about it and how to get involved here:
mailto:billdixon.rockpool1@gmail.com
There was a problem with the 2021 data in that there was no easy comparison, because the 1996 data had been collected in September and the 2021 data in July. Seasonal factors could therefore play a large part in any differences that might be evident.
The contrast in the results however made me determined to have another look at a time when a better comparison could be had. That time was September 7th 2021 and the earlier data was collected on September 9th 1996. Two days difference over a 25 year period I deemed to be close enough.
Here are the findings:
Pool 1 07.09.21
Total number of seaweed species: 9 (1996), 9 (2021)
Total animal species: 3 (1996), 9 (2021)
Total species: 12 (1996), 18 (2021)
The number of seaweed species remained constant at 9 over the 25 year period but the species complement had changed radically. There were 4 species that may have persisted over 25 years. (It’s possible that they might have disappeared and reappeared as well of course).
Two of the constant species were red encrusting algae (the stuff that looks like a thin layer of pinkish-red cement on the bottom of many rock pools). The other two were the closely related green seaweeds Ulva fenestrata (sea lettuce) and U. intestinalis (gutweed). These will be familiar to anyone who has glanced at a rock pool in summer, the former appearing as flat, wavy green fronds and the latter as thin green tubes (hence its species name intestinalis).
Notable among the newcomers was Sargassum muticum, a species that was not found in Pembrokeshire until 1997. I had the honour of working on this introduced species for my PhD back in the dark ages of the 1980s. It originated in Japan and was found in the 1970s on The Isle of Wight and spread from there. It’s now well established all over Pembrokeshire and much further north. Fuller details can be found here:
It’s an interesting species that I would say has had a bad, even xenophobic press (ALIEN WEED STEALS BRITISH JOBS). My work found that it can enhance diversity by providing a surface for many other species to attach to. In this pool, there were individuals of the spiral tube worm (Spirorbis spp) attached to S. muticum. Spirorbis was very common on Jetty Beach until quite recently, whence it had become scarce. Maybe the presence of a large brown alga like Sargassum is giving it a chance to re-establish by providing an attachment surface. Other species may take advantage of this too.
The four seaweed species that were not present in this pool in 2021 were all red algae. Red algae are particularly sensitive to strong light and environmental fluctuations. They tend to live on the lower parts of shores or in the sub-littoral (permanently underwater) for this reason. Could it be that their reduced presence in this pool is a reflection of wider environmental fluctuations in the form of light, temperature, salinity etc than occurred back in 1996?
A big surprise was the increase in animal species, from 3 in 1996 to 9 in 2021.
September 1996 was only 8 months or so after the Sea Empress oil spill. Maybe the low number of animal species was because animal populations wiped out by oil were still recovering?
In July 1997 (see Blog Number 90) I found 16 seaweed species. It’s possible that the abrasive action of more seaweed fronds back in the 1990s reduced the number of animal species able to survive in the pool. It is well known that frond abrasion can reduce dramatically the survival of species with larvae that settle out from the plankton and have to stick themselves down (like limpets and barnacles, for example). A further possibility is that fewer animal species survived in a pool dominated by seaweeds because of increased competition for space.
Pool 2 07.09.21
Total seaweed spp: 3 (1996), 6 (2021)
Total animal spp: 5 (1996), 10 (2021)
Total spp: 8 (1996), 16 (2021)
2021 saw a doubling of species numbers for both seaweeds and animals since September 1996. Although the numbers of species were not large (3 seaweed species in 1996 to 6 in 2021 and 5 animal species in 1996 to 10 in 2021) this has to be better than a reduction and I would say a good thing. One does not often hear news of increasing species diversity these days.
The numbers of species for the counts of 2021 (July and September) were quite similar but also showed a small increase between July and September. The dominant seaweed was tubular Ulva (gutweed, formerly Enteromorpha spp).
A species not found in this pool in 1996 was Phorcus lineatus (toothed top shell). This species has seen dramatically increased numbers in recent years (Archer-Thomson, J.A.T. and Morrell, S.L. 2021).
Phorcus lineatus is a southern species that was reaching its northern limit around Pembrokeshire. Its recent increase in both numbers and range are seen by some as indicators of climate change. There were 7 individuals in the pool in 2021 (also 15 of its close relative Steromphala umbilicalis (purple top shell)).
Interestingly, the shells of P. lineatus were serving as an attachment surface for another species not seen here before; the barnacle Austrominius modestus. This latter species originated in New Zealand and was first seen in Chichester Harbour during the 1940s. It was thought to have arrived on ships’ hulls or as larvae in ballast water. It has since spread successfully all around Britain and Europe. It is tolerant of turbid water and varying salinity. Such conditions are found in upper parts of Milford Haven. The result has been that it is thought to have out-competed native species like Semibalanus balanoides and Chthamalus stellatus and is now the dominant barnacle in the upper parts of Milford Haven. It is much less common in the outer reaches of The Haven, where the water is clearer and fully saline all the time.
As a result of finding A. modestus in Pool 2, I inspected a part of The Dale Fort Jetty where I knew there had been a population of A. modestus a couple of years ago. I could not find any. This is typical of barnacle populations. They vary a lot over short periods of time. The jetty was now dominated by S. balanoides (including a repaired section which was uncolonised concrete three years ago). Could this be the start of an A. modestus takeover bid? One might expect them to do better in the fluctuating environment of a rock pool than native species. However, barnacles do not tend to do well in rock pools anyway. This is often put down to environmental variability, but I believe predation by shrimps and possibly crabs to be a major factor too. Increased careful observation and experiment is the only way to address such questions.
Pool 3 07.09.21
Total seaweed spp: 13 (1996), 6 (2021)
Total animal spp: 7 (1996), 6 (2021)
Total spp: 20 (1996), 12 (2021)
The story of Pool 3 was not such a happy one as transpired in the other two pools. Seaweed species numbers were halved between 1996 and 2021. Animal species numbers remained quite similar going down by only one species between 1996 and 2021.
The results for this pool comparing July 2021 with April 1997 (see Blog 90) were however much worse than this, so it may well be that seasonal factors were significant.
Pool 3 is located higher up the shore than the other 2 pools and it could well be that the more extreme variations in the physical environment that result from this are playing a part in the increased variability seen in this pool.
Pool 3 was dominated by Ulva spp (mostly U. fenestrata and U. intestinalis).
Small individuals of Fucus spiralis (twisted wrack) were found attached under the water in Pool 3. This is very unusual. F. spiralisis thought to require a period of desiccation and it’s not likely to get that in a rock pool.
The period of drying out is said to reduce the growth of fungi that would otherwise kill the seaweed. Could it be that conditions in this pool are so variable that the drying out that usually sees off the fungus is not required? Alternatively, it could be that I have misidentified it because young Fucoids are very difficult to tell apart.
As in Pool 2 the introduced barnacle Austrominius modestus was present in Pool 3 for the first time. There was also a single individual native barnacle Chthamlaus stellatus present on a small rock just above the water.C. stellatus is often said to be confined to exposed shores which just goes to show that they don’t take any notice of where books say they should be.
Also as in Pool 2, but in this case more numerous, there was a settlement of very small Spirorbis (spiral tubeworm) and also some tiny, newly settled Spirobranchus sp. (keeled tubeworm)(ex Pomatocerus).
Overall, in terms of numbers of species two of the pools (Pools 1 and 2) have seen an increase in diversity over 25 years. Pool 3 has seen a reduction in diversity, especially with regard to seaweeds.
Why bother with any of this? What does it matter if some obscure rock pools on the shores of Milford Haven change over time? It matters for several reasons. For a start, a seashore that is not exploited for food (or whatever) is about as natural an ecosystem as can be had in a place like Britain. Virtually everything else we can see has got like it is because of our activities. Secondly, because it’s not just seashores that are of interest.
There is currently much concern about the decline in biodiversity in habitats all over the world. Anyone who has driven a car over the last 49 years (yes, sadly I’m that old) will remember the huge quantities of insects that were sacrificed on their windscreens.
When I was a young motorcyclist, I remember in the summer, having to look out for streams or ponds, so I could stop to wash the insects off my visor. That doesn’t happen these days. There are far, far fewer insects around than there used to be. I suspect the massive use of pesticides to be at least partially responsible.
The perceived decline in insect species is currently a major concern all over the world and given the fundamental importance of insects to terrestrial ecosystems it should be. A recent review of 73 investigations concluded that up to 40% of insect species could be lost in the next few decades.
Why are insects important?
They pollinate plants, control other insects and plant pests, are vital parts of decomposer systems, feeding on dead creatures and recycling nutrients. They contribute to making soil, in which plants grow. They aerate the soil by burrowing and tunnelling and fertilize it with their poo. They are used in medicine (maggots used to remove decaying flesh), they are eaten directly by all manner of creatures including humans (yum), bees make honey, wasps make Marmite, bees make wax, wasps make trouble, etc., etc., etc. I’m sure you can think of many other functions of insects. (Incidentally, the bit about wasps is disputed by some authorities).
There is much discussion and dispute about all this because there is not much actual data on insect numbers. How can you say whether changes are important (or even happening at all) if you don’t really have a clue about what is normal or usual? We don’t even really know if there is such a thing as normal.
I understand that insects are not everybody’s cup of tea. I used them here as an example because like most insects we are terrestrial creatures ourselves and are more familiar with terrestrial ecosystems than marine ones. However:
The sea constitutes 98% of the habitable space for life on earth. Phytoplankton do about 50% of the world’s photosynthesis. Macroalgae (big seaweeds largely on seashores) do about 20%. Pretty much everything else in the sea lives on the products of this. This also means that about 70% of the oxygen derived from photosynthesis has marine origins. Readers interested in breathing will understand that this might be of profound significance to them.
Nominative determinism* would suggest that Homo sapiens being the wise Hominids (sapienta means wisdom) that we are would have realised the importance of keeping an eye on the ecosystems that we rely on so that we can tell when things might be going astray.
Initially, all that’s needed is for people to look at and record what’s there and how it might vary with time. This has hardly ever been done anywhere. I can’t imagine why because the fame, wealth and glory I have obtained from staring at the rocks of Milford Haven (for 36 years now) are incalculable. Actually, I’m lying about this. There’s not much fame or glory involved, which is one good reason why it’s not been done very often; but that doesn’t mean it’s not really important.
If you don’t look, how can you know what’s normal? If you don’t know what’s normal, how can you say when things are going wrong? The people from The Field Studies Council (who until recently employed me) and especially those who continue to encourage me to write this blog understand this.
The structure of science does not encourage this sort of work. You are not going to win a Nobel Prize by staying in one place for a long time and counting the limpets. You might however be able to say when things begin to change; or even suggest causal factors like pollution, climate change, biotic interactions and so on.
Field Centres are in a really good position to do this sort of work. I’m pleased to be informed that bookings at Dale Fort are looking strong and the worlds’ most iconic field centre may yet survive Covid.
Monitoring doesn’t cost much more than time, practice and persistence. I believe that field centre staff should be encouraged to stay in their jobs long-term and be allowed enough non-teaching time to learn the names and habits of the creatures that live in their part of the world and to monitor them quantitatively. They will then be in a position to know if things are changing or going astray. They might even be able to say why. Then, they will be of even more use to their students and they will contribute massively towards bringing environmental understanding to all.
* Nominative determinism refers to where the name of someone determines their role in life (e.g. Hugh G.Wrection might be a scaffolder, specializing in large buildings).
Reference:
Archer-Thomson, J.A.T. and Morrell S.L., (2021),
Milford Haven Waterway Environmental Surveillance Group
Rocky Shore Surveillance 2020. Milford Haven Port Authority
Look out for Blog 92. I don’t know what it will be about, but I’m determined to get to a hundred.
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