the&sea$empress&incident&and&the&limpets&of&frenchman’s&steps,&...

ARCHER-‐‑THOMSON (2016). FIELD STUDIES (http://fsj.field-‐‑studies-‐‑council.org/)

© Field Studies Council (02/09/2016)

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THE SEA EMPRESS INCIDENT AND THE LIMPETS OF FRENCHMAN’S STEPS, TWENTY YEARS ON.

JOHN ARCHER-‐‑THOMSON

Honey Hook Cottage, Lower Freystrop, Haverfordwest, Pembrokeshire, SA62 4ET. UK

Students and staff from FSC Dale Fort Field Centre have studied limpet populations on Frenchman’s Steps shore for over thirty years. Variations in population density and age structure have been measured and in particular the effects of the Sea Empress oil spill have been investigated. The oil pollution reduced the population density significantly and affected smaller (younger) limpets in particular, especially those on the lower half of the shore. Since then further changes in limpet population density have occurred. These are interpreted in the light of long-‐‑term oscillations in populations of barnacles and dogwhelks, which are related to the use (and subsequent banning) of Tri-‐‑Butyl-‐‑Tin anti-‐‑fouling paint. Long-‐‑term monitoring, using data collected by student groups, can be useful in highlighting variations in population density over time as well as being a worthwhile educational experience in its own right for the students and staff involved in the data collection.

FIGURE 1. Frenchman’s Steps , Pembrokeshire (Grid Ref. SM822053).

INTRODUCTION

Limpets of the Genus Patella are very important organisms on European rocky shores. Their differential grazing activity on a wide range of microorganisms (biofilm) and seaweeds, including Fucus spp., and encrusting red seaweeds, affects the shore community so significantly that limpets have been called a “keystone species” in that their effect on community composition is greater than would be predicted from their abundance or biomass alone (Jenkins et al., 1999, Little et al., 2009). Factors that affect limpet abundance are therefore likely to affect the shore as a whole, which makes any long-‐‑term monitoring programme of limpets potentially interesting.

Meaningful statements about the effect of an event on a rocky shore population depend on knowledge of what the population’s status was before the event. It is also imperative to know about “natural” variations in the population before deductions can be made concerning “unnatural” ones. However, very few suitable long-‐‑term data sets exist, for any ecosystem, to allow such deductions to be made with any confidence.

When the Sea Empress oil spill occurred (see Figure 2) in February 1996, long-‐‑term data sets on the numbers and size-‐‑range of the common limpet Patella vulgata L, collected by student groups from FSC Dale Fort, did exist. Details of the methodology are given below. Comparisons were made between pre-‐‑ and immediately post-‐‑pollution results and the conclusions were presented in Field Studies (Archer-‐‑Thomson, 1999). Useful background information on the effects of oil spills on rocky shores, the Sea Empress oil spill, chemical details of the oil and the biology of limpets can be found in this paper, but see Branch (1981) for a comprehensive overview of limpet biology and Crump et al.,



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(1998) for an account of the oil spill on West Angle Bay, Pembrokeshire. In summary, the oil spill reduced the numbers of limpets in total (Figure 3) with especially noticeable declines in smaller size classes on the lower part of the shore, as a result the modal class for the population rose from a “normal” value in the 10-‐‑14.99 mm size class, in all pre-‐‑pollution data sets, to the 15-‐‑19.9 mm size class in April 1996 (Figure 4). In April 1997 numbers of limpets on the study shore had returned to what might be considered “normal” again (Figure 5) but the modal class was still shifted to the right (Figure 6). By April 1998 numbers were towards the high side of the “normal” range (Figure 7) and the modal class had returned to “normal” as well (Figure 8). Seemingly, the population had made a full recovery, in terms of numbers and size distribution, within two years: a surprisingly quick revival given the magnitude of the event. Student data sets vary in quality. However, if students are told that they will be contributing to long-‐‑term monitoring, conscientious data collection is far more likely and has occurred in this instance. Two sets of observations, a fortnight apart, by different A-‐‑level groups, in April 1996, gave similar and statistically significant results (Archer-‐‑Thomson, 1999) (Figure 3).

Having realised the value of the limpet data, monitoring continued in April of each year, to gain further insight into what might represent “normal” fluctuations of this population. To help with the consistency of recording, final-‐‑year MSc students from the University of Leuven, who visit FSC Dale Fort annually, were used to collect the data. In more recent years FSC Dale Fort teaching staff have collected the data. There is now an unbroken (except 2005) set of data for the Frenchman’s Steps population up to April 2016.

FIGURE 2. Frenchman’s Steps and surrounding coastline. On 15th February 1996 the Sea Empress oil tanker grounded near St Ann’s Head, 2 days later high winds drove her onto the rocks. Over 70,000 tonnes of oil was spilt. (Map from Archer-‐‑Thomson, 1999)

MATERIALS AND METHODS

The site chosen was Frenchman’s Steps, Grid Ref. SM822053 (Figure 2). This is a sheltered rocky shore, with a north-‐‑north-‐‑easterly aspect (Figure 1), Ballantine’s Exposure Grade 4 (Ballantine, 1961). Data were collected by groups of students attending field courses at FSC Dale Fort Field Centre and latterly by FSC Dale Fort teaching staff. All data collection was directly supervised by the author. An interrupted belt transect, sampled at 0.75 m height intervals, was established from a fixed starting height 2.25 m Above Chart Datum (ACD). This is where the bedrock starts; below this the substrate is mobile and unsuitable habitat for limpets. Data collection continued up the shore until the upper distributional limit of limpets was reached. 50 cm by 50 cm quadrats were placed at 0.5 m (horizontal) intervals along a tape measure laid out at each height. In each quadrat the longest diameter of every limpet was measured and recorded in its appropriate 5 mm size class. To prevent measuring the same limpet more than once, each shell was lightly marked with chalk.

The number of quadrats used each year varied with the numbers of students involved in the data collection so the results were standardised to give totals for ten quadrats at each height. No changes to the methodology have been made over the years to facilitate comparisons with past data (Archer-‐‑Thomson, 1999).

Limpet population data are likely to show a degree of seasonality, especially as recruitment to the shore occurs in the autumn, so it was decided to standardise data collection to April of each year where possible; however, some data sets are from March and May.



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FIGURE 3. Limpet numbers at Frenchman’s Steps at the given heights ACD for two April 1996 data sets and three pre-‐‑pollution

examples (1985-‐‑1989).

FIGURE 4. Limpets at Frenchman’s Steps: size frequency data for a typical pre-‐‑pollution data set (A; April 1985) versus an April 1996 example (B). Modal class size is in red.

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FIGURE 5. Limpet numbers at Frenchman’s Steps at the given heights ACD for two April 1996 data sets and three pre-‐‑pollution examples (1985-‐‑1989), and a one-‐‑year post-‐‑pollution example (1997).

FIGURE 6. Limpets at Frenchman’s Steps: size frequency data for a April 1996 data set (A) versus (B) an April 1997 (one-‐‑year post pollution). Modal class size is in red.

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FIGURE 7. Limpet numbers at Frenchman’s Steps at the given heights ACD for two April 1996 data sets and three pre-‐‑pollution

examples (1985-‐‑1989), and two post-‐‑pollution examples (1997-‐‑one-‐‑year post pollution, 1998 – two-‐‑year post pollution).

FIGURE 8. Limpets at Frenchman’s Steps: size frequency data for an April 1996 data set (A) versus (B) an April 1998 (two-‐‑year post

pollution). Modal class size is in red.

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RESULTS AND ANALYSIS

Table 1 shows a data set for 30 April 1998. Table A contains class data from seven quadrats (student groups), whereas Table B gives the standardised results, as if ten quadrats had been used.

Table 2 gives (standardised) results for the number of limpets found in ten 50 x 50 cm quadrats at various heights up the shore. The three data sets before 1996 were chosen because they were gathered in April and deemed typical for the shore. The two sets of data for 1996, (1) and (2), were recorded by two different A-‐‑level groups, on the 3 and 29 of April respectively. Analysis of these results is presented in Archer-‐‑Thomson (1999). Table 3 similarly provides (standardised) data for size frequencies. TABLE 1. Original (A) and standardised (B) data for limpet numbers and size range from Frenchman’s Steps on 30 April 1998. (A) data from seven quadrats (student groups). (B) the converted (standardised) data for the equivalent of ten 50 x 50 cm quadrats. The shaded cells show:

the height at which most limpets were found (3.75m ACD). the mode for the size class data (10.00-‐‑14.99mm). the shift in the modal class at each height to progressively larger limpet sizes as height up the shore increases.

(See Archer-‐‑Thomson (1999) for an explanation).

A: Data from seven quadrats (student groups) Vertical height above chart datum / m Size class / mm 2.25 3 3.75 4.5 5.25 6 6.75 TOTALS

< 4.99 7 36 61 29 4 0 0 137 5.0−9.99 26 118 116 52 17 6 0 335 10.0−14.99 24 100 136 103 43 4 0 410 15.0−19.99 12 90 104 91 39 9 0 345 20.0−24.99 12 69 50 89 53 12 0 285 25.0−29.99 2 25 17 21 32 22 0 119 30.0−34.99 9 8 5 6 17 10 0 55 35.0−39.99 3 1 0 0 5 7 0 16 40.0−44.99 5 2 7 45.0−49.99 1 1 50.0−54.99 55.0−59.99 60.0−64.99 65.0−69.99

TOTALS 101 449 489 391 210 70 0 1710 B: Converted (standardized) data for the equivalent of ten 50 x 50 cm quadrats Vertical height above chart datum / m Size class / mm 2.25 3 3.75 4.5 5.25 6 6.75 TOTALS

< 4.99 10 51 87 41 6 0 195 5.0−9.99 37 169 166 74 24 9 479 10.0−14.99 34 143 194 147 61 6 585 15.0−19.99 17 129 149 130 56 13 494 20.0−24.99 17 99 71 127 76 17 407 25.0−29.99 3 36 24 30 46 31 170 30.0−34.99 13 11 7 9 24 14 78 35.0−39.99 4 1 7 10 22 40.0−44.99 7 3 10 45.0−49.99 1 1 50.0−54.99 55.0−59.99 60.0−64.99 65.0−69.99

TOTALS 143 642 698 558 300 100 2441



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TABLE 2. The number of limpets found in ten 50cm x 50 cm quadrats at each 75 cm vertical height interval up the shore at Frenchman’s Steps, in April . ( *Data collected in March. No limpets found above 6.75 m ACD).

April data

Vertical height above chart datum / m Totals 2.25 3 3.75 4.5 5.25 6 6.75

1985 191 504 693 556 333 49 0 2327 1986 152 407 480 338 228 55 0 1660 1989 487 600 607 556 241 94 0 2585 1996 (1) 72 177 335 313 248 75 5 1225 1996 (2) 26 174 320 346 264 36 0 1166 1997 64 330 432 519 235 145 0 1725 1998 143 642 698 558 300 100 0 2441 1999 333 567 657 529 354 118 0 2558 2000 320 472 558 553 351 152 0 2406 2001 119 383 510 630 293 121 0 2056 2002 174 423 723 560 389 103 0 2372 2003 98 584 711 552 238 78 0 2261 2004 129 396 479 540 269 70 2 1885 2005 No data collected for this year 2006 228 364 421 490 293 104 0 1900 2007 190 410 423 328 228 15 0 1594 2008 208 422 392 330 266 146 30 1794 2009 220 222 424 498 290 18 0 1672 2010 159 775 818 862 493 268 0 3375 2011 163 811 1261 1126 553 203 0 4116 2012 168 716 856 1122 432 156 0 3450 2013 257 703 777 645 442 225 0 3048 2014 347 748 715 548 337 168 0 2863 2015* 396 806 728 560 318 114 0 2922 2016 374 846 648 616 388 94 0 2966

Table 4 presents the statistical analysis of the abundance data sets from Table 2. The total number of limpets at each height, for any given year, is compared with those from every other year. It should be noted that this type of data is difficult to analyse statistically. Biological data of this nature are unlikely to meet the requirements of parametric statistics (e.g. assuming normally distributed data, with equal variances) so non-‐‑parametric tests were used. Because the variation in numbers, with height up the shore was large (as a consequence of the environmental gradient on rocky shores from fully marine conditions at the bottom to near terrestrial ones at the top of the shore) the analysis had to be based on a matched-‐‑pair system (2.25 m ACD versus 2.25 m ACD for the years in question etc.). Although multivariate techniques could have been used, these would have been inaccessible to A-‐‑level students. The Wilcoxon Matched Pairs test was deemed the most suitable test. It is noted, however, that given the small sample size (seven heights maximum), this test would only record a statistically significant difference if the quadrat totals in all the heights from one year were greater (or lesser) than those from another. Inherent variation at site one (where the substrate varies from year to year) makes this constraint significant. Even so, the analysis did largely show significant differences where expected. An excellent description of the limitations associated with this statistics test is given in Fowler et al. (1998). Table 4 has been colour coded to aid interpretation. Normally, the Null Hypothesis is rejected, coded S, or accepted, coded NS, at the 5% significance level or better. In Table 4 the results have been coded (S) if the Null Hypothesis could be rejected at the 10% significance level. The decision to indicate rejection at this level is because of the inherent variability in the data (increased by substrate variation at site one, for example) and because the Wilcoxon test ideally needs a minimum sample size of six non-‐‑zero differences. Many of the data sets give six non-‐‑zero differences, some give seven, but operation here is at the lower end of the ideal sample size spectrum. Thus, (S) indicates a tendency in the data and is therefore still a useful guide.

A look at Table 4 shows a cluster of significant differences (S) for the 1996 (oil pollution) data sets as expected but also another cluster of significant difference (Ss) for the 2010 data and later, which highlights how different the 2010, and later, population densities are.

When working at the 5% significance level, there is a risk of rejecting a Null Hypothesis (H0), when it is true, five times out of a hundred or one time in twenty. This is referred to as a type 1 error. Table 4 contains a great many Wilcoxon Matched Pairs test results and thus the risk of a type 1 error is increased. A Friedman test analyses the data set in table two as a whole, whilst still operating on matched pairs (It is like a matched pairs Analysis of Variance



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TABLE 3. The number of limpets in each 5 mm size class on the transect up the shore at Frenchman’s Steps. Data standardised for ten 50 x 50 cm quadrats at each height. Cells shaded:

show the modal class. (* Data collected in March. No limpets found bigger than 69.99 mm).

Size class in mm April data

< 4.99

5.00-‐‑9.99

10.00-‐‑14.99

15.00-‐‑19.99

20.00-‐‑24.99

25.00-‐‑29.99

30.00-‐‑34.99

35.00-‐‑39.99

40.00-‐‑44.99

45.00-‐‑49.99

50.00-‐‑54.99

55.00-‐‑59.99

60.00-‐‑64.99

65.00-‐‑69.99 Totals

1985 96 476 658 494 327 171 72 18 13 1 1 2327 1986 55 239 438 410 266 145 67 27 10 3 1660 1989 324 586 590 469 240 200 108 41 14 14 2585 1996 (1) 33 157 285 327 221 94 72 19 11 5 1225 1996 (2) 8 91 248 376 286 115 29 8 3 1 1166 1997 19 172 360 472 359 187 112 34 6 3 1 1725 1998 195 479 585 494 407 170 78 22 10 1 2441 1999 57 543 702 489 375 169 125 73 13 9 1 1 1 2558 2000 109 366 668 589 316 197 84 39 16 12 5 2 1 2 2406 2001 41 373 639 424 292 171 57 46 10 1 1 1 2056 2002 262 452 617 476 324 144 57 24 12 3 1 2372 2003 47 492 679 554 214 186 55 25 3 4 2 2261 2004 48 292 559 474 232 145 64 44 16 2 7 2 1885 2005 No data collected this year 2006 54 396 639 435 230 94 35 6 6 2 3 1900 2007 75 337 470 305 210 76 59 30 21 3 8 1594 2008 76 413 622 338 199 77 48 16 4 1 1794 2009 50 264 424 454 234 134 56 28 20 2 6 1672 2010 75 547 1072 775 457 216 120 71 33 9 3375 2011 253 1120 1204 841 335 213 79 43 21 8 1 4116 2012 174 608 1236 740 382 166 76 44 16 4 4 3450 2013 142 617 865 722 348 183 95 40 23 13 3048 2014 55 558 862 742 387 177 70 5 7 2 2863 2015* 26 588 846 820 370 164 42 44 14 6 2 2922 2016 10 280 1090 884 508 128 46 6 8 4 2 2966

would be to a series of matched pair t tests, suitable if the data were normally distributed, or if sample sizes were larger). The result of the Friedman test on the data as a whole (see FRIEDMAN tab in the Excel spreadsheet of RAW data in the appendix*) allows the rejection of the H0 (that there is no significant difference between the medians of the data sets) at P< 0.001, a very highly significant result. The chance of getting a result this significant by chance is less than one in a thousand. Although the Friedman analysis reduces the chance of a type 1 error significantly, it only indicates the data sets as a whole are different, and does not show which data sets (years) are significantly different from which others. Thus, the many Wilcoxon tests are necessary despite the increased risk of type 1 errors. (* For interested readers RAW limpet survey data may be found in an additional file supplied with this on-‐‑line paper.)

DISCUSSION

Explanations for the patterns in the data, from 1985 to 1998, have been presented previously (Archer-‐‑Thomson, 1999) and summarised below.

The total number of limpets at each height [Table 2]. Limpet densities vary with height up the shore. Numbers are low at the base of the study area, 2.25 m ACD, but increase with height to a maximum value in the middle shore (at approximately 3.75 m ACD), decreasing above this again to zero by about 6.75 m ACD. Portions of the lowest site are covered with pebbles, the amount and extent varying from year to year, which explains some of the variation at the bottom of the shore.



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TABLE 4. Wilcoxon Matched Pairs Test results for limpet numbers over the given years (April data). Data standardised for ten 50 x 50 cm quadrats at each height. The shaded cells show: NS Non-‐‑significant result, Accept H0 (of no significant difference between the median number of limpets at the 5% significance level). S Reject H0 at 5% significance level. (S) Reject H0 at 10% significance level (see text for explanation as you would normally Accept the H0 here).

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2016

1985 NS NS (S) S NS NS NS NS NS NS NS NS NS S NS (S) (S) (S) (S) S NS NS NS

1986 S NS NS NS (S) S S NS S NS NS NS NS NS NS S S S S S S S

1989 (S) (S) NS NS NS NS NS NS NS NS NS S NS (S) NS NS NS NS NS NS NS

1996 (1) NS NS S S S S S NS (S) S NS S NS S S S S S S S

1996 (2) (S) S S S S S (S) S S NS (S) (S) S S S S S S S

1997 NS NS S (S) NS NS NS NS NS NS NS S S S S S (S) (S)

1998 NS NS NS NS NS S NS (S) NS NS S S S S S S NS

1999 NS NS NS NS (S) S S (S) S NS NS NS (S) NS NS NS

2000 NS NS NS S S S (S) S NS NS NS (S) NS NS NS

2001 NS NS NS NS NS NS NS S S S S NS NS NS

2002 NS S NS NS NS (S) (S) (S) (S) S NS NS NS

2003 NS NS NS NS NS S S S S (S) S (S)

2004 NS NS NS NS S S S S S S S

2006 NS NS NS (S) (S) NS S S S (S)

2007 NS NS (S) (S) (S) S S S S

2008 NS NS NS NS S (S) NS NS

2009 (S) (S) (S) S S S S

2010 NS NS NS NS NS NS

2011 NS NS NS NS NS

2012 NS NS NS NS

2013 NS NS NS

2014 NS NS

2015 NS

2016

There is a decrease in numbers towards the top of the shore, because conditions become increasingly harsh for a marine snail. Temperature and salinity variation and dehydration create stresses that all get worse as emersion (periods spent out of water) times increase. Limpets graze selectively on green algae (both macro and microscopic), lichens and young fucoids. They often feed at night when the tide is out (Branch, 1981), or at any time when the sea is calm and the tide is in. Feeding time and food supply are not necessarily reduced in the upper shore but a limpet might feed less if already stressed by other environmental (abiotic) factors. In short, abiotic factors probably set the upper distributional limits (Branch, 1981). Numbers decrease towards the bottom of the shore because of inter-‐‑specific competition from macro algae in more sheltered regions and from barnacles in more exposed ones. Exposure to wave action will vary considerably on shores in different locations. A headland site receives far more wave energy than an embayed one, but there are subtle variations along small horizontal distances as the slope and topography vary and this can contribute over the study area width of approximately ten metres. Thus, lower distributional limits are probably set by biotic (living) factors. It follows that optimal conditions, between the two extremes mentioned above, exist for limpets in the middle shore. Over the thirty years in which data have been gathered, the trends described above have held true with no significant deviation. The total number of limpets on the shore over time [Table 2].

Looking at the population as a whole numbers were significantly reduced by the Sea Empress oil spill, recovering to normal again within a year or two. From 1998 onwards, the population fluctuated within what had become expected (“normal”?) limits up until 2010. In 2010 and 2011 limpet numbers exceeded anything hitherto



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experienced (Figure 9). Up until 2010 it was assumed that “normal” fluctuations in the study limpet population had been established. Data from 2010 and 2011 threw this assumption into doubt but the question now became one of whether the record population densities were “natural” or if there was another explanation that might be regarded as “unnatural”?

FIGURE 9. Total number of limpets on Frenchman’s Steps sample site, 1985-‐‑2016.

FIGURE 10. (Left) Dogwhelk Nucella lapillus plus egg capsules from Frenchman’s Steps. (Right). Typical dogwhelk Nucella lapillus density in recent years (photograph taken September 2009).

In an attempt to explain the record high limpet densities of 2010 and 2011, it is necessary to include anecdotal evidence and field observations from the author’s own 30-‐‑year-‐‑plus experience of the shores around FSC Dale Fort. In 1982 there were virtually no dogwhelks (Nucella lapillus) on the shores between the Field Centre and Dale village (Figure 2). Local shores might have yielded two or three animals in an area of 1500 m2. Dogwhelks (Figure 10) are fairly common rocky shore animals in the UK, but at the time their numbers were severely depleted by Tri-‐‑Butyl-‐‑Tin (TBT) anti-‐‑fouling paint, which was widely used to prevent fouling organisms (barnacles, seaweeds, etc.) from growing on the hulls of pleasure boats and larger commercial vessels. In the 1980s unusual changes in the sexual characteristics of dogwhelks were noticed in estuaries and other areas where small boats, painted with TBT anti-‐‑fouling paint, were concentrated (Little et al., 2009). Studies showed

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(Gibbs et al., 1988) that female dogwhelks were growing a non-‐‑functional male reproductive organ, which blocked their oviduct, preventing successful reproduction. This condition was termed ‘imposex’. As a result, over about 20 years, many local snail populations became extinct. TBT paint was so toxic that imposex could be initiated in newly hatched snails at concentrations of only 2 ng of tin/l of seawater (Little et al., 2009). Fortunately, TBT anti-‐‑fouling paints were banned from small craft (<25 m) in 1985, from all craft in 2003 and all traces of the paint had to be gone from hulls by 2008, by International Maritime Organisation statute.

After the ban, dogwhelk populations began to recover on the shores around Dale Fort Field Centre, and they are now host to possibly unsustainably high densities (Figure 10). Dogwhelks typically eat barnacles (the acorn barnacle Semibalanus balanoides, being the favoured species (Crothers, 1985)) of which there is an abundance at the study site. A suggestion for the unusually high limpet densities in 2010 and 2011 is that the dogwhelks may have had such an impact on the local barnacle population that inter-‐‑specific competition for space has been reduced in the limpets’ favour, allowing far more to survive on the shore than hitherto. This fits in with theories about rocky shore community structure and the inter-‐‑relationships between organisms such as barnacles, limpets and dogwhelks (Jenkins et al., 1999). It is noted that after 2011 (see Figure 9) the number of limpets started to decrease again. A speculative explanation can be given below, although further data will be needed to confirm if this trend back to “normal” continues in future years. Dogwhelks are known to also eat young limpets (Branch, 1981). In 2011 there were unusually large numbers of limpets in the 5.00-‐‑9.99 mm size class (Figure 11). In 2012, however, when the limpet population had decreased from an all time high of 4,116 animals to 3,450, the numbers of limpets in the 5.00-‐‑9.99 mm size class had decreased substantially (Figure 12). It is suggested that, having had a significant impact on the barnacle population, dogwhelks were now predating on small, thinner-‐‑shelled limpets as well, which might explain the limpet population moving towards “normal” again. Between 2012 and 2013 (Figure 13) there was a reduction in the total number of limpets on the shore (Figure 9) and a marked drop in the 10.00-‐‑14.99 mm size class in particular (Figure 14). This might represent the upper size limit for limpets to be preyed on by dogwhelks (dogwhelks favour small prey if available (Crothers, 2012)) but if barnacle prey is still in short supply, limpets of this size may still be a suitable option. Since 2013 the total population density of limpets seems to have stabilised albeit at a higher than “normal” level. An interesting result from the 2016 data shows an increase again in the 10.00-‐‑14.99 mm size class (Figure 14). It will be useful to monitor this size class in future years to see if this represents the start of a new trend or if this is merely a short-‐‑term fluctuation. Throughout this article the terms “normal”, “abnormal”, “natural” and “unnatural” have been in inverted commas because of the difficulties in establishing whether a particular shore state is affected by human activity or not. Since the study of the Frenchman’s Steps population started in the 1980s and was significantly affected by Sea Empress oil (definitely “unnatural”), it was assumed that the population state was “normal” before that pollution event. However, looking at the data again in the light of TBT effects it is entirely possible that the lack of dogwhelks in the 1980s allowed a higher than previous population density of barnacles to exist and therefore the limpet densities of the time were suppressed. As dogwhelk numbers recovered after the TBT ban, barnacle densities may have been depressed allowing limpet numbers to increase to “normal” levels again. Thus, the recent (2010 and onwards) data may not represent an unnaturally high limpet density at all but a recovery to pre-‐‑TBT levels. In summarising the trends in the above data, it is possible to recognise four different states for the limpet population on Frenchman’s Steps between 1985 and 2016 (Figure 9). A pre-‐‑oil pollution state, fluctuating between a total of 1,500 and approximately 2,500 individuals. An oil pollution state of approximately 1,200 individuals. A post-‐‑oil pollution state, but pre-‐‑2010, of 1,500 to 2,500 individuals again and a post-‐‑2010 state of 2,800 to 4,100 individuals. Figure 15 summarises these four states and indicates whether the differences are statistically significant, or not, with the use of 95% confidence limits as indicators. With the obvious exception of the 1996 data, it is impossible to know whether these other states indicate “normal”, “abnormal”, “natural” or “unnatural” conditions for this population. It will be fascinating to see what transpires over the next decade or so as the various dogwhelk, barnacle and limpet interactions play out. Hopefully oil pollution will not play a part in the shore dynamics again. Size frequency data [Table 3]. The modal class for 21 out of the 25 size frequency data sets is the 10.00-‐‑14.99 mm one (Figures 8, 11, 12 & 13 for example). Exceptions to this “rule” occurred when the Sea Empress oil pollution displaced the modal class to the right (15.00-‐‑19.99 mm size class) in 1996 and 1997 (Figures 4 & 6). There was also anomalous data in 2009 for which no explanation can be given by the author. Limpets from the genus Patella grow throughout their lives with growth rates varying greatly with food supply (Branch, 1981 and Ballantine, pers. comm.). For this particular shore it is reasonable to assume that the largest limpets are the oldest and the smallest the youngest. Food supply and feeding time vary with height up the shore (Branch, 1981) so this assumption must be treated with some caution but it should hold true in general. There are fewer larger, older limpets because they die of old age, disease, etc. There are very few small, young, limpets in most of the



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FIGURE 11. Size frequency data set, limpets of Frenchman’s Steps, 19th April 2011. Modal class size is in red.

FIGURE 12. Size frequency data set, limpets of Frenchman’s Steps, 8th May 2012. Modal class size is in red.

FIGURE 13. Size frequency data set, limpets of Frenchman’s Steps, 11th April 2013. Modal class size is in red.

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data sets. This could be for a number of reasons. Firstly, very young limpets grow through the first few size classes relatively quickly (Branch, 1981). Secondly, students collecting the data are more likely to miss the very small limpets, as they are harder to see or may even be mistaken for barnacles. Limpets settle from the plankton when their shell is approximately 0.2 mm long (Lewis & Bowman, 1975) and favour small, damp microhabitats such as crevices and pools. This may effectively make them invisible for sampling whilst there. The sample area at Frenchman’s Steps was chosen to maximise the area of suitable rock surface and minimise the area of pools and crevices thus reducing this complication as far as possible. It is interesting to note that the population density data have varied significantly over the study period, showing four distinct phases, whereas the overall pattern in size frequency data is more consistent, varying only during, and in the two-‐‑year period after, the Sea Empress incident (2009 excepting). Reasons for the shift in the modal class, during and after the oil spill are detailed in Archer-‐‑Thomson (1999).

FIGURE 14. Annual April shore totals (10-‐‑14.99 mm size class) for limpets of Frenchman’s Steps (1985-‐‑2016).

FIGURE 15. Mean numbers of limpets, for the given heights ACD, plus 95% confidence limits summary data.

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ARCHER-THOMSON (2016). FIELD STUDIES (http://fsj.field-studies-council.org/)

Taxonomic note.There are three species of limpet in the genus Patella that occur on rocky shores in south-west Britain, Patella

vulgata Linnaeus, common limpet; Patella ulyssiponensis Gmelin, china limpet; and Patella intermedia Murray in Knapp, black-footed limpet. An excellent guide to their identification and biology is given in Fish & Fish (2011). Identification of the three species in the field with student groups is, to all practical purposes, impossible. It involves taking limpets off the rock to look at their foot colour and this invariably kills them. This would be unacceptably destructive on a site where long-term monitoring is intended and, more importantly, ethically unacceptable, especially with groups of students who are being taught to respect the environment in which they study. Fortunately P. vulgata is by far the most common limpet on the shore at Frenchman’s Steps (Ballantine pers. com. and author’s own experience), the other two species favour more wave exposed locations and the lower part of the shore. Patella ulyssiponensis also favours rock pools in the middle shore, where it feeds on encrusting red algae, but again there are no pools at the chosen sample heights. Therefore although there may be representatives of the other two species of limpet in the study area, it is likely their abundance is low and so will not affect the results significantly.

Biological note.Limpets have recently made the news (bbc.co.uk, science & environment, 18th Feb 2015) as they have the honour of hosting the strongest biological material ever tested having ousted spider silk from the top spot (Barber et al., 2015). Limpets feed using a tongue-like structure called a radula, which they scrape over the rock surface as they feed on various species of green seaweed, young stages of brown seaweeds, microscopic algae and other components of what is referred to as “biofilm” covering the rock surface. The radula is essentially a ribbon covered in teeth that are less than a millimetre in length (see excellent high magnification images of a limpet radula in Cremona, 2014, page 21). The teeth are made up of fibres consisting of an iron-based mineral called goethite laced through a protein base. Because the mineral fibres are so thin they are not prone to structural flaws that would normally weaken materials. The remarkable feature of these fibres is that the extreme strength is scale-independent, normally structures, like bridges and arches for example, have to be of a certain size to achieve their desired tensile strength; not so limpet radula teeth. The material is so strong that it can be compared to a single string of spaghetti holding up 1,500 one-kilogram bags of sugar! China limpets exploit the strength of their radula teeth to eat various species of pink paint weed in turn the paint weed has its growing point well below the surface so, even with a growth rate of less than a millimetre a year, it can withstand limpet grazing.

CONCLUDING REMARKS

Numbers of limpets on the shore at Frenchman’s Steps show considerable variation over the years that they have been studied by Dale Fort students and educational staff. Four main states have been identified namely, pre-oil pollution (1985-1989 data sets), pollution and immediate aftermath (1996-1997 data sets), post-oil pollution (1998-2009 data sets) and post 2010 (2010-2016 data sets). With the obvious exception of the oil pollution results assigning “normal/ abnormal” status to any of the data sets is problematic because of the plethora of other variables involved.

Size frequency data show a remarkable consistency across the years with the “norm” only being disrupted by a major oil spill and then for only two years.

Simple fieldwork, carried out with student groups armed with nothing more complex than quadrats, calipers or 15 cm rules, chalk, recording sheets, optical levels and metre rules can yield fascinating data. These results, however, reveal patterns that can be difficult to interpret with any certainty because of the number of variables acting on the shore community. This includes supply-side considerations to do with recruitment to the population and planktonic phases of shore organisms. One of the many features that keep the monitoring of this shore exciting is that the interpretation of the patterns keeps changing as each new data set is collected. Roll on the next thirty years!

DEDICATION AND THANKS

I would like to dedicate this paper to the memory of Dr. Bill Ballantine M.B.E. who knew more about limpets than most, was a staunch supporter of FSC Dale Fort from the 1950s and beyond, and an inspiration for the cause of marine conservation worldwide, latterly from his base in New Zealand. Hopefully he still investigates limpet biology, albeit on a distant shore.

I would also like to thank the various student groups, especially those from the University of Leuven, and the education staff of FSC Dale Fort for their help in gathering these data over the years. It would have been spectacularly tedious without you!

REFERENCES

Archer-Thomson, J.H.S., (1999). The Sea Empress incident and the limpets of Frenchman’s Steps. Field Studies, 9, 531-546.Ballantine, W.J., (1961). A biologically-defined exposure scale for the comparative description of rocky shores. Field Studies, 1 (3), 1-19.

http://fsj.field-studies-council.org/



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Barber, J.H., Lu, D. & Pugno, N.M., (2015). Extreme strength observed in limpet teeth. Journal of the Royal Society Interface, 12: http://dx.doi.org/10.1098/rsif.2014.1326

Branch, G.M., (1981). The biology of limpets: physical factors, energy flow and ecological interactions. Oceanography and Marine Biology: an Annual Review, 19, 235-‐‑280.

Crothers, J.H., (1985). Dog-‐‑whelks: an introduction to the biology of Nucella lapillus (L.). Field Studies, 6, 291-‐‑360. Crothers, J.H., (2012). Snails on rocky sea shores. Naturalists’ Handbook 30. Pelagic Publishing, Exeter. Cremona, J., (2014). Seashores: an ecological guide. Crowood Books, Wiltshire. Fish, J.D. & Fish, S., (2011). A student’s guide to the seashore, 3rd edn. Cambridge University Press, Cambridge. Crump, R. G., Morley, H.S. & Williams, A.D., (1998). West Angle Bay, a case study. Littoral monitoring of permanent quadrats before

and after the Sea Empress oil spill. Field Studies, 9, 497-‐‑511. Fowler, J., Cohen, L. & Jarvis, P., (1998). Practical Statistics for Field Biology, 2nd edn. Wiley, England. Gibbs, P.E., Pascoe, P.L., & Burt, G.R. (1988). Sex change in the female dog-‐‑whelk, Nucella lapillus, induced by tributyltin from

antifouling paints. Journal of the Marine Biological Association of the UK, 68, 715-‐‑731. Jenkins, S.R., Hawkin, S.J., & Norton, T.A. (1999). Direct and indirect effects of a macroalgal canopy and limpet grazing in structuring

a sheltered intertidal community. Marine Ecology Progress Series, 188, 81-‐‑92. Lewis, J.R. & Bowman, R.S., (1975). Local habitat induced variations in the population dynamics of Patella vulgata. Journal of

Experimental Marine Biology and Ecology, 17, 165-‐‑203. Little, C., Williams, G.A. & Trowbridge, C.D., (2009). The Biology of Rocky Shores, 2nd edn. Biology of Habitats. Oxford University Press,

Oxford.

FURTHER INFORMATION Link for raw data

http://www.field-studies-council.org//media/2908899/limpet-raw-data-up-to-and-including-2016.xlsx

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