Regular readers (if any there are) will recall that my last blog suggested that the next one (this one) might well be concerned with 26 years of rocky shore monitoring. Well, it is; up to a point.
A few months ago I had an email from a man called Bill Dixon. Bill informed me that he had recently moved to Solva and that he was interested in setting up a rock pool monitoring project. He asked me for comments and suggestions and we began a correspondence that continues to this day.
As an example of the sort of rock pool monitoring I’d done in the past, I sent Bill some data that I had collected in 1996. This was from three rock pools that I had monitored for about a year following The Sea Empress oil spill.
Bill and I both being of a certain age, we felt it was important that we get both of our vaccinations and wait until the Welsh Government said it was OK before we met in person. Eventually, this all happened and we finally met a couple of weeks ago. The aim was for me to show him what I did to locate the pools and then how I recorded the species found in them.
There was some doubt in my mind that I would be able to find the pools after a quarter of a century. However, compass bearings from 1996 did the trick and we found the three pools. I was fairly sure that any data we collected would be of limited value in comparison terms because the date was 14th July and the nearest date I had from the past was April 1997. Imagine looking at your garden in early April and comparing it with mid-July. In our seasonal climate it would look very different. Seashores also change seasonally.
However, the main point was to show Bill my methodology, so we collected some data anyway. I also thought it would be useful to enable me to relocate the pools so that I could go back and look again in September. I have data from September 1996, so some similar data from September 2021 ought to enable an interesting comparison. The effects of seasonal factors would be at least partially negated and the effects of 26 years might become more evident.
The pools were found and photographed and the inhabitants recorded. I was surprised to find huge changes from 1996 to now. Even allowing for seasonal factors I would not have expected such differences, hence this blog.
Here’s a summary of what I found:
Pool 1
Pool 1
Total seaweed spp: 16 (1997) 9 (2021)
Total animal spp: 5 (1997) 8 (2021)
Total spp: 21 (1997) 17 (2021)
Obviously, we’d expect inter-seasonal differences, so there can be no simple comparison of the figures from 14th July 2021 with results from much earlier in the year and 24 years earlier.
Notwithstanding all that however, the differences seem to be huge.
The number of seaweed species diminished from 16 in 1997 to 9 in 2021. The pool was dominated in 2021 by tubular Ulva spp. (mostly U. intestinalis). In April 1997, there was just a 5% coverage of U. lactuca. There was no U. intestinalis at all. There were only 3 algal species in common between 1997 and 2021. 6 species occurred in 2021 that were not found in the earlier survey. Encrusting red species were found at comparable cover values of 70% and 80% respectively.
The number of animal species showed an opposite trend. There were 5 species found in 1997 and 8 in 2021. 5 previously unrecorded species (for this pool) were found in 2021. Notably, 2021 saw 9 Phorcus lineatus compared with none in 1997. This increase could be attributed to climate change. There is no doubt that numbers of P. lineatus have increased massively on this shore in recent years (Archer-Thomson and Morrell, 2021). P. lineatus is also a southern species that has recently extended its range north into colder, but maybe warming waters. However, this species is known to migrate up and down the shore seasonally (up-shore in warm weather, down-shore in cold weather), so the effect could easily be a seasonal one, rather than a reflection of long-term climate change. Since I have data for September 1996, it might be possible to come to more definite conclusions. Of course in September 1996, the pools were probably still recovering from The Sea Empress oil spill. This might complicate the issue, but nobody said that understanding what’s going on in rock pools was simple.
Pool 2
Pool 2
Total seaweed spp: 7 (1997) 4 (2021)
Total animal spp: 4 (1997) 8 (2021)
Total spp: 11 (1997) 12 (2021)
As with Pool 1, a reduction in the number of seaweed species was seen (from 7 in 1997 to 4 in 2021). Similarly to Pool 1, the dominant weed was tubular Ulva (predominantly U. intestinalis). There were 3 new seaweed species found in 2021.
There were 4 animal species present in 1997 and 9 in 2021. Again, a similar pattern to Pool 1. Also, Phorcus lineatus appeared with a density of 9 individuals in the pool. A big change and a very similar pattern to Pool 1.
Pool 3
Pool 3
Total seaweed spp: 14 (1997) 4 (2021)
Total animal spp: 5 (1997) 2 (2021)
Total spp: 18 (1997) 6 (2021)
There were 14 species of seaweed in 1997 and 4 in 2021. A similar result to Pools 1 and 2 but even more so.
Also similar was the domination of the pool by species of Ulva.
It’s difficult to attribute a cause to this, it may be just a seasonal effect.
There is another possibility and that is the enrichment of the water with nutrients.
Since 1997, one of the largest poultry farms in Britain has been developed on the north shore of Milford Haven. The excrement of the birds is thought to have contributed large quantities of phosphates and nitrates to the waters of The Haven.
Ulva spp are opportunistic fast-growing species that might take advantage of this nutrient enrichment at the expense of slower-growing species.
There were 5 animal species in 1997 and only 2 in 2021. This is the opposite trend to what was seen in pools 1 and 2. Nevertheless, diversity dropped dramatically over the 24 years.
Phorcus lineatus appeared in this pool for the first time but in lower numbers (2) compared with Pools 1 and 2.
The results of this survey indicate much larger changes than I’d expected. When I resurvey the pools in September 2021, it might then be possible to determine what constitute seasonal effects and what might be longer-term differences.
If you would like to join Bill Dixon’s Pembrokeshire rock pool survey team, you can email him at:
billdixon.rockpool1@gmail.com
In the meantime, keep an eye out for the next blog which might well tell you all about my rocky shore monitoring; (on the other hand Nunzilla is becoming impatient…).
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
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