Dale Fort Blog Number 34

4 09 2014

Rocky Shore Monitoring at Dale Fort

1996 – 2014

Part the Second

Species Diversity on the 3 Transects 1996 – 2014

1) Dale Fort Jetty Beach

Jetty total spp 1996 -2014The numbers of species were low immediately after the 1996 oil spill. This was not a causal effect. The first survey of the Dale Fort Jetty Beach transect was carried out before any oiling occurred (year zero on the graph above). In fact by one year after the spill numbers of species had actually increased. Numbers peaked in 2013 and declined slightly in 2014

2) Castle Beach

Total spp castle 1996 - 2014  The year after the oil spill species numbers had increased and following that remained very consistent until 2014 (last point on the graph above) when numbers peaked at 39 species.

3) Monkhaven

Monk toitak spp 1996 - 2014No effects of the oil spill on total species numbers at Monkhaven were evident. The apparent decline in 2003 (6th point from left on the graph above) was due to huge swells preventing safe access to the lower three sites. For the past decade species numbers have been extremely consistent at Monkhaven.




Total Number of Species Found on all 3 Sites, 1996 – 2014

3 spp graphsThe general pattern in terms of species diversity seems to be one of increasing stability with increasing exposure. This seemed strange in that the more stressful environment seems to have the most consistent number of species. Therefore, it was decided to look at a species which had very variable numbers see if the pattern of stability with increasing exposure was repeated at a single species level.

Melarhaphe neritoides

Melarhaphe neritoides is a small snail that lives on the upper parts of rocky shores, browsing on micro-algae and lichens. It favours exposed conditions but is also found at the sheltered Dale Fort Jetty site.

It was chosen because its numbers vary a lot from year to year. If similar patterns of stability are seen in this species’ abundance as the patterns of stability seen for all species at the sites (see above) it suggests that these patterns are real and not an artefact of sampling. The general pattern is indeed the same. Jetty Beach was the most variable by far and Castle Beach and Monkhaven were remarkably consistent. There was a peak in numbers at both Castle Beach and Monkhaven in 2003.

Melarhaphe 3 sites 1996 - 2014Limpets

Limpets are large, obvious, common, long-lived mainly herbivorous animals that occupy an extensive vertical range on rocky shores of all degrees of exposure. They are therefore ideal subjects for rocky shore monitoring.


Abundance of Patella vulgata (common limpet) on the 3 transects.

1) Dale Fort Jetty Beach

Limpets Jetty 1996 -2014

In terms of total numbers there appeared to be a cycle of 10 – 12 years from peak to minimum abundance of limpets on Jetty Beach. An increase over the next few years would provide evidence that this is a true cyclic phenomenon.

The numbers of animals at each site (lower graph) are rather variable. It is noticeable that the first year of monitoring (red line) had the lowest numbers in the middle shore and this may be an effect of oil deposition. Limpet casualties were evident during 1996.

2) Castle Beach

Limpets Castle 1996 -2014

At Castle Beach there was less evidence of cyclic behaviour in terms of total numbers of limpets. Any cycle that might be there would seem to be a shorter one of 5 – 7 years or so. This may be a reflection of increased exposure and intra-specific competition resulting in reduced longevity for the population as a whole at Castle Beach.

The numbers at each site on the Castle Beach transect were extremely consistent from year to year. The earliest, latest, and middle year records all tracked the 19 year mean very closely (see lower graph).

3) Monkhaven

Monk limpets 1996 -2014Following the possible patterns of Dale Fort Jetty Beach and Castle Beach the total numbers of limpets on the Monkhaven transect showed some evidence of cyclic behaviour. Interestingly, there was a possible trend of diminution of length of cycle with increasing exposure. The total numbers cycle at Monkhaven seems to be one of 4 – 5 years, slightly shorter than the cycle at Castle Beach which in turn was slightly shorter than the one at Dale Fort Jetty Beach (see Dale Fort Jetty Beach upper chart).

In common with the trends in total numbers the numbers of animals at each site at Monkhaven showed the most consistent pattern of all with very little change from year to year (see Monkhaven lower graph).

The Limpets of Site 5 Castle Beach

All sites are recorded photographically to enable further analysis. A middle shore site at Castle Beach was selected for a detailed limpet-size investigation. This site proved easy to photograph consistently.   Initially this was done with photographic film. Prints were examined under a Wild binocular microscope and the maximum length of all positively identified animals was measured using Vernier Callipers. Actual size was determined using the 0.5m sides of the quadrats as a scale. From 2002 photographs were digital and large prints negated the need for a microscope.

Limpets site 5 castle 1996 - 2014Mean maximum diameter of limpets at Site 5 on the Castle Beach transect seems to be exhibiting cyclic behaviour. There are about 8 – 10 years between the largest mean maximum diameter and smallest mean maximum diameter. It seemed reasonable that mean maximum diameter might be negatively correlated with limpet density.   The figures seem to indicate that peaks in limpet density might correspond with troughs in mean maximum diameter. This is particularly so between 2006 and 2013. However, regression analysis for the years 2006 to 2013 indicates this not to be the case (see below)




















Limpet size density CB site 5 2006 2013There seems to be no simple correlation between limpet size and density, a conclusion similar to that of Ballantine (1961):


” There is no direct relationship between mean size and population density (expressed as weight per occupied area) in P. vulgata.”


W.J. BALLANTINE (1961) PhD thesis


Other workers have found the opposite:


“For site A, a Spearman’s rank correlation coefficient of -0.84 confirms that the inverse

relationship between mean limpet size and limpet density was significant at the 1% level.”


Field Studies, 10, (2003) 579 – 599




It is difficult to make any firm conclusions from the above. It seems reasonable to suppose though that there is no simple relationship between limpet size and density. Inter-specific biotoc interactions are probably important as are interactions with the physical




environment. It was noted that the lowest barnacle density recorded coincided with the smallest limpet mean maximum diameter and a low limpet density (see below).

Barnacle density site 5 castle 1996 - 2014This would imply a that there might be extra space available on the rock for species to occupy in 2014. Limpet numbers might therefore expand over the next few years. On the other hand barnacles tend to settle in the spring and summer months while limpets tend to settle in the autumn. Could it be that barnacles will already be occupying the space by the time of the next limpet settlement? The most abundant mid-shore barnacle is Semibalanus balanoides. This species requires a month of mean water temperature below 7.2 o C before it can breed ( MarLin website, 2014). This temperature was not achieved during 2014 (Data logger next to site 5). Could this reduce the breeding success of S. balanoides?   S. balanoides is an osmoconformer. 2013 – 2014 saw huge rainfall, could this have increased barnacle mortality? If as some authorities believe sea temperatures are rising, then might S. balanoides be replaced at lower shore levels by the warm water species Chthamalus stellatus? These questions and many more will only be answered by further monitoring.



Gibbula umbilicalis (purple topshell) at Castle Beach

This species was of interest because it is thought to be a warm water species that is currently extending its range northwards. This has been seen by some authorities as evidence that sea temperatures are rising. It is a small herbivorous snail that occupies the middle to lower parts of sheltered to moderately exposed rocky shores.

Purple tops at castle 1996 - 2014 There was a drop to one individual about a year after the oil spill. This was followed by an increase that fluctuated from year to year but overall numbers climbed to a maximum of 15 after 10 years. G.umbilicalis seemed to be increasing year on year. However, 2007 and 2008 changed all that, because the numbers were decreasing again. 2009 to 2011 saw a rapid increase and numbers peaked at 29 (2009 – 10 was the coldest winter for decades). 2011 to 2013 saw another increase and 2014 a decrease. The causal factors responsible for this variation are unknown. G. umbilicalis is however an extremely mobile species. Factors like weather conditions are likely to have a huge influence on count data e.g. As a result of animals seeking out appropriate microhabitats like crevices during dry, warm weather. This renders them very difficult to detect. 2014 was the stormiest year for decades and the reduction in numbers might be due to increased mortality due to excessive wave action.


There is a paucity of long-term, reliable, quantitative data even from shores that have been studied for decades.

The most important variable in most data sets is the people that collected the data.

The Dale Fort data set has the unusual feature that the same person (the author) has collected the data from the beginning. This should ensure maximum consistency.

Methodologies should strive for consistency and repeatability to try to reduce the effects of personnel changes.

The very strong ecotone caused by the rise and fall of the tide overwhelms most changes you can represent using multivariate analysis.

If/when the next oil spill occurs we will be able to make much more definite statements about the effects, particularly on limpets but also for several other species.

Seashores are one of the few natural habitats that remain in the UK. Their inhabitants indicate the state of the seas and as we depend, for our very existence, on the seas they are worthy of study.

Continuing to add to this long-term data set will facilitate the recognition and interpretation of any anthropogenic or other changes in the future.





























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