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Exposure to wave action: Measurements of an important ecological parameter on rocky shores on Anglesey
J. exp. mar. Biol. Ecol., 1968, Vol. 2, pp. 46-63; North-Holland
EXPOSURE
TO WAVE ACTlON:
ECOLOGICAL
PARAMETER
Publishing Company, Amsterdam
MEASUREMENTS
OF AN IMPORTANT
ON ROCKY SHORES ON ANGLESEY
W. EIFION JONES AND ANDREAS DEMETROPOULOS ’ Marine
Science Laboratories,
Menai Bridge,
Anglesey,
Wales
Abstract:
The ways in which the fauna and flora of rocky shores may be affected by wave action are discussed and a simple dynamometer is described by means of which a series of readings has been obtained in a bay on the west coast of Anglesey. These readings have been compared with theoretical values for the maximum dynamic pressure produced by the waves concerned; their agreement shows that such dynamometers are a satisfactory indication of the severity of wave action. The maximum readings obtained have been correlated with the distribution of plants and animals at the same stations. It appears that the typical sheltered shore is very much restricted to the lowest values on the dynamometer scale and that there is a very wide range of values corresponding to very exposed conditions. It is believed that these dynamometers give a simple method of comparing the intensity of wave action on different shores: they are easily and cheaply constructed.
Both the variety of organisms on rocky shores and the zonation pattern in which they are arranged are influenced by a series of environmental factors. Some of these, like climate, may be broadly similar over considerable areas (Lewis, 1964); others, such as shade or the influence of freshwater streams, are more localized and may vary widely over small distances. Amongst local factors it is generally agreed that the severity of wave action to which plants and animals are exposed is of great importance but, unlike most other factors, this is difficult to represent in quantitative terms. It is not too difficult, granted sufficient experience, to rank a series of shores in order of increasing exposure to wave action by observation of their physiography, flora, and fauna, but it is impossible by this means to state how much more exposed is one shore than another. This is not to deny the value of subjective the scales of exposure useful in providing
proposed
a framework
estimates
by Ballantine
of exposure;
(1961) and Lewis
for the description
and comparison
on the contrary, (1964) are most of shores.
Both
these workers propose a scale of exposure based on the flora and fauna of rocky shores of varying exposure; Ballantine divides his series of shores into eight degrees of exposure from ‘extremely exposed’ to ‘extremely sheltered’ and Lewis suggests five classes, based on very widely ranging observations, which include the extremes of shelter and exposure. In each case biological criteria allow any rocky shore to be allocated a place in the classification but it must not be thought that in either scheme the ‘degrees’ are regularly spaced along a linear scale. 1 Present address, Department of Fisheries, Nicosia, Cyprus. 46
THE MEASUREMENT
Attempts Moore
by ecologists
at an objective
(1935) based his estimate
47
OF WAVE ACTION
assessment
on the number
of wave action
have been few.
of days per 100 during
which on-
shore winds were blowing: Southward (1953) measured the height reached by the swash above the predicted tide level, a measurement of considerable significance when considering the elevation of shore zonation on exposed shores but of limited value in a comparison
of shores having
different
aspects
or climates.
In the present work an attempt has been made to measure the forces of breaking waves and to correlate these measurements with observations of the distribution of organisms on the same shores. Before describing the experiments it may be useful to outline the reasoning by which it was decided what ought to be measured. The effects of breaking waves may be divided into two main categories. Wetiing The swash of a breaker flows upshore from the break point to a level determined partly by the surface characteristics of the shore, partly by the dimensions (height, wavelength, velocity, etc.) of the wave just before it breaks, and partly by the wind blowing at the time. Thus, increasing exposure implies an increase in wave force and wind and a raising of the level affected by swash and spray. This is the explanation of the elevation of shore zonation observed by numerous workers (see, Burrows, et ul., 1954; Jones, 1959; Lewis, 1964). Damage The possible
damaging
effects of breakers
may be summarized
as follows.
Abrasion. (i) By particles in suspension; these may be of considerable size when waves are large, and the scouring effect of sand and shingle may be the dominant factor on some shores. In the present study it was decided to avoid shores which were obviously affected in this way. (ii) By the surrounding rock surface; this applies mainly to those plants whose flexibility allows them to be whipped against the rock. Pressure. The hydrostatic pressure exerted by the water in a wave may momentarily be greatly augmented by shock pressures produced as the air trapped in the hollow face of the breaker is compressed (Bagnold, 1939). Such pressures are not directional; they act only by a genera1 compression of the plants or animals, but since these are essentially liquid-filled and incompressible no damage need be expected. Where, as in the case of Himanthalia receptacles, or Entcronzorpha thalli, gas spaces occur, the flexibility of the thallus and the uniformity of the applied pressure seem likely to ensure that the plants are not damaged. Drag. As used here, this term includes both the frictional force of water moving over the surface of the organism and the dynamic pressure exerted against it by the water. Both will act in the direction of flow and will tend to dislodge the organism from its anchorage. The magnitude of the force will increase with the velocity attained by the water during the impact of the wave. The velocity of water particles is a function
48
W. EIFION
JONES
AND
ANDREAS
of the size of the wave but may also be expected sures develop and the water mass is explosively will, however,
be rare, unpredictable,
DEMETROPOULOS
to reach higher values if shock presdisrupted.
Such augmented
velocities
and of brief duration.
In the opinion of the authors, drag as defined above probably represents the greatest hazard to life on a shore not subject to great abrasion. It may be expected to act at all stages in the life history
of an attached
organism
(possibly
preventing
the settlement of some organisms) and to increase in magnitude as the organism grows, particularly since, in other than encrusting forms, growth implies an elevation of the body away from the rock surface and so into more and more rapidly moving water. It was decided, therefore, that the pull exerted by the water should be the component of wave action to be measured. By measuring this directly, account is automatically taken of local factors such as the shelter from neighbouring features, the slope, and depth of the bottom offshore, because all these affect the velocity attained by water in a breaking wave. The slope of the shore may still require separate assessment if the elevation of the spray-affected zones is to be considered, While direct measurements of wave forces do not appear to have been attempted by ecologists, they have, as might be expected, been carried out by a number of civil engineers. Dynamic pressures were first measured in the classical experiments of Stevenson (1849) who recorded pressures as high as 7000 lb. ftm2 (3.2 kg cm- 2). Later experiments include a series by Franzius & Sonne (1901), Gaillard (1904) and de Rouville, Besson & Petry (1938) during whose studies at Dieppe higher pressures than those recorded by Stevenson were recorded momentarily. Shock pressures of this kind were also studied experimentally by Bagnold (1939) but, as has been suggested above, these pressures, though of paramount importance in the design of breakwaters or other marine works, are probably of less significance to ecologists. (For reviews of this work see Defant, 1961, and Wiegel, 1964). APPARATUS
The dynamometers used in this study were designed to record the maximum drag produced in the period during which the instrument was exposed on the shore. No indication could be obtained of the frequency of impact, but is seemed likely that the maximum force exerted would be of greater ecological importance than a number of smaller forces. Fig. 1 shows the final form of the dynamometer; they were modified during the course of the investigation although the essential component was retained throughout. This latter was a standard brass-faced Salter’s 50 lb. spring balance in which the hook was replaced by a 7 inch (17.5 cm) length of nylon line, at the other end of which was a steel disc of 2 sq. in (12.9 cm”) area (1.59 in, 4.04 cm diameter). This disc functioned as a drogue on which the moving water acted, extending the spring and producing a reading on the balance. It was arranged that the maximum reading should be recorded. The meter in its protective case was attached to the rock by a short
THE
MEASUREMENT
OF WAVE
ACTION
length of chain and was thus free to swing in any direction drag regardless
of the direction
The first method
49
and to record the maximum
in which it was applied.
used to record the maximum
reading
was a scriber moving
over
a smoked or painted surface on the face of the balance, but later this was replaced by a pivoted arm which remained in the extreme position to which it was pushed by the balance
pointer.
The protective
case was at first of steel secured by four bolts,
but this proved inconvenient in the field and in the final design the case was a section of domestic ‘Vulcathene’ piping, heat-sealed at one end. Both the case and the meter were attached to the chain by a single rod secured by a split pin. This allowed very
-b
h-
i-d
Fig. 1. Simple spring dynamometer used in wave force measurements on the Anglesey coast: a, hardwood plug 1” (2.5 cm) square, inserted into hole 3” (7.5 cm) deep; b, 3/8” (0.96 cm) diameter steel coach screw on which a chain link is welded; c, D-shackle with pin secured by split pin; d, steel stirrup through which is inserted the retaining pin holding the meter and its case, the pin being secured by a split pin; e, ‘Vulcathene’ tube closed by welding at one end; this protective case is 73” (18.5 cm) long and the retaining pin 7” (17.5 cm) from the open end; f, drogue, a mild steel disc, 1.6” (4.06 cm) diam. welded or riveted to a mild steel tube 1” (2.5 cm) long, shown in section in f’; the nylon cord is 7” (17.5 cm) long, including the portion in the drogue tube, in which it is a tight fit; the drogue is secured by a heat-sealed knot in the cord; g, ring at end of meter; reduced in size and re-welded to fit into case; the retaining pin passes through this ring; h, ‘Salter’ No. 3 50 lb. spring balance with hollow rivets replaced by solid rivets; i, semi-circular brass striker soldered to balance indicator; j, pivoted pointer secured by ‘Nyloc’ or other self-locking nut and tensioned against spring washer (details in section in j ‘).
50
W. EIFION
JONES
AND
ANDREAS
DEMETROPOULOS
rapid removal, reading, resetting, and replacement of the meter on the shore, an important consideration in bad weather. The recording pointer and the chain shackles were secured by ‘Nyloc’ lock nuts which were found to retain their adjustment very well. The anchorage was also modified. At first a steel plate was attached to the rock by two No. 14 screws inserted into lead ‘Rawlplugs’. These proved unsatisfactory owing to the waves’ capacity to loosen the screws; later the anchor consisted of a steel loop embedded in ‘Ciment Fondu’ and, finally, a 3 in coach screw, on which a chain link was welded, was inserted into a hardwood plug. The drilling of thz holes for these anchors in the pre-Cambrian rock of Anglesey constituted one of the major di~cl~lti~s of the investigation.
9 8a8 7b7a 7
65a
meter
5 stations
43
21a
1
Fig. 2. Dynamometer stations at Porth Trecastell, Anglesey. A. Map of the Trecastell showing the dynamometer stations; Stations la, 5a, 7a, 7b and 8a season; Station 4 was abandoned after this section of the rock was removed and maximum readings obtained at each meter station in the second season according to their position on the map.
northern shore of Porth were added in the second by wave action. B. Mean (see Table I) and plotted
THE MEASUREMENT
OF WAVE ACTION
51
LOCATION
The site chosen for the investigation was on the northern side of Porth Trecastell, a bay on the western side of Anglesey. This shore, as wi!l be seen from the maps (Figs 2 and 31, is approximately straight and faces south. The bay is open to the Atlantic in a sector between 200” and 230” which covers the direction from which the dominant winds reach the west coast of Britain. On either side of this sector the winds reach the bay over a more limited fetch (Fig. 3). Within the bay shelter increases from the seaward end of the headland towards the beach and so provides a series of stations, shown in Fig. 2, with different exposure conditions but sinmar in most other respects, including aspect, climate, salinity conditions, times of low and high water, and tidal range. The substratum is aiso
Fig. 3, Fetch across open water towards Perth Trecastell. Average fetch in three sectors opening from Forth Trecasteli: directions are relative to true north; the north shore of the bay faces fully into sector b; the lie of the bay results in some shelter towards sectors n and c.
similar, the rocks being highly metamorphosed pre-Cambrian schists and gneisses with occasional igneous intrusions. The slope of the shore is reasonably uniform. Few natural rock formations approach the ideal of a steep, unbroken rock face of constant slope; some breaks in the rocks between the lower and upper littoral occur, particularly at the less exposed end of the shore, and this must be allowed for in the interpretation on the results. The rock face ended in sand at all stations. Only at Station 1 (the most sheltered} did sand extend higher than LWMST; at all others it was lower, about 15 ft (5 m) below LWMST at Station 7 and 25 ft (8 m) below at Station 9.
W. EIFION JONES AND ANDREAS
52
The meters were sited at what was intended from the nearest Ordnance
DEMETROPOULOS
to be MSL, as determined
bench mark. A Watts ‘Quickset
by levelling
level was used throughout.
Later it was found that the altitude of this bench mark as recorded on the O.S. map was no longer correct and, on the basis of subsequent tidal observations in the bay, it was established MSL. Stations
that the stations
were between
0.8 and 1.9 ft (24 and 58 cm) below
4a, 5a, 7a and 7b (Fig. 2) were used in the second
only. A vertical series of anchors
was also established
at Station
season
(1963-64)
7.
METHODS
The dynamometers wer-e visited and re-set whenever conditions were suitable. Useful readings could not be obtained under calm conditions, while very rough weather made it dangerous, and sometimes impossible, to obtain readings at the more exposed stations. Meters were sometimes damaged and occasionally lost, particularly in the first season before the anchoring system was perfected. The simplicity and cheapness of the instrument were thus of some importance. The biological data were collected in two ways: (1) quantitative estimates of the flora and fauna were made along 0.5 m belt transects across the entire littoral region at selected dynamometer stations, and (2) measurements of the upper and lower limits of vertical distribution of the more important species were made at all stations. These limits were determined by means of a staff and level, using the meter anchors as the datum point and then adjusting the levels t.o an arbitrary datum 8 ft (2.44 m) below MSL, this being approximately the level of extreme LWST. RESULTS PHYSICAL DATA
Table
I gives the readings
provements of readings terpolated
obtained
in the second
season
of observations.
lm-
in the method of anchoring the meters resulted in a more complete set than in the first season but some gaps still occur. Values have been infor these as indicated,
such values being calculated
by plotting
the read-
ings obtained at any station against those obtained on corresponding dates at the next station. These plots gave good approximations to straight lines. If a reading was missing at a particular station it could be obtained by noting the contemporary values at each neighbouring station in turn, reading the corresponding values from the graphs and taking the mean of the two values obtained. Before accepting these results as a valid measure of ‘exposure’ it was necessary to establish that they had a genuine relationship to the intensity of wave action and that the variations obtained were not merely due to chance. The evidence for the reliability of the results is as follows. First, the highest readings were, in general, obtained where subjective judgement indicated that wave action would be most intense. Secondly, when the readings for each date are plotted against the distance
THE
MEASUREMENT
OF WAVE
ACTION
53
TABLE I
Dynamometer
readings
obtained
Stations
1
la
2
32
Oct. 22nd Dec. 2nd Dec. 13th Jan. 9th Jan. 10th Jan. 16th Jan. 28th Jan. 29th Jan. 30th Feb. 7th Mar. 12th Mar. 16th Mar. 26th Apr. 6th Apr. 22nd Mean Maximum
3 4 2 5l 1 1 5 2 3 6 6 3 4 111 5 3.4 6
6 8 42 10 3 3 8 3.5 5
6.5 8
18 15.5 6 16 6 5 13 6 11 22 18 10 13 4 17 12 22
9 5 8
8 2.5 2 6 3 4 8 7 8 8
8 6.1 10
8 5.5 8
10
5 -~ 16 16 6 21.5 7 11 17 7 6 22.5 15 17 16 8 18 13.6 22.5
during
October
1963 - April
1964.
5a
6
7
7a
lb
8
17 15 6 22 7 6 14 6 10 18 17 17.5 17 5 18 13 22
9 14 5.5 19.5 5 3 14 4 11 15 24 16 17 2 17 11.7 24
19 20 8 28 8 6 20 6 18 22 25 20 24 4 23 16.7 28
10 8 3 11.5 3 2 8 3 6 9 9 8 10 3 11 6.9 11.5
14 9 5 18 3 8 12 5 10 11 14.5 12 15 6 18.5 10.7 18.5
18 22 13.5 29 5 12 18 6 10 15.5 22 17 19.5 5 30 15.8 30
’ Figures in italics are interpolated (see text). 2 Station 4 was not used in this series since the section by wave action at the end of the previous season.
of rock
on which
8a -~ IY 18 11 28 7 6 21 9 22 18 28 20 20 8 33 17.9 33
9 ~~~_ 16 16 8 25.5 5 6 18.5 12 14 16 24 18 20 8 30 15.8 30
it was set was removed
of the corresponding stations from the most sheltered end of the bay, the pattern of distribution of wave action along the shore thus produced is strikingly similar in each case, although the actual magnitudes differ according to weather and sea conditions on the different days. This is illustrated in Fig. 2 where the exposure pattern is plotted for both the mean and maximum readings in relation to a map of the station positions. The inference is drawn that the variation in the readings is not a chance effect but that the values obtained are related to the ‘exposure’ of the stations. Thirdly, the readings were compared with the theoretical forces which might be expected from the waves producing them. This comparison with wave dimensions could not be made directly since no wave recorder was available; however, from meteorological data provided by the RAF station at Valley, a coastal station approximately 3 miles (5 km) north of Porth Trecastell, it was possible to calculate the probable maximum dimensions of waves to be expected. The data required were wind speed, duration and direction and these were related to the appropriate fetch as indicated in Fig. 3. Darbyshire (1957) has concluded that an increase in fetch above 200 miles has no further effect in increasing wave dimensions, and a fetch greater than this has been assumed to apply over sector b. It has also been assumed that the wind data supplied by Valley apply over an area large enough to include the 200 mile fetch; this is supported by an examination of synoptic charts of the North Atlantic though exceptions no doubt occur. Using these data, wave heights and periods were obtained from the graphs of Darbyshire and Draper (1963)
54
W. EIFION
which include
JONES
Longuet-Higgins’
AND
ANDREAS
DEMETROPOULOS
wave height factor by which an increment
depending
on wind duration is added to the predicted wave height to give the maximum likely to occur under the prevailing conditions. The results of these calculations are given in Table II. The maximum
dynamic
crest can be calculated
pressure,
P, produced
from Gaillard’s
by the moving
water at the wave
(1904) equation:
v being the orbital velocity of the particles in the wave, c the velocity of the wave front, p the water density, and k an empirical coethcient which depends on the suddenness of the applied load and cannot exceed 2. In his experiments, Gaillard obtained values of k = 1.2. In terms of wave height (H) and period (T) this formula may be written:
If P is expressed in ib./sq. in, then, since the drogue discs are 2 sq. in in area, the meter reading may be expected to approximate to 2P, neglecting the friction of the water on the line and providing a correct value of k is chosen. In Table II the values for 2P, predicted by Gaillard’s formula are shown, taking k = 1. In Fig. 4 these are plotted
against the corresponding
wave heights and on the
TABLE II
Calculated
wave heights,
Wind speed (knots)
Oct. Dec. Dec. Jan. Jan. Jan. Jan. Jan. Jan. Feb. Mar. Mar. Mar. Apr. Apr.
22nd 2nd 13th 9th 10th 16th 28th 29th 30th 7th 12th 16th 26th 6th 22nd
16 20 16 29 10 26 14 24 18 22 18 28 8 25
periods
Direction (see Fig. 3)
b C
; b Swell only a a : C C
a :
and equivalent
dynamometer
readings.
Maximum 1 wave height
Wave period
(ft)
(set)
24 9 6 18 6
10.08 6.85 4.26 25.56 3.33 -
6 5 4 8 3.5 -
18.0 7.4 7.8 36.0 6.0 -
18 15 15 24 18 24 12 24 24
18.46 8.40 16.80 12.96 9.94 6.48 20.70 2.16 21.60
7 5 7 6.3 5 4 7.5 3 7.8
25.6 13.0 24.2 20.0 11.4 10.2 30.2 5.2 32.8
Duration (h)
Calculated meter readings
i!
1 Wave dimensions calculated on the basis of meteorological data and the graphs of Darbyshire and Draper (1963). 2 Calculated dynamometer readings (see text) are twice the value given by Gaillard’s (1904) formula, taking k = 1.
THE
MEASUREMENT
OF WAVE
ACTION
55
C I’ :
/ ,*’
3@ 0
4 I .-? z E 20. ;
l
/’
i0 E I h 10
0
,’ 0 ’
//
0
,’
0
/’
/’
9‘
,,‘O
Fig. 4. readings height period due to
,’
P , *’ /’
0
I r’
.o
0
,,’ 0
’
0 0
0
I ,CJ,
10
0 0
/
0
4
,,,,,&
2
Wave 4
’
fekt 2b
height - metres
6
’
’ 8
,
Relationship between calculated wave height and dynamometer reading: - 0 - dynamometer recorded at Station 8a (the most exposed) plotted against the calculated maximum wave on the same date; - - 0 - - theoretical dynamometer readings based on wave height and on the same dates and using Gaillard’s (1904) formula, taking k = 1; scatter is presumably swell which could not be assessed and to the fact that the graph relates force to wave height only.
Wave 2
height 4
feet - metres
20 6
8
Fig. 5. Relationship between wave height and dynamometer readings at all stations: curves obtained by plotting data from Table I as in Fig. 4; numbering indicates the stations: T is the theoretical curve as in Fig. 4.
56
W. EIFION
JONES
AND
same diagram
the meter
readings
ANDREAS
recorded
DEMETROPOULOS
at Station
8a (the most
exposed)
are
similarly plotted. It will be seen that there is a good correspondence. For the smaller waves a value of k = 1.3 gives a close fit to the experimental curve, while for the higher
values
k is less than
when the calculated
1. This is as might
be expected;
the readings
waves were small are those most likely to be increased
obtained by swell,
which is ignored by the wave prediction formulae. At the other end of the scale the larger waves are more affected by shoaling water than the smaller; they break sooner and are less likely to exert their full force at the meter station. This is emphasized in Fig. 5 where the curves for all stations are drawn together with the theoretical curve, as in Fig. 4. It will be seen that whilst the curves for all stations approach the theoretical curve when waves are small, they rise less and less closely to it as the wave size increases. The closer the station is to the inner end of the bay, the greater is this divergence. This small increase in wave impact which results from a great increase in the size of waves is what is implied in the term ‘shelter’. Station 3, well inside the bay, shows a closer approach to the theoretical curve; this may be explained by its being situated on a rock face steeper than at other stations and protruding beyond the general line of the shore. It may be concluded that the good agreement between theoretical and observed readings shows that this form of dynamometer is a reliable indicator of wave force. READINGS
AT DIFFERENT
LEVELS
In order to investigate how wave impact depended on the level of the shore at which the meter was set, a vertical series was established at Station 7 where the rock slope was uniform (about 30” to the horizontal) for much of the littoral. The meters were at vertical intervals of 2 ft (0.61 m) above and below the datum meter, which is taken as 1 ft below MSL. It was found too dangerous to service the meters below TABLE III Dynamometer Approx. relative Jan. Jan. Feb. Feb. Feb. Feb. Mar. Apr. June
height to MSL
5th 25th 7th 8th 18th 26th 25th 5th 1st
Mean Maximum 1 Figures
in italics
readings
obtained
-1 -0.3
from
a vertical
jl to.3
series at Station +3 to.9
7 in the I963 season. -1~5 -1.5
-.-7 ft -;2.1 m
15 4 16 II 1 14 16 28 22 2
15 6 10 9 16 14 28 24 5
10 6 12 10 10 8 20.5 17 5
13 2 10 13 10 12 23 22 4
10 4 12 12 12 13 23 28 5
14.5 28
13.9 28
11.2 20.5
12.1 23
13.2 28
are interpolated
(see text).
THE MEASUREMENT
this level on days when
useful
readings
OF WAVE ACTION
could
be obtained.
57
The results
from
the
meters at levels to a little above MHWST are shown in Table III. They suggest that, at least to this level, there is no significant fall in the maximum wave force. Since the maximum dynamic pressure is to be expected near the crest of the wave where the forward orbital velocity is greatest, this result is not surprising. By the same reasoning
the maximum
wave impact
should
decrease
towards
the lower lit-
toral since the crests of the largest, most energetic waves cannot break at that level at any state of the tide. This is supported by the increase in numbers of species and density of growth in the lower littoral and sublittoral fringe. It is not suggested that these results are conclusive; further readings at all levels are needed. CORRELATION OF THE DYNAMOMETER READINGS AND ECOLOGICAL PATTERN It is proposed
to comment mainly on the presence or absence of species and on
their vertical limits; it seems best to postpone consideration
of detailed quantitative
-MHWST
5
.MLWNT
c34. I”
Fig. 6. Distribution of some plant species related to maximum dynamometer readings. Vertical limits of distribution of plants at each station are plotted at the maximum dynamometer reading obtained at that station: upper limits of Fucus uesiculosus probably lowered at the sheltered stations by local steepening of the rock at that level; occasional individuals occur outside the limits shown which relate to the main bulk of the populations. f upper limit; .$ lower limit. Porp’, epilithic upper Porphyra umbilicalis zone; Porp”, lower epiphytic zone; Lit, Lichina; F. ves, Fucus vesiculosus; F. ser, F. serratus; Gig, Gigartina stellata; L. sac., Laminaria saccharina; L. dig, L. digitata; Al, Maria. Calm water tidal levels are approximate (see text).
58
variations
W. EIFION
JONES
AND
ANDREAS
until more shores can be included.
limits of distribution be irregularities
of various
because
Table
IV shows the upper
species at each meter position.
of local variations
there are breaks in the rock platform true of Stations 4 (abandoned during there is a region
DEMETROPOULOS
in shelter
and lower
There must inevitably
and slope, particularly
where
between the lower and upper littoral. This is the second season) and 5, while at Station la
of steeper slope in the midlittoral.
Figs. 6 and 7 show these limits
plotted against the appropriate meter readings. The most obvious anomalies occur at a meter reading of 24 lb. and may be related to the broken nature of the shore at Station 5. In order to present a more generalized picture, the curves have been smoothed (Fig. 8) and this diagram, it is suggested, can be taken as an indication of TABLE IV
Upper and lower limits of some All levels are given in feet above
Stations
1
Pelvetia Porphyra (epilithic) Porphyra (epiphytic) Lichina FUCUS vesiculosus F. serratus
plants and animals at dynamometer an arbitrary datum approximating limit; 1, lower limit.
la
2
3
14.5 12.5
r
12.0 7.5
5
5a
6
7
7a
7b
8
8a
9
15.2 12.3 _ -
15.1 12.3 _ _
15.2 12.6 _ -
14.4 12.8 _ -
13.5 12.5 _ -
14.2 12.8 -
14.0 13.7
16.0 13.5
15.3 13.0
11.0 7.3 12.7 11.6
11.2 7.9 13.7 12.3
12.0 7.4 14.8 12.1
10.0 6.7 13.6 11.9
11.3 7.4 14.3 11.9
12.4 6.5 14.5 12.8
12.8 7.0 16.2 12.7
12.3 5.8 -
10.2 6.9 2.4
10.5 3.9 2.1
10.9 4.9 1.6
2.3 9.0 1.9
9.7 3.5 1.9
11.4 3.9 -
11.5 3.7 -
11.3 3.6 --
LI 14.6 1 13.2 u-z_ ]_
-
14.8 12.3 _ -
I” 17.9 LI 12.3 1 10.8
7.1 9.1 14.0 11.8
6.4 9.0 13.2 10.2
12.0
10.5 7.5 13.6 11.1
8.5 5.0 5.7
10.6 6.0 1.2
10.3 6.5 2.2
: u
4.2 7.7 3.9
4.6 8.8 5.3
r
2.5 5.1 2.5-
2.1 6.5
_ -
stations in Porth Trecastell. to extreme LWMST; u, upper
Gigartina Laminaria saccharina L. digitata
; ” 1 l”z
Alaria Littorina neritoides L. saxatilis Chthamalus
3
2.2 5.9 _ -----__-----
1.9
I”- u 19.8 1 4.0 u 10.8 1 10.1 Ll 10.2 1 2.9
2.1
6.4 1.2 _
2.2
2.8 6.1 _
1.6
7.4 1.9 _
2.0
5.1 1.6 _
1.9
6.6 1.2 _
1.6
0.3 6.1 _
1.7
0.9 5.4 _
1.3
0.7 6.5 _
1.9
0.7
0.7
2.2
2.3
2.7
34.9 12.8 34.9 3.8 14.3 9.1 12.4 2.4
12.7
-
-
-_
_. -
-_
-_
-_
-_
25.5 15.5 25.5 3.3 11.3 7.0 9.5 3.1
25.5 10.0 25.5 3.5 10.3
12.0
24.5 11.0 24.5 3.9 12.0
28.9 12.0 28.9 4.0 11.9
28.9 12.0 28.9 3.0 12.8
26.9 13.6 26.9 3.2 12.4
28.0 13.4 24.0 2.9 13.7
12.8
10.7 3.1
11.3 3.2
12.0 2.4
33.9 13.7 33.9 3.7 13.8 10.7 12.0 3.0
11.4 3.1
12.0 2.9
12.9 2.5
9.1 3.4
7.3 1.0 _
0.5
-
3.4 12.2 7.5 11.5 3.0
0.8 8.0 _
2.5 14.1
1 Blanks indicate that no reading was made at that station, usually because the points lay too far from the meter to have a similar exposure to it or were below LWM. ’ Absence is indicated by a hyphen. 3 The lower limit of Chthamalus was determined only where shown.
3.7 14.7 12.3 2.3
concerned
THE
MEASUREMENT
the way that the distribution erately exposed from the results
of shore
OF WAVE
organisms
59
ACTION
is related to wave action
on mod-
shores in this region of the west coast of Britain. It is apparent that the generally accepted views of the effects of wave action on
rocky shore organisms
are confirmed.
Some comments
,....’
_..,.:* * ** ,~.’
&S
’
,
. ..
.
c
--__,’ t Lsax
/ :
.....“.
,+
...i :t
j’
...“.
-. -._&ner
are appropriate.
1’
i$ Ii
:
AHWST
:.,
::’ .,
.
WSL Bal
VlLWST 20 3’0 10 MAXIMUM DYNAMOMETER READING - LBS. Fig. 7. Distribution of some animal species related to dynamometer readings. distribution of animals at each station are plotted at the maximum dynamometer at that station. f upper limit; 4 lower limit. L. ner, Littorina neritoides; L. sax, Chthamalus stellatus; Bal, Balanus balanoides; lower limit of Chthamalus recorded only. Calm water tidal levels are approximate (see text).
Vertical limits of readings obtained L. sczxatilis; Ch, at given stations
There is an elevation of the littoral with increasing wave action. This is particularly noticeable in the case of upper shore organisms. An increase of 25 lb. (11.4 kg) in the meter readings is related to a rise of 8 ft (2.44) m in the upper limits of Littorina neritoides (L.) and Porphyra umbilicalis (L.) J. Ag., a 4 ft (1.22 m) elevation of the upper limit of Fucus vesiculosus L., and 3 ft (0.92 m) for Gigartina steflata (Stack.) Batt. At the lower extremity of the littoral this is less marked; Laminaria digit&a (Huds.) Lamour has a more or less constant upper limit until it falls with the appearance of AZaria esculenta (L.) Grev. Some species are absent. Fucus spiralis L. and Ascophyllum nodosum (L.) Le Jol. were not recorded on any transect line though both occurred in local shelter in various
60
places.
W. EIFION
Both are known
to note how limited
JONES
AND
ANDREAS
to have limited
DEMETROPOULOS
tolerance
is their range in terms
of exposure;
of the dynamometer
canaliculata (L.) Dcne. et Thuret has a much greater either of the former species and this is clearly shown be emphasized
that this species is present
it is only surprising readings.
Pelvetia
tolerance of wave action than by the results, though it must
only as scattered
plants in its zone.
-4
MLWNT
MLWST I..
5
15 DYNAMOMETER
Fig. 8. Distribution of on data in Table IV but umbilicalis; PORPH”, stellarus; BAL.,
0
20
25
30
READING
littoral plants and animals related to dynamometer readings. Diagram based smoothing out local irregularities: PORPH’, epilithic upper zone of Porphyra lower epiphytic zone; L. SACC, Laminaria saccharina; CH., Chthamalus Balanus balanoides. Calm water tidal levels are approximate (see text).
Himanthalia elongata (L.) Gray was absent at Porth Trecastell but it is unlikely that this can be attributed directly to wave action. The species occurs at Trearddur Bay, 8 miles (13 km) northeast of Porth Trecastell, in conditions similar, in terms of flora and fauna, to transects with meter readings in the range 20-30 at Porth Trecastell. But Trearddur Bay has deeper water close inshore and conditions at Porth Trecastell are generally more turbid.
THE MEASUREMENT
The change in the zonation Fucus serratus
OF WAVE ACTION
61
levels is not directly related to change in meter readings.
L., for instance,
shows an abrupt
fall in its upper limit between
meter
readings of 10 lb. and 12-14 lb., then continues as an essentially sublittoral species into much more exposed conditions. This may be due to increasing activity of limpets
but observation
of seaweed
colonization
of a 2 m strip cleared
of limpets
between Stations 6 and 7 does not confirm this. The apparent levelling off of Lichina spp. between meter readings 12 lb. and 24 lb. seems to be at least partly the result of local irregularities. In order to prevent the diagram becoming overcrowded, some species are omitted. The most important of the perennial species are Laurenciapinnatzjida (Huds.) Lamour, which is present with Gigartina and has a higher upper limit on the less exposed shores; Rhodymenia palmata (L.) Grev., a common epiphyte at the lower levels: and encrusting corallines, which are present at the lower levels, particularly on the more exposed sites. Some plants and animals deserve special mention. Porphyra umbilicalis has a zone of mainly epiphytic plants overlapping the upper level of Fucus uesiculosus and also an epilithic form which occurs only at the more exposed stations and at a higher level. This dual distribution is widespread (Conway, personal communication) but not easy to explain. Putella aspera (Lamarck) increased in numbers with Increasing exposure, from 16/m2 at Station la to 80/m2 at Station 8a. At the sheltered end of the shore P. vulgata L. and P. intermedia Jeffreys (which were not reliably distinguished in the field) vastly outnumbered P. aspera, which occurred only below MLWNT. At Station 3 (meter reading 24 lb.) P. uspera was present to MSL and at 8a (36 lb.) almost up to MHWNT. Below a level of 6 ft above datum P. aspera was the dominant limpet at this station. The density of both Chthamalus stellatus (Poli) and Balanus balunoides (L.) increased with increasing exposure. The upper limit of Chthamalus rose 4.1 ft between Stations 1 and 9, that of Bulanus by 2.13 ft. The two species overlap considerably on this shore and the lower limit of Chthamafus and upper limit of Balunus were hard to determine.
The overlap
was noticeably
smaller
towards
the exposed
station.
An upper limit for Verrucaria maura Wahlenb. could not be determined as this lichen met the cliff-top vegetation above most stations. Lichina patches (L. pygmaea Ag. with some L. confines Ag.) were conspicuous at all stations, except Station9 at the seaward end of the headland. Small mussels, Mytilus edulis L., occurred in patches at various stations, from up to 1 ft above MSL at Station la to 4 ft above MSL at Station 8a. The population density and average size of individuals also increased towards the exposed stations. Littorina saxatilis (Olivi) in the larger forms subsp. tenebrosa (Montagu) and subsp. jugosa (Montagu) were similar in their distribution to L. neritoides. The smaller form subsp. saxatilis (Johnston) usually occurs in the empty shells of Balanus and was found down to the lower limit of that barnacle on the shore. The wide Littorina saxatilis zone in the diagram includes all forms. Subsp. rudis (Maton)
62
W. EIFION JONES AND ANDREAS
DE~ETROPOULOS
occurred in Iocal shelter but was very rare (two specimens at Station 1) on the transect lines. CONCLUSIONS
This work has demonstrated that an investigation of wave forces as an ecological parameter is worthwhile; a simple instrument appears capable of producing meaningful results which can add something to our knowledge of the factors affecting distribution on the shore, Now that our knowledge of the distribution patterns is becoming so detailed (Lewis, 1964) it is necessary to evaluate these factors and this
EPlLlTHlC PORPHYRA HWST
MSL
DYNAMOMETER
READING
Fig. 9. Distribution of littoral plants in relation to exposure. Diagram indicating distribution trends in relation to exposure as represented by an extrapolation from dynamometer readings at Porth Trecastell and other records (see text): F. spp., Fucus spiralis; f. wsiculosus and f. linearis the bIaddered and bladderless form of &kcrrs r;esiculosccv respectively; Asc, AscophyG~~ nodosu~n; L. sacc, Laminnricz saecharinn.
can best be done, as has been attempted here, by investigating them one at a time in places where the other factors, as far as can be judged, vary as little as possible. Work on waves force obviously needs to be extended to a wider range of shores than has so far been attempted and it is hoped that the adoption by other workers of this or a similar instrument may lead to an accumulation of useful quantitative information. As an indication of the kind of picture which is emerging we suggest, in a speculative extrapolation, that the zonation pattern on the full range of shores of western Britain, including the most exposed (Powell & Chamberlain 1956; Lewis, 1964), may be as in Fig. 9. Wave forces in this were calculated largely using data from Bigelow & Edmondson (1947) supplemented by that of Burrows et al. (1954) and Jones (1959).
THE MEASUREMENT OF WAVE ACTION
63
ACKNOWLEDGEMENTS We wish to acknowledge
the help of many
members
of the staff of the Physical
Oceanography and Marine Biology Departments at Menai Bridge and particularly Professor J. Darbyshire for his advice on many aspects of waves, Mr. J. G. Harvey for considerable sions on breaking
help in meteorological
matters,
Mr. J. Simpson
waves, the staff of our workshop
our boat crew for their assistance
for valuable
for their constant
discus-
helpfulness,
and
on the shore. REFERENCES
R. A., 1939. Interim report on wave pressure research. J. Civil Engrs, Lond., Vol. 7, pp. 202-226. BALLANTINE, W. J., 1961. A biologically defined exposure scale for the comparative description of rocky shores. Fld Stud., Vol. 1, pp. 1-19. BIGELOW, H. B. & W. T. EDMONDSON, 1947. Wind waces at sea, breakers and surf. Hydrographic Office U. S. Navy Pub. No. 602, 177 pp., Washington. BURROWS, E. M., E. CONWAY,S. M. LODGE & H. T. POWELL, 1954. The raising of intertidal algal zones on Fair Isle. J. Ecol., Vol. 42, pp. 283-288. DARBYSHIRE, J., 1957. A note on the comparisons of proposed wave spectrum formulae. Dt. hydrogr. BAGNOLD,
Z., Bd 10, S. 184-190. DARBYSHIRE, M. & L. DRAPER, 1963. Forecasting
wind-generated sea waves. Engineering, Lond., Vol. 195, pp. 482484. DEFANT, A., 1961. Physical oceanography, Vol. 2. Pergamon Press, London, 598 pp. FRANZIUS, L. & E. SONNE, 1901. Wasserbau am Meere und in Strommiindungen. Handbuch der lngenieurwissenschaften, Leipzig, Bd 3, Heft 3, 751 S. GAILLARD,D., 1904. Wave action in relation to engineering structures. Prof. Pap. Corps Eng., U.S. Army, No. 31, 194 pp. JONES,W. E., 1959. The effects of exposure on the zonation of algae on Bardsey Island. Rep. Eardsey Bird Fld Obs., Vol. 7, pp. 41-46. LEWIS, J. R., 1964. The ecology of rocky shores. The English Universities Press, London, 323 pp. MOORE, H. B., 1935. The biology of Balanus balanoides. IV. Relation to environmental factors. J. mar. biol. Ass. U.K., Vol. 20, pp. 279-307. POWELL,H. T. & Y. M. CHAMBERLAIN, 1956. Plant life on Rockall. In, Rockall, edited by J. Fisher, Geoffrey Bles, London, 200 pp. ROUVILLE, M. A. DE, P. BESSON& P. PETRY, 1938. Etat actuel des etudes internationales sur les efforts dus aux lames. Annls Ponts Chauss., T. 108, pp. 5-113. SOUTHWARD,A. J., 1953. The ecology of some rocky shores in the south of the Isle of Man. Proc. Trans. Lpool biol. Sot., Vol. 59, pp. l-50. STEVENSON, T., 1849. Account of experiments upon the force of the waves of the Atlantic and German Oceans. Trans. R. Sot. Edinb., Vol. 16, pp. 23-32. WIEGEL, R. L., 1964. Oceanographical engineering. Prentice Hall, New York, 532 pp.
EXPOSURE
TO WAVE ACTlON:
ECOLOGICAL
PARAMETER
Publishing Company, Amsterdam
MEASUREMENTS
OF AN IMPORTANT
ON ROCKY SHORES ON ANGLESEY
W. EIFION JONES AND ANDREAS DEMETROPOULOS ’ Marine
Science Laboratories,
Menai Bridge,
Anglesey,
Wales
Abstract:
The ways in which the fauna and flora of rocky shores may be affected by wave action are discussed and a simple dynamometer is described by means of which a series of readings has been obtained in a bay on the west coast of Anglesey. These readings have been compared with theoretical values for the maximum dynamic pressure produced by the waves concerned; their agreement shows that such dynamometers are a satisfactory indication of the severity of wave action. The maximum readings obtained have been correlated with the distribution of plants and animals at the same stations. It appears that the typical sheltered shore is very much restricted to the lowest values on the dynamometer scale and that there is a very wide range of values corresponding to very exposed conditions. It is believed that these dynamometers give a simple method of comparing the intensity of wave action on different shores: they are easily and cheaply constructed.
Both the variety of organisms on rocky shores and the zonation pattern in which they are arranged are influenced by a series of environmental factors. Some of these, like climate, may be broadly similar over considerable areas (Lewis, 1964); others, such as shade or the influence of freshwater streams, are more localized and may vary widely over small distances. Amongst local factors it is generally agreed that the severity of wave action to which plants and animals are exposed is of great importance but, unlike most other factors, this is difficult to represent in quantitative terms. It is not too difficult, granted sufficient experience, to rank a series of shores in order of increasing exposure to wave action by observation of their physiography, flora, and fauna, but it is impossible by this means to state how much more exposed is one shore than another. This is not to deny the value of subjective the scales of exposure useful in providing
proposed
a framework
estimates
by Ballantine
of exposure;
(1961) and Lewis
for the description
and comparison
on the contrary, (1964) are most of shores.
Both
these workers propose a scale of exposure based on the flora and fauna of rocky shores of varying exposure; Ballantine divides his series of shores into eight degrees of exposure from ‘extremely exposed’ to ‘extremely sheltered’ and Lewis suggests five classes, based on very widely ranging observations, which include the extremes of shelter and exposure. In each case biological criteria allow any rocky shore to be allocated a place in the classification but it must not be thought that in either scheme the ‘degrees’ are regularly spaced along a linear scale. 1 Present address, Department of Fisheries, Nicosia, Cyprus. 46
THE MEASUREMENT
Attempts Moore
by ecologists
at an objective
(1935) based his estimate
47
OF WAVE ACTION
assessment
on the number
of wave action
have been few.
of days per 100 during
which on-
shore winds were blowing: Southward (1953) measured the height reached by the swash above the predicted tide level, a measurement of considerable significance when considering the elevation of shore zonation on exposed shores but of limited value in a comparison
of shores having
different
aspects
or climates.
In the present work an attempt has been made to measure the forces of breaking waves and to correlate these measurements with observations of the distribution of organisms on the same shores. Before describing the experiments it may be useful to outline the reasoning by which it was decided what ought to be measured. The effects of breaking waves may be divided into two main categories. Wetiing The swash of a breaker flows upshore from the break point to a level determined partly by the surface characteristics of the shore, partly by the dimensions (height, wavelength, velocity, etc.) of the wave just before it breaks, and partly by the wind blowing at the time. Thus, increasing exposure implies an increase in wave force and wind and a raising of the level affected by swash and spray. This is the explanation of the elevation of shore zonation observed by numerous workers (see, Burrows, et ul., 1954; Jones, 1959; Lewis, 1964). Damage The possible
damaging
effects of breakers
may be summarized
as follows.
Abrasion. (i) By particles in suspension; these may be of considerable size when waves are large, and the scouring effect of sand and shingle may be the dominant factor on some shores. In the present study it was decided to avoid shores which were obviously affected in this way. (ii) By the surrounding rock surface; this applies mainly to those plants whose flexibility allows them to be whipped against the rock. Pressure. The hydrostatic pressure exerted by the water in a wave may momentarily be greatly augmented by shock pressures produced as the air trapped in the hollow face of the breaker is compressed (Bagnold, 1939). Such pressures are not directional; they act only by a genera1 compression of the plants or animals, but since these are essentially liquid-filled and incompressible no damage need be expected. Where, as in the case of Himanthalia receptacles, or Entcronzorpha thalli, gas spaces occur, the flexibility of the thallus and the uniformity of the applied pressure seem likely to ensure that the plants are not damaged. Drag. As used here, this term includes both the frictional force of water moving over the surface of the organism and the dynamic pressure exerted against it by the water. Both will act in the direction of flow and will tend to dislodge the organism from its anchorage. The magnitude of the force will increase with the velocity attained by the water during the impact of the wave. The velocity of water particles is a function
48
W. EIFION
JONES
AND
ANDREAS
of the size of the wave but may also be expected sures develop and the water mass is explosively will, however,
be rare, unpredictable,
DEMETROPOULOS
to reach higher values if shock presdisrupted.
Such augmented
velocities
and of brief duration.
In the opinion of the authors, drag as defined above probably represents the greatest hazard to life on a shore not subject to great abrasion. It may be expected to act at all stages in the life history
of an attached
organism
(possibly
preventing
the settlement of some organisms) and to increase in magnitude as the organism grows, particularly since, in other than encrusting forms, growth implies an elevation of the body away from the rock surface and so into more and more rapidly moving water. It was decided, therefore, that the pull exerted by the water should be the component of wave action to be measured. By measuring this directly, account is automatically taken of local factors such as the shelter from neighbouring features, the slope, and depth of the bottom offshore, because all these affect the velocity attained by water in a breaking wave. The slope of the shore may still require separate assessment if the elevation of the spray-affected zones is to be considered, While direct measurements of wave forces do not appear to have been attempted by ecologists, they have, as might be expected, been carried out by a number of civil engineers. Dynamic pressures were first measured in the classical experiments of Stevenson (1849) who recorded pressures as high as 7000 lb. ftm2 (3.2 kg cm- 2). Later experiments include a series by Franzius & Sonne (1901), Gaillard (1904) and de Rouville, Besson & Petry (1938) during whose studies at Dieppe higher pressures than those recorded by Stevenson were recorded momentarily. Shock pressures of this kind were also studied experimentally by Bagnold (1939) but, as has been suggested above, these pressures, though of paramount importance in the design of breakwaters or other marine works, are probably of less significance to ecologists. (For reviews of this work see Defant, 1961, and Wiegel, 1964). APPARATUS
The dynamometers used in this study were designed to record the maximum drag produced in the period during which the instrument was exposed on the shore. No indication could be obtained of the frequency of impact, but is seemed likely that the maximum force exerted would be of greater ecological importance than a number of smaller forces. Fig. 1 shows the final form of the dynamometer; they were modified during the course of the investigation although the essential component was retained throughout. This latter was a standard brass-faced Salter’s 50 lb. spring balance in which the hook was replaced by a 7 inch (17.5 cm) length of nylon line, at the other end of which was a steel disc of 2 sq. in (12.9 cm”) area (1.59 in, 4.04 cm diameter). This disc functioned as a drogue on which the moving water acted, extending the spring and producing a reading on the balance. It was arranged that the maximum reading should be recorded. The meter in its protective case was attached to the rock by a short
THE
MEASUREMENT
OF WAVE
ACTION
length of chain and was thus free to swing in any direction drag regardless
of the direction
The first method
49
and to record the maximum
in which it was applied.
used to record the maximum
reading
was a scriber moving
over
a smoked or painted surface on the face of the balance, but later this was replaced by a pivoted arm which remained in the extreme position to which it was pushed by the balance
pointer.
The protective
case was at first of steel secured by four bolts,
but this proved inconvenient in the field and in the final design the case was a section of domestic ‘Vulcathene’ piping, heat-sealed at one end. Both the case and the meter were attached to the chain by a single rod secured by a split pin. This allowed very
-b
h-
i-d
Fig. 1. Simple spring dynamometer used in wave force measurements on the Anglesey coast: a, hardwood plug 1” (2.5 cm) square, inserted into hole 3” (7.5 cm) deep; b, 3/8” (0.96 cm) diameter steel coach screw on which a chain link is welded; c, D-shackle with pin secured by split pin; d, steel stirrup through which is inserted the retaining pin holding the meter and its case, the pin being secured by a split pin; e, ‘Vulcathene’ tube closed by welding at one end; this protective case is 73” (18.5 cm) long and the retaining pin 7” (17.5 cm) from the open end; f, drogue, a mild steel disc, 1.6” (4.06 cm) diam. welded or riveted to a mild steel tube 1” (2.5 cm) long, shown in section in f’; the nylon cord is 7” (17.5 cm) long, including the portion in the drogue tube, in which it is a tight fit; the drogue is secured by a heat-sealed knot in the cord; g, ring at end of meter; reduced in size and re-welded to fit into case; the retaining pin passes through this ring; h, ‘Salter’ No. 3 50 lb. spring balance with hollow rivets replaced by solid rivets; i, semi-circular brass striker soldered to balance indicator; j, pivoted pointer secured by ‘Nyloc’ or other self-locking nut and tensioned against spring washer (details in section in j ‘).
50
W. EIFION
JONES
AND
ANDREAS
DEMETROPOULOS
rapid removal, reading, resetting, and replacement of the meter on the shore, an important consideration in bad weather. The recording pointer and the chain shackles were secured by ‘Nyloc’ lock nuts which were found to retain their adjustment very well. The anchorage was also modified. At first a steel plate was attached to the rock by two No. 14 screws inserted into lead ‘Rawlplugs’. These proved unsatisfactory owing to the waves’ capacity to loosen the screws; later the anchor consisted of a steel loop embedded in ‘Ciment Fondu’ and, finally, a 3 in coach screw, on which a chain link was welded, was inserted into a hardwood plug. The drilling of thz holes for these anchors in the pre-Cambrian rock of Anglesey constituted one of the major di~cl~lti~s of the investigation.
9 8a8 7b7a 7
65a
meter
5 stations
43
21a
1
Fig. 2. Dynamometer stations at Porth Trecastell, Anglesey. A. Map of the Trecastell showing the dynamometer stations; Stations la, 5a, 7a, 7b and 8a season; Station 4 was abandoned after this section of the rock was removed and maximum readings obtained at each meter station in the second season according to their position on the map.
northern shore of Porth were added in the second by wave action. B. Mean (see Table I) and plotted
THE MEASUREMENT
OF WAVE ACTION
51
LOCATION
The site chosen for the investigation was on the northern side of Porth Trecastell, a bay on the western side of Anglesey. This shore, as wi!l be seen from the maps (Figs 2 and 31, is approximately straight and faces south. The bay is open to the Atlantic in a sector between 200” and 230” which covers the direction from which the dominant winds reach the west coast of Britain. On either side of this sector the winds reach the bay over a more limited fetch (Fig. 3). Within the bay shelter increases from the seaward end of the headland towards the beach and so provides a series of stations, shown in Fig. 2, with different exposure conditions but sinmar in most other respects, including aspect, climate, salinity conditions, times of low and high water, and tidal range. The substratum is aiso
Fig. 3, Fetch across open water towards Perth Trecastell. Average fetch in three sectors opening from Forth Trecasteli: directions are relative to true north; the north shore of the bay faces fully into sector b; the lie of the bay results in some shelter towards sectors n and c.
similar, the rocks being highly metamorphosed pre-Cambrian schists and gneisses with occasional igneous intrusions. The slope of the shore is reasonably uniform. Few natural rock formations approach the ideal of a steep, unbroken rock face of constant slope; some breaks in the rocks between the lower and upper littoral occur, particularly at the less exposed end of the shore, and this must be allowed for in the interpretation on the results. The rock face ended in sand at all stations. Only at Station 1 (the most sheltered} did sand extend higher than LWMST; at all others it was lower, about 15 ft (5 m) below LWMST at Station 7 and 25 ft (8 m) below at Station 9.
W. EIFION JONES AND ANDREAS
52
The meters were sited at what was intended from the nearest Ordnance
DEMETROPOULOS
to be MSL, as determined
bench mark. A Watts ‘Quickset
by levelling
level was used throughout.
Later it was found that the altitude of this bench mark as recorded on the O.S. map was no longer correct and, on the basis of subsequent tidal observations in the bay, it was established MSL. Stations
that the stations
were between
0.8 and 1.9 ft (24 and 58 cm) below
4a, 5a, 7a and 7b (Fig. 2) were used in the second
only. A vertical series of anchors
was also established
at Station
season
(1963-64)
7.
METHODS
The dynamometers wer-e visited and re-set whenever conditions were suitable. Useful readings could not be obtained under calm conditions, while very rough weather made it dangerous, and sometimes impossible, to obtain readings at the more exposed stations. Meters were sometimes damaged and occasionally lost, particularly in the first season before the anchoring system was perfected. The simplicity and cheapness of the instrument were thus of some importance. The biological data were collected in two ways: (1) quantitative estimates of the flora and fauna were made along 0.5 m belt transects across the entire littoral region at selected dynamometer stations, and (2) measurements of the upper and lower limits of vertical distribution of the more important species were made at all stations. These limits were determined by means of a staff and level, using the meter anchors as the datum point and then adjusting the levels t.o an arbitrary datum 8 ft (2.44 m) below MSL, this being approximately the level of extreme LWST. RESULTS PHYSICAL DATA
Table
I gives the readings
provements of readings terpolated
obtained
in the second
season
of observations.
lm-
in the method of anchoring the meters resulted in a more complete set than in the first season but some gaps still occur. Values have been infor these as indicated,
such values being calculated
by plotting
the read-
ings obtained at any station against those obtained on corresponding dates at the next station. These plots gave good approximations to straight lines. If a reading was missing at a particular station it could be obtained by noting the contemporary values at each neighbouring station in turn, reading the corresponding values from the graphs and taking the mean of the two values obtained. Before accepting these results as a valid measure of ‘exposure’ it was necessary to establish that they had a genuine relationship to the intensity of wave action and that the variations obtained were not merely due to chance. The evidence for the reliability of the results is as follows. First, the highest readings were, in general, obtained where subjective judgement indicated that wave action would be most intense. Secondly, when the readings for each date are plotted against the distance
THE
MEASUREMENT
OF WAVE
ACTION
53
TABLE I
Dynamometer
readings
obtained
Stations
1
la
2
32
Oct. 22nd Dec. 2nd Dec. 13th Jan. 9th Jan. 10th Jan. 16th Jan. 28th Jan. 29th Jan. 30th Feb. 7th Mar. 12th Mar. 16th Mar. 26th Apr. 6th Apr. 22nd Mean Maximum
3 4 2 5l 1 1 5 2 3 6 6 3 4 111 5 3.4 6
6 8 42 10 3 3 8 3.5 5
6.5 8
18 15.5 6 16 6 5 13 6 11 22 18 10 13 4 17 12 22
9 5 8
8 2.5 2 6 3 4 8 7 8 8
8 6.1 10
8 5.5 8
10
5 -~ 16 16 6 21.5 7 11 17 7 6 22.5 15 17 16 8 18 13.6 22.5
during
October
1963 - April
1964.
5a
6
7
7a
lb
8
17 15 6 22 7 6 14 6 10 18 17 17.5 17 5 18 13 22
9 14 5.5 19.5 5 3 14 4 11 15 24 16 17 2 17 11.7 24
19 20 8 28 8 6 20 6 18 22 25 20 24 4 23 16.7 28
10 8 3 11.5 3 2 8 3 6 9 9 8 10 3 11 6.9 11.5
14 9 5 18 3 8 12 5 10 11 14.5 12 15 6 18.5 10.7 18.5
18 22 13.5 29 5 12 18 6 10 15.5 22 17 19.5 5 30 15.8 30
’ Figures in italics are interpolated (see text). 2 Station 4 was not used in this series since the section by wave action at the end of the previous season.
of rock
on which
8a -~ IY 18 11 28 7 6 21 9 22 18 28 20 20 8 33 17.9 33
9 ~~~_ 16 16 8 25.5 5 6 18.5 12 14 16 24 18 20 8 30 15.8 30
it was set was removed
of the corresponding stations from the most sheltered end of the bay, the pattern of distribution of wave action along the shore thus produced is strikingly similar in each case, although the actual magnitudes differ according to weather and sea conditions on the different days. This is illustrated in Fig. 2 where the exposure pattern is plotted for both the mean and maximum readings in relation to a map of the station positions. The inference is drawn that the variation in the readings is not a chance effect but that the values obtained are related to the ‘exposure’ of the stations. Thirdly, the readings were compared with the theoretical forces which might be expected from the waves producing them. This comparison with wave dimensions could not be made directly since no wave recorder was available; however, from meteorological data provided by the RAF station at Valley, a coastal station approximately 3 miles (5 km) north of Porth Trecastell, it was possible to calculate the probable maximum dimensions of waves to be expected. The data required were wind speed, duration and direction and these were related to the appropriate fetch as indicated in Fig. 3. Darbyshire (1957) has concluded that an increase in fetch above 200 miles has no further effect in increasing wave dimensions, and a fetch greater than this has been assumed to apply over sector b. It has also been assumed that the wind data supplied by Valley apply over an area large enough to include the 200 mile fetch; this is supported by an examination of synoptic charts of the North Atlantic though exceptions no doubt occur. Using these data, wave heights and periods were obtained from the graphs of Darbyshire and Draper (1963)
54
W. EIFION
which include
JONES
Longuet-Higgins’
AND
ANDREAS
DEMETROPOULOS
wave height factor by which an increment
depending
on wind duration is added to the predicted wave height to give the maximum likely to occur under the prevailing conditions. The results of these calculations are given in Table II. The maximum
dynamic
crest can be calculated
pressure,
P, produced
from Gaillard’s
by the moving
water at the wave
(1904) equation:
v being the orbital velocity of the particles in the wave, c the velocity of the wave front, p the water density, and k an empirical coethcient which depends on the suddenness of the applied load and cannot exceed 2. In his experiments, Gaillard obtained values of k = 1.2. In terms of wave height (H) and period (T) this formula may be written:
If P is expressed in ib./sq. in, then, since the drogue discs are 2 sq. in in area, the meter reading may be expected to approximate to 2P, neglecting the friction of the water on the line and providing a correct value of k is chosen. In Table II the values for 2P, predicted by Gaillard’s formula are shown, taking k = 1. In Fig. 4 these are plotted
against the corresponding
wave heights and on the
TABLE II
Calculated
wave heights,
Wind speed (knots)
Oct. Dec. Dec. Jan. Jan. Jan. Jan. Jan. Jan. Feb. Mar. Mar. Mar. Apr. Apr.
22nd 2nd 13th 9th 10th 16th 28th 29th 30th 7th 12th 16th 26th 6th 22nd
16 20 16 29 10 26 14 24 18 22 18 28 8 25
periods
Direction (see Fig. 3)
b C
; b Swell only a a : C C
a :
and equivalent
dynamometer
readings.
Maximum 1 wave height
Wave period
(ft)
(set)
24 9 6 18 6
10.08 6.85 4.26 25.56 3.33 -
6 5 4 8 3.5 -
18.0 7.4 7.8 36.0 6.0 -
18 15 15 24 18 24 12 24 24
18.46 8.40 16.80 12.96 9.94 6.48 20.70 2.16 21.60
7 5 7 6.3 5 4 7.5 3 7.8
25.6 13.0 24.2 20.0 11.4 10.2 30.2 5.2 32.8
Duration (h)
Calculated meter readings
i!
1 Wave dimensions calculated on the basis of meteorological data and the graphs of Darbyshire and Draper (1963). 2 Calculated dynamometer readings (see text) are twice the value given by Gaillard’s (1904) formula, taking k = 1.
THE
MEASUREMENT
OF WAVE
ACTION
55
C I’ :
/ ,*’
3@ 0
4 I .-? z E 20. ;
l
/’
i0 E I h 10
0
,’ 0 ’
//
0
,’
0
/’
/’
9‘
,,‘O
Fig. 4. readings height period due to
,’
P , *’ /’
0
I r’
.o
0
,,’ 0
’
0 0
0
I ,CJ,
10
0 0
/
0
4
,,,,,&
2
Wave 4
’
fekt 2b
height - metres
6
’
’ 8
,
Relationship between calculated wave height and dynamometer reading: - 0 - dynamometer recorded at Station 8a (the most exposed) plotted against the calculated maximum wave on the same date; - - 0 - - theoretical dynamometer readings based on wave height and on the same dates and using Gaillard’s (1904) formula, taking k = 1; scatter is presumably swell which could not be assessed and to the fact that the graph relates force to wave height only.
Wave 2
height 4
feet - metres
20 6
8
Fig. 5. Relationship between wave height and dynamometer readings at all stations: curves obtained by plotting data from Table I as in Fig. 4; numbering indicates the stations: T is the theoretical curve as in Fig. 4.
56
W. EIFION
JONES
AND
same diagram
the meter
readings
ANDREAS
recorded
DEMETROPOULOS
at Station
8a (the most
exposed)
are
similarly plotted. It will be seen that there is a good correspondence. For the smaller waves a value of k = 1.3 gives a close fit to the experimental curve, while for the higher
values
k is less than
when the calculated
1. This is as might
be expected;
the readings
waves were small are those most likely to be increased
obtained by swell,
which is ignored by the wave prediction formulae. At the other end of the scale the larger waves are more affected by shoaling water than the smaller; they break sooner and are less likely to exert their full force at the meter station. This is emphasized in Fig. 5 where the curves for all stations are drawn together with the theoretical curve, as in Fig. 4. It will be seen that whilst the curves for all stations approach the theoretical curve when waves are small, they rise less and less closely to it as the wave size increases. The closer the station is to the inner end of the bay, the greater is this divergence. This small increase in wave impact which results from a great increase in the size of waves is what is implied in the term ‘shelter’. Station 3, well inside the bay, shows a closer approach to the theoretical curve; this may be explained by its being situated on a rock face steeper than at other stations and protruding beyond the general line of the shore. It may be concluded that the good agreement between theoretical and observed readings shows that this form of dynamometer is a reliable indicator of wave force. READINGS
AT DIFFERENT
LEVELS
In order to investigate how wave impact depended on the level of the shore at which the meter was set, a vertical series was established at Station 7 where the rock slope was uniform (about 30” to the horizontal) for much of the littoral. The meters were at vertical intervals of 2 ft (0.61 m) above and below the datum meter, which is taken as 1 ft below MSL. It was found too dangerous to service the meters below TABLE III Dynamometer Approx. relative Jan. Jan. Feb. Feb. Feb. Feb. Mar. Apr. June
height to MSL
5th 25th 7th 8th 18th 26th 25th 5th 1st
Mean Maximum 1 Figures
in italics
readings
obtained
-1 -0.3
from
a vertical
jl to.3
series at Station +3 to.9
7 in the I963 season. -1~5 -1.5
-.-7 ft -;2.1 m
15 4 16 II 1 14 16 28 22 2
15 6 10 9 16 14 28 24 5
10 6 12 10 10 8 20.5 17 5
13 2 10 13 10 12 23 22 4
10 4 12 12 12 13 23 28 5
14.5 28
13.9 28
11.2 20.5
12.1 23
13.2 28
are interpolated
(see text).
THE MEASUREMENT
this level on days when
useful
readings
OF WAVE ACTION
could
be obtained.
57
The results
from
the
meters at levels to a little above MHWST are shown in Table III. They suggest that, at least to this level, there is no significant fall in the maximum wave force. Since the maximum dynamic pressure is to be expected near the crest of the wave where the forward orbital velocity is greatest, this result is not surprising. By the same reasoning
the maximum
wave impact
should
decrease
towards
the lower lit-
toral since the crests of the largest, most energetic waves cannot break at that level at any state of the tide. This is supported by the increase in numbers of species and density of growth in the lower littoral and sublittoral fringe. It is not suggested that these results are conclusive; further readings at all levels are needed. CORRELATION OF THE DYNAMOMETER READINGS AND ECOLOGICAL PATTERN It is proposed
to comment mainly on the presence or absence of species and on
their vertical limits; it seems best to postpone consideration
of detailed quantitative
-MHWST
5
.MLWNT
c34. I”
Fig. 6. Distribution of some plant species related to maximum dynamometer readings. Vertical limits of distribution of plants at each station are plotted at the maximum dynamometer reading obtained at that station: upper limits of Fucus uesiculosus probably lowered at the sheltered stations by local steepening of the rock at that level; occasional individuals occur outside the limits shown which relate to the main bulk of the populations. f upper limit; .$ lower limit. Porp’, epilithic upper Porphyra umbilicalis zone; Porp”, lower epiphytic zone; Lit, Lichina; F. ves, Fucus vesiculosus; F. ser, F. serratus; Gig, Gigartina stellata; L. sac., Laminaria saccharina; L. dig, L. digitata; Al, Maria. Calm water tidal levels are approximate (see text).
58
variations
W. EIFION
JONES
AND
ANDREAS
until more shores can be included.
limits of distribution be irregularities
of various
because
Table
IV shows the upper
species at each meter position.
of local variations
there are breaks in the rock platform true of Stations 4 (abandoned during there is a region
DEMETROPOULOS
in shelter
and lower
There must inevitably
and slope, particularly
where
between the lower and upper littoral. This is the second season) and 5, while at Station la
of steeper slope in the midlittoral.
Figs. 6 and 7 show these limits
plotted against the appropriate meter readings. The most obvious anomalies occur at a meter reading of 24 lb. and may be related to the broken nature of the shore at Station 5. In order to present a more generalized picture, the curves have been smoothed (Fig. 8) and this diagram, it is suggested, can be taken as an indication of TABLE IV
Upper and lower limits of some All levels are given in feet above
Stations
1
Pelvetia Porphyra (epilithic) Porphyra (epiphytic) Lichina FUCUS vesiculosus F. serratus
plants and animals at dynamometer an arbitrary datum approximating limit; 1, lower limit.
la
2
3
14.5 12.5
r
12.0 7.5
5
5a
6
7
7a
7b
8
8a
9
15.2 12.3 _ -
15.1 12.3 _ _
15.2 12.6 _ -
14.4 12.8 _ -
13.5 12.5 _ -
14.2 12.8 -
14.0 13.7
16.0 13.5
15.3 13.0
11.0 7.3 12.7 11.6
11.2 7.9 13.7 12.3
12.0 7.4 14.8 12.1
10.0 6.7 13.6 11.9
11.3 7.4 14.3 11.9
12.4 6.5 14.5 12.8
12.8 7.0 16.2 12.7
12.3 5.8 -
10.2 6.9 2.4
10.5 3.9 2.1
10.9 4.9 1.6
2.3 9.0 1.9
9.7 3.5 1.9
11.4 3.9 -
11.5 3.7 -
11.3 3.6 --
LI 14.6 1 13.2 u-z_ ]_
-
14.8 12.3 _ -
I” 17.9 LI 12.3 1 10.8
7.1 9.1 14.0 11.8
6.4 9.0 13.2 10.2
12.0
10.5 7.5 13.6 11.1
8.5 5.0 5.7
10.6 6.0 1.2
10.3 6.5 2.2
: u
4.2 7.7 3.9
4.6 8.8 5.3
r
2.5 5.1 2.5-
2.1 6.5
_ -
stations in Porth Trecastell. to extreme LWMST; u, upper
Gigartina Laminaria saccharina L. digitata
; ” 1 l”z
Alaria Littorina neritoides L. saxatilis Chthamalus
3
2.2 5.9 _ -----__-----
1.9
I”- u 19.8 1 4.0 u 10.8 1 10.1 Ll 10.2 1 2.9
2.1
6.4 1.2 _
2.2
2.8 6.1 _
1.6
7.4 1.9 _
2.0
5.1 1.6 _
1.9
6.6 1.2 _
1.6
0.3 6.1 _
1.7
0.9 5.4 _
1.3
0.7 6.5 _
1.9
0.7
0.7
2.2
2.3
2.7
34.9 12.8 34.9 3.8 14.3 9.1 12.4 2.4
12.7
-
-
-_
_. -
-_
-_
-_
-_
25.5 15.5 25.5 3.3 11.3 7.0 9.5 3.1
25.5 10.0 25.5 3.5 10.3
12.0
24.5 11.0 24.5 3.9 12.0
28.9 12.0 28.9 4.0 11.9
28.9 12.0 28.9 3.0 12.8
26.9 13.6 26.9 3.2 12.4
28.0 13.4 24.0 2.9 13.7
12.8
10.7 3.1
11.3 3.2
12.0 2.4
33.9 13.7 33.9 3.7 13.8 10.7 12.0 3.0
11.4 3.1
12.0 2.9
12.9 2.5
9.1 3.4
7.3 1.0 _
0.5
-
3.4 12.2 7.5 11.5 3.0
0.8 8.0 _
2.5 14.1
1 Blanks indicate that no reading was made at that station, usually because the points lay too far from the meter to have a similar exposure to it or were below LWM. ’ Absence is indicated by a hyphen. 3 The lower limit of Chthamalus was determined only where shown.
3.7 14.7 12.3 2.3
concerned
THE
MEASUREMENT
the way that the distribution erately exposed from the results
of shore
OF WAVE
organisms
59
ACTION
is related to wave action
on mod-
shores in this region of the west coast of Britain. It is apparent that the generally accepted views of the effects of wave action on
rocky shore organisms
are confirmed.
Some comments
,....’
_..,.:* * ** ,~.’
&S
’
,
. ..
.
c
--__,’ t Lsax
/ :
.....“.
,+
...i :t
j’
...“.
-. -._&ner
are appropriate.
1’
i$ Ii
:
AHWST
:.,
::’ .,
.
WSL Bal
VlLWST 20 3’0 10 MAXIMUM DYNAMOMETER READING - LBS. Fig. 7. Distribution of some animal species related to dynamometer readings. distribution of animals at each station are plotted at the maximum dynamometer at that station. f upper limit; 4 lower limit. L. ner, Littorina neritoides; L. sax, Chthamalus stellatus; Bal, Balanus balanoides; lower limit of Chthamalus recorded only. Calm water tidal levels are approximate (see text).
Vertical limits of readings obtained L. sczxatilis; Ch, at given stations
There is an elevation of the littoral with increasing wave action. This is particularly noticeable in the case of upper shore organisms. An increase of 25 lb. (11.4 kg) in the meter readings is related to a rise of 8 ft (2.44) m in the upper limits of Littorina neritoides (L.) and Porphyra umbilicalis (L.) J. Ag., a 4 ft (1.22 m) elevation of the upper limit of Fucus vesiculosus L., and 3 ft (0.92 m) for Gigartina steflata (Stack.) Batt. At the lower extremity of the littoral this is less marked; Laminaria digit&a (Huds.) Lamour has a more or less constant upper limit until it falls with the appearance of AZaria esculenta (L.) Grev. Some species are absent. Fucus spiralis L. and Ascophyllum nodosum (L.) Le Jol. were not recorded on any transect line though both occurred in local shelter in various
60
places.
W. EIFION
Both are known
to note how limited
JONES
AND
ANDREAS
to have limited
DEMETROPOULOS
tolerance
is their range in terms
of exposure;
of the dynamometer
canaliculata (L.) Dcne. et Thuret has a much greater either of the former species and this is clearly shown be emphasized
that this species is present
it is only surprising readings.
Pelvetia
tolerance of wave action than by the results, though it must
only as scattered
plants in its zone.
-4
MLWNT
MLWST I..
5
15 DYNAMOMETER
Fig. 8. Distribution of on data in Table IV but umbilicalis; PORPH”, stellarus; BAL.,
0
20
25
30
READING
littoral plants and animals related to dynamometer readings. Diagram based smoothing out local irregularities: PORPH’, epilithic upper zone of Porphyra lower epiphytic zone; L. SACC, Laminaria saccharina; CH., Chthamalus Balanus balanoides. Calm water tidal levels are approximate (see text).
Himanthalia elongata (L.) Gray was absent at Porth Trecastell but it is unlikely that this can be attributed directly to wave action. The species occurs at Trearddur Bay, 8 miles (13 km) northeast of Porth Trecastell, in conditions similar, in terms of flora and fauna, to transects with meter readings in the range 20-30 at Porth Trecastell. But Trearddur Bay has deeper water close inshore and conditions at Porth Trecastell are generally more turbid.
THE MEASUREMENT
The change in the zonation Fucus serratus
OF WAVE ACTION
61
levels is not directly related to change in meter readings.
L., for instance,
shows an abrupt
fall in its upper limit between
meter
readings of 10 lb. and 12-14 lb., then continues as an essentially sublittoral species into much more exposed conditions. This may be due to increasing activity of limpets
but observation
of seaweed
colonization
of a 2 m strip cleared
of limpets
between Stations 6 and 7 does not confirm this. The apparent levelling off of Lichina spp. between meter readings 12 lb. and 24 lb. seems to be at least partly the result of local irregularities. In order to prevent the diagram becoming overcrowded, some species are omitted. The most important of the perennial species are Laurenciapinnatzjida (Huds.) Lamour, which is present with Gigartina and has a higher upper limit on the less exposed shores; Rhodymenia palmata (L.) Grev., a common epiphyte at the lower levels: and encrusting corallines, which are present at the lower levels, particularly on the more exposed sites. Some plants and animals deserve special mention. Porphyra umbilicalis has a zone of mainly epiphytic plants overlapping the upper level of Fucus uesiculosus and also an epilithic form which occurs only at the more exposed stations and at a higher level. This dual distribution is widespread (Conway, personal communication) but not easy to explain. Putella aspera (Lamarck) increased in numbers with Increasing exposure, from 16/m2 at Station la to 80/m2 at Station 8a. At the sheltered end of the shore P. vulgata L. and P. intermedia Jeffreys (which were not reliably distinguished in the field) vastly outnumbered P. aspera, which occurred only below MLWNT. At Station 3 (meter reading 24 lb.) P. uspera was present to MSL and at 8a (36 lb.) almost up to MHWNT. Below a level of 6 ft above datum P. aspera was the dominant limpet at this station. The density of both Chthamalus stellatus (Poli) and Balanus balunoides (L.) increased with increasing exposure. The upper limit of Chthamalus rose 4.1 ft between Stations 1 and 9, that of Bulanus by 2.13 ft. The two species overlap considerably on this shore and the lower limit of Chthamafus and upper limit of Balunus were hard to determine.
The overlap
was noticeably
smaller
towards
the exposed
station.
An upper limit for Verrucaria maura Wahlenb. could not be determined as this lichen met the cliff-top vegetation above most stations. Lichina patches (L. pygmaea Ag. with some L. confines Ag.) were conspicuous at all stations, except Station9 at the seaward end of the headland. Small mussels, Mytilus edulis L., occurred in patches at various stations, from up to 1 ft above MSL at Station la to 4 ft above MSL at Station 8a. The population density and average size of individuals also increased towards the exposed stations. Littorina saxatilis (Olivi) in the larger forms subsp. tenebrosa (Montagu) and subsp. jugosa (Montagu) were similar in their distribution to L. neritoides. The smaller form subsp. saxatilis (Johnston) usually occurs in the empty shells of Balanus and was found down to the lower limit of that barnacle on the shore. The wide Littorina saxatilis zone in the diagram includes all forms. Subsp. rudis (Maton)
62
W. EIFION JONES AND ANDREAS
DE~ETROPOULOS
occurred in Iocal shelter but was very rare (two specimens at Station 1) on the transect lines. CONCLUSIONS
This work has demonstrated that an investigation of wave forces as an ecological parameter is worthwhile; a simple instrument appears capable of producing meaningful results which can add something to our knowledge of the factors affecting distribution on the shore, Now that our knowledge of the distribution patterns is becoming so detailed (Lewis, 1964) it is necessary to evaluate these factors and this
EPlLlTHlC PORPHYRA HWST
MSL
DYNAMOMETER
READING
Fig. 9. Distribution of littoral plants in relation to exposure. Diagram indicating distribution trends in relation to exposure as represented by an extrapolation from dynamometer readings at Porth Trecastell and other records (see text): F. spp., Fucus spiralis; f. wsiculosus and f. linearis the bIaddered and bladderless form of &kcrrs r;esiculosccv respectively; Asc, AscophyG~~ nodosu~n; L. sacc, Laminnricz saecharinn.
can best be done, as has been attempted here, by investigating them one at a time in places where the other factors, as far as can be judged, vary as little as possible. Work on waves force obviously needs to be extended to a wider range of shores than has so far been attempted and it is hoped that the adoption by other workers of this or a similar instrument may lead to an accumulation of useful quantitative information. As an indication of the kind of picture which is emerging we suggest, in a speculative extrapolation, that the zonation pattern on the full range of shores of western Britain, including the most exposed (Powell & Chamberlain 1956; Lewis, 1964), may be as in Fig. 9. Wave forces in this were calculated largely using data from Bigelow & Edmondson (1947) supplemented by that of Burrows et al. (1954) and Jones (1959).
THE MEASUREMENT OF WAVE ACTION
63
ACKNOWLEDGEMENTS We wish to acknowledge
the help of many
members
of the staff of the Physical
Oceanography and Marine Biology Departments at Menai Bridge and particularly Professor J. Darbyshire for his advice on many aspects of waves, Mr. J. G. Harvey for considerable sions on breaking
help in meteorological
matters,
Mr. J. Simpson
waves, the staff of our workshop
our boat crew for their assistance
for valuable
for their constant
discus-
helpfulness,
and
on the shore. REFERENCES
R. A., 1939. Interim report on wave pressure research. J. Civil Engrs, Lond., Vol. 7, pp. 202-226. BALLANTINE, W. J., 1961. A biologically defined exposure scale for the comparative description of rocky shores. Fld Stud., Vol. 1, pp. 1-19. BIGELOW, H. B. & W. T. EDMONDSON, 1947. Wind waces at sea, breakers and surf. Hydrographic Office U. S. Navy Pub. No. 602, 177 pp., Washington. BURROWS, E. M., E. CONWAY,S. M. LODGE & H. T. POWELL, 1954. The raising of intertidal algal zones on Fair Isle. J. Ecol., Vol. 42, pp. 283-288. DARBYSHIRE, J., 1957. A note on the comparisons of proposed wave spectrum formulae. Dt. hydrogr. BAGNOLD,
Z., Bd 10, S. 184-190. DARBYSHIRE, M. & L. DRAPER, 1963. Forecasting
wind-generated sea waves. Engineering, Lond., Vol. 195, pp. 482484. DEFANT, A., 1961. Physical oceanography, Vol. 2. Pergamon Press, London, 598 pp. FRANZIUS, L. & E. SONNE, 1901. Wasserbau am Meere und in Strommiindungen. Handbuch der lngenieurwissenschaften, Leipzig, Bd 3, Heft 3, 751 S. GAILLARD,D., 1904. Wave action in relation to engineering structures. Prof. Pap. Corps Eng., U.S. Army, No. 31, 194 pp. JONES,W. E., 1959. The effects of exposure on the zonation of algae on Bardsey Island. Rep. Eardsey Bird Fld Obs., Vol. 7, pp. 41-46. LEWIS, J. R., 1964. The ecology of rocky shores. The English Universities Press, London, 323 pp. MOORE, H. B., 1935. The biology of Balanus balanoides. IV. Relation to environmental factors. J. mar. biol. Ass. U.K., Vol. 20, pp. 279-307. POWELL,H. T. & Y. M. CHAMBERLAIN, 1956. Plant life on Rockall. In, Rockall, edited by J. Fisher, Geoffrey Bles, London, 200 pp. ROUVILLE, M. A. DE, P. BESSON& P. PETRY, 1938. Etat actuel des etudes internationales sur les efforts dus aux lames. Annls Ponts Chauss., T. 108, pp. 5-113. SOUTHWARD,A. J., 1953. The ecology of some rocky shores in the south of the Isle of Man. Proc. Trans. Lpool biol. Sot., Vol. 59, pp. l-50. STEVENSON, T., 1849. Account of experiments upon the force of the waves of the Atlantic and German Oceans. Trans. R. Sot. Edinb., Vol. 16, pp. 23-32. WIEGEL, R. L., 1964. Oceanographical engineering. Prentice Hall, New York, 532 pp.