Biological Journal
ofthe
Linnean Society (1994), 51: 123-147. With 26 figures
Intertidal zonation of animals and plants on
rocky shores in the Bristol Channel and Severn
Estuary-the northern shores
C. METTAM
School of Pure and Applied Biology, University of Wales College of Cardzf, PO Box
915, Cardif CFI I T L
The vertical distribution of common species of macroalgae and fauna on rocky shores extending
through the length ofthe estuary is described. In the west, the shores are fully marine and exposed to
Atlantic waves. In an eastward direction, greater shelter and an increasing tidal range occur along a
gradient of salinity and turbidity. The flora and fauna change along this gradient. No single
biological feature signals a transition from the Bristol Channel to the Severn Estuary but four
arbitrary regions are recognized: a ‘marine’ section eastwards to Swansea, a ‘transitional’ section
between Swansea and Cardiff, an ‘estuarine’ section u p to Newnham, and the tidal river Severn.
ADDITIONAL KEY WORDS:-South
Wales - zonation
-
estuarine gradient.
CONTENTS
Introduction . . . . . . . . . . . .
Materials and methods
. . . . . . . . .
Results . . . . . . . . . . . . . .
The changing intertidal environment . . . . .
The changing biota . . . . . . . . .
Discussion
. . . . . . . . . . . .
Changes along an environmental gradient . . . .
Changes with time . . . . . . . . .
Changes following a tidal power barrage . . . .
Acknowledgements . . . . . . . . . .
References
. . . . . . . . . . . .
Appendix,
. . . . . . . . . . . .
.
.
.
.
.
.
. .
. .
. . . . .
. . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. .
. .
. .
. .
. .
. .
. .
.
.
.
.
.
.
.
.
.
. .
. .
. .
. .
. .
. .
. .
123
124
125
125
130
142
142
144
144
145
145
146
INTRODUCTION
This is an account of the northern coastline of the Bristol Channel and Severn
Estuary, complementing descriptions of the southern coast (Crothers, 1976;
Little & Smith, 1980; Smith & Little, 1980). It is based on surveys carried out
between 1972 and 1974, supplemented by more recent observations. The aim is
to provide, in a broad sweep, a view of the change in common shore species
along a gradient of changing conditions. North and south coasts differ in several
important ways. For example, where cliffs back the shore, the beach platform of
the southern coast is shaded but the northern coast is exposed to the sun. The
western approaches are both exposed to waves rolling in from the Atlantic but,
0024-4066/94/001123
+ 25 S08.00jO
123
0 1994 The Linnean Society of London
124
C. METTAM
in a region of prevailing southwest winds, the northern coast receives the greater
impact. Water circulation, controlled by tidal action, wind drift (Pingree &
Griffiths, 1980) and the Coriolis effect, also brings different conditions to north
and south shores. Low salinity water tends to escape seawards along the northern
shore, particularly at times of high discharge from rivers, when a measurable
reduction in salinity occurs along the entire length of the northern Bristol
Channel coastline (Stevens, 1986). This does not happen on the southern side
where, instead, comparatively clear, saline water from the Atlantic is drawn
eastwards towards Porlock Bay and there is a relatively abrupt transition
between the marine and estuarine waters. Isohalines tend to lie diagonally across
the Channel, compressed together on the southern shore and stretched out
westwards along the northern shore. A similar pattern is seen in isotherms,
isophotes (from Secchi disc extinction depths) and isopleths of chlorophyll a
(Williams & Collins, 1985). High levels of nutrients in the seaward drift of
estuarine water are consumed as the lessening turbidity permits phytoplankton
growth (Abdullah et al., 1973).
As the Bristol Channel narrows into the Severn Estuary, its shores have an
increasing tidal range and increasingly strong currents, salinities are increasingly
variable and the tidal curve becomes increasingly asymmetrical. The northern
coast of the Bristol Channel and Severn Estuary thus presents a gradient of
change from saline waters off the west coast of Wales (Hughes, 1966) to the fresh
waters of the River Severn. Other rivers entering Estuary and Channel at
intervals along the coast do not obscure this gradient.
While there are detailed, descriptive accounts of the flora and fauna of rocky
shores in west Wales (Ballantine, 1961; Moyse & Nelson-Smith, 1963;
Nelson-Smith, 1967) their changing distributions eastwards along the coast have
not previously been described.
MATERIALS AND METHODS
Animals and plants were recorded from transects from west Wales to
Gloucester (Figure 1, Appendix 1) referred to in the text by a number (1-25) in
square brackets. Transects were more closely spaced where biological changes
were greatest and avoided locally barren areas. Aerial photographs were useful
in selecting some transect sites. The coast between Swansea Bay and Portskewett
was surveyed photographically from a Cessna light aircraft at low tide, with
normal Kodachrome and false-colour, infra-red sensitive Ektachrome film in a
hand-held, 35 mm Pentax camera.
Transect lines were established from high water to low water of spring tides,
with 0.5 metre vertical intervals, mostly by using a simple quadrat-levelling
device (Mettam, 1983), although a surveying level was used on transects [17]
and [21]. Heights were related to Chart Datum as revised in the 1974 Tide
Tables (Admiralty Hydrographic Department, 1973).
Species were recorded using an abundance scale, which tends to override
small-scale patchyness and enables a rapid comparison to be made of different
shores over a large area (Baker & Crothers, 1987). A five rank scale was used;
(Appendix 2), recording from seaward-facing surfaces and excluding rock pools
and crevices.
ROCKY SHORE ZONATION
0
kilometres
125
50
A
B$
('4$25
Figure I . Bristol Channel and Severn Estuary. Locations of 25 transect sites on the northern shore
(listed in Appendix 1) are shown by lines perpendicular to the coastal outline. Location of sites A-E,
see Table 1) for collection of water samples shown by arrows. The line of the proposed barrage is
shown by a pecked line.
Although the general environmental features of the Severn Estuary have been
known since the pioneering work of Bassindale (1943a, b), tidally changing
conditions on the shores have not. Salinity, water temperature, air temperature
and turbidity were therefore recorded a t five stations (Figure I , Table 1) during
spring and neap tides, on dates representing winter minimum and summer
maximum of salinity. The techniques used have been described by Little &
Smith (1980).
RESULTS
The changing intertidal environment
The main physical changes along the estuarine gradient are outlined below.
Tidal levels
The mean high and low tidal levels for spring and neap tides (MHWS,
MHWN, MLWN, MLWS), from the Admiralty Tide Tables, in Figs 5-26
'TABLE
1. Locations in the estuary where water samples
were taken
Northern coast
A Newnham
B Lydney
C Beachley
D Goldcliff
E Lavernock
Nearest transect
~ 3 1
[21-221
~ 9 1
~ 7 1
~ 5 1
Southern coast
.~
Sharpness
Aust
Portishead
Sand Bay/Weston
C . METTAM
I26
depict the tidal range increasing in the narrowing estuary except where the
rising bed of the estuary progressively excludes the lower tidal zones. Tidal range
and absolute low tide level are most extreme near [19]. Further upstream, for
some distance, neap tides have a low water below that of the springs. Low water
level upstream of [20] is influenced by river discharges and the uppermost two
transects lie in the realm of the tidal river, only reached by spring tides.
Salinity, temperature and suspended solids
Tidal curves for two positions in the Estuary (Avonmouth and Sharpness) for
each sampling occasion are shown in Fig. 2. High salinities followed the tidal
incursion (compare Fig. 3 with Fig. 2), but highest salinities lagged some time
after the highest tide in the upper estuary, suggesting a strong axial flood of
saline water which turned shorewards on the ebb. This lateral stratification may
account for differences on opposite sides of the estuary, especially as the ebbing
water mass is deflected in different directions by bottom topography (Mettam,
1979a). Figure 3 shows salinity changes during neap tides only. O n the spring
tides the main difference was a further incursion of saline water into the upper
reaches. The spring tide salinity in winter was more than 5%0 for most of the
tidal cycle at Lydney and briefly reached 3%0 a t Newnham, but in summer the
A
6
I-
4
2
0
-2
2
-4
c,
D
6
4
2
0
-2
-4
-6
Figure 2. 'l'idal curves for (A) neap tide, (S)spring tide in winter; (C) neap tide and ( D ) spring tide
in summer on the days when water samples were taken. A = Avonmouth; S = Sharpness. Dots show
measured tidal heights at Sharpness Dock. Dates were: (A) 2.3.1977; (B) 7.2.1977; (C) 9.8.1977;
( D ) 2.8.1977.
C . METTAM
128
maximum salinity at Newnham exceeded 21%0while Lydney was fairly stable at
between 22 and 24%0 (Fig. 4). O n all occasions the greatest change in salinity
during a tidal cycle at any position occurred when the maximum salinity was
about 20%0.
T h e tidal incursion brought in relatively warmer water in winter and cooler
water in summer. Overall, differences were slight but the northern coast, where
intertidal flats of the Welsh Grounds may have warmed the overlying water in
summer, was sometimes a degree or more warmer than the southern coast
(Fig. 4) particularly at Goldcliff [17], at the seaward end of extensive intertidal
flats, on a summer's day when air temperatures stayed around 26°C for several
hours.
The detailed distribution of suspended sediments can be seen from aerial
photographs (Collins, 1983; Mettam, 1979a). A typical range of values might be
from < 0.01 gl-1 in the Bristol Channel to 1 .O gl-I in the Severn Estuary
(Collins, 1983). During the tidal cycle much greater peak values occurred:
suspended solids frequently exceeded 1 .O gl-I; occasional much higher values,
> 6.0 gl-' (Fig. 4), may have been due to resuspension of sediment from the
surface of intertidal flats.
A
s
30.
h
.-*
c
.rd
m
A
25-
20-
15-
10.
1'2
24'
Hours
Figure 4. Examples of changing salinity (A), temperature (B) and suspended solids (C) over a tidal
cycle of spring tide in summer at five levels in the estuary ('Table 1).
1
241
I*
221
lk
I
I
-&.
\.
Figure 4.
a'
Hours
.+
0
20 Lavcrnock
I
.-..-.-
--.-.=-.-.-.
-
24'
Hours
Wcslon
I
130
C. METTAM
Wave exposure and scour
The western section of the Bristol Channel is exposed to the Atlantic, but wave
exposure varies according to aspect. The increasing tidal range enhances the
abrasive effect of sand on wave exposed rocks by increasing horizontal
movement of water, but reduces the time that waves affect any particular level of
the shore. The rather flat profile of some beach platforms may also reduce wave
impact but increase tidal scouring. These complex effects have not been
quantified.
Surfers are ‘biological indicators’ of exposure to waves and surfers know that
certain states of the tide regularly produce local intensifications of waves on
particular beaches. Surfing is possible throughout the year along the northern
coast eastwards to Llantwit Major and surf conditions are described by
Thompson (1972). At Rhossili, on the Gower peninsular, waves often reach 3 m.
Similar, or larger (5 m maximum), waves occur at Kenfig. Southerndown has a
regular surf of 1-2 m wave height whilst at Llantwit Major, a low surf, less than
1 m, occurs when conditions are right. One might therefore expect frequent
strong wave action on southwest facing shores as far eastwards as Sker Point [5]
but declining towards [9] with no regular surf beyond. After [ 151 the shores face
into the estuary and away from prevailing winds.
The changing biota
The terms used to describe the main zones (sublittoral, eulittoral, littoral
fringe) are those of Lewis (1964). T h e envelope of standard tidal levels (MHWS,
MHWN, MLWN, MLWS) has been adopted as a framework on which the
distributions of common species of flora and fauna are plotted as kite diagrams.
Transect sites are spaced along the horizontal axis such that two adjacent kites
indicating maximum abundance will touch. I n comparison with the actual
distance between sites (Fig. 1) the diagram is stretched out where transects lie
close together and compressed where they are widely separated.
Colonizers of Primary space: Macroalgae
False-colour aerial photographs showed discrete zones of algae on many
transects. Fucoids were present on most as far east as [22]. Cover was restricted
to the lower shore on some south-facing sites, e.g. [5], [6], [8] and [9]. I n the
greater shelter at [7], the middle part of the shore was bare of algae but fucoid
zones were well developed on both upper and lower levels. I n the shelter of the
Estuary, fucoids dominated rocky shores [18], [19]. The full range of larger
species occurred only in bays at [7], [ 101 and on the estuarine end of the
gradient where the larger species had broadly overlapping zones. Distribution
limits of algal species are described by Benson-Evans et a/. (1974) and Smith
(1980) and only a selection of larger species is described here.
Laminarians were limited to the western shores (Fig. 5). Alaria esculenta was
not found on the mainland transects. Mumbles Head was the eastward limit of
sublittoral fringe kelp forest (Laminaria digitata with occasional L. hyperborea) . A
few L. digitata at Porthcawl [6] were confined to rock pools.
131
ROCKY SHORE ZONATION
0
v1
-2-
z3 - 4 -
-61
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
I
I
I
I
I
I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 5. Relative abundance of laminarians (more than one species) at different tidal levels on 25
transect sites (Appendix 1).
The most widespread fucoid, Fucus serratus, occupied the lower shore of most
transects but showed a noticeable shift in its zonation from west to east (Fig. 6).
On western shores, it was found below MHWN, rising upshore in bays [4],[7],
[lo] to an upper limit mid-way between M T and MHWN that was maintained
on the estuarine transects. The upper limit was depressed on the exposed point at
[12]. In the absence of laminarians, its lower limit extended into the sublittoral
but,. from [12] eastwards, it was elevated towards MLWN. Elevation was
greatest at [19] and at [20], where there was a low, bare zone of rock.
Fucus vesiculosus occupied a zone between MLWN and MHWN but, on weedy
shores, it overlapped extensively with F. serratus. Fucus uesiculosus was absent on
many transects but present on others where Ascophyllum nodosum was absent. Its
zone was difficult to define, because of the patchy distribution, but did not
8t-
6-
-6
-
1
l
1
1
1
1
1
l
1
1
l
1
1
1
1
1
1
1
1
I
I
l
I
I
I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 6. Relative abundance of Fucus ssrratus at different tidal levels on 25 transect sites
(Appendix I ) .
132
C. METTAM
6-
-6
-
-
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 7. Relative abundance of Fucus vesiculosus at different tidal levels on 25 transect sites
(Appendix I ) ,
change systematically along the coast (Fig. 7). A seemingly exceptional elevation
at [ 111 may be a confusion of identity with F. spiralis.
Ascophyllum nodosum occupied a narrow zone around MHWN on shores where
it was sparse, but extended further upshore on the estuarine transects (Fig. 8)
where individual fronds were long and luxuriant, contrasting with the abraded,
stumpy growths to the west. On the estuarine transects, A . nodosum formed a
many-layered growth with high canopy cover. Strong growth occurred a t [ 181
where F. vesiculosus was restricted to a narrow zone and at [20] there was a zone
of pure Ascophyllum separating F. vesiculosus from F. spiralis. Ascophyllum also
showed an elevation of its lower limit in the turbid zone of the estuary.
Colonizers of primary space: soft-bodied animals
Reefs of Sabellaria alveolata featured on many transects (Fig. 9). Reef growth is
favoured by wave action and the presence of sand, which the worms use in the
construction of their tubes. Some reefs were poorly developed and eroded
remains under the algal canopy testified to formerly more extensive occupation
of some shores. At Southerndown and St Mary's Well, reefs were eroded like
miniature battlements. At Limpert Bay, much of the reef was down to a partly
colonized crust over the rock. Reefs at [14] and near transect [15] contained no
living worms. At Sker Point [5], Subellaria reef was uncommon and the nearly
vertical rock face of the lower shore there was occupied by a reef-like structure of
Pomatoceros lamarckii tubes (Fig. 10). At [6], Sabellaria was found mainly on the
crest of ridges and Pomatoceros on the vertical sides of gullies on the same
rocks.
ROCKY SHORE ZONATION
133
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 8. Relative abundance of Ascoplryllum nodosum at different tidal levels on 25 transect sites
(Appendix 1).
0
m
Lal
c
-2-
E -4-
-6
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
I
l
l
I
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 9. Relative abundance of Sabelfaria afueolala at different tidal levels on 25 transect sites
(Appendix I ) . Unfilled kites = uninhabited reef.
The absence of macroalgae on some estuarine shores exposed species which
were probably overlooked on other transects. T h e burrowing worm Polydora
ciliata was especially conspicuous towards low water at [12], where the bare
rocks were coated with fine particles accumulated by the worms around the
mouths of their burrows. T h e sublittoral fringe, where laminarians were absent,
was colonized by encrusting sponges; Halichondria sp. and Hymeniacidon perleve in
particular formed relatively massive encrustations, especially on exposed, south
facing shores between [6] and [9]. By transect [14], where encrusting sponges
had all but disappeared, upright growths of the branching sponge Haliclona
octoculata (also present on shores to the west) were conspicuous on otherwise bare
C. METTAM
134
r-
Ordnance
datum
0
rn
4
-2-
J- - - I
-4-
-
-6
MLWN
I
I
I
I
I
I
I
I
I
I
-
I
I
1
MLWS
-
I
I
l
l
I
I
1
1
1
I
I
I
rock. Rounded boulders on the south side of Sully Island [I41 were covered
sheets of the colonial tunicate Dendrodoa grossularia, dull brown in colour rather
than the normal orange. Other species conspicuous a t [I41 included the
hydroids Tubularia indivisa, especially on vertical faces, and Serlularia cupressifrmis
on flatter, wetter areas. The latter made exceptionally good growth a t [ 171 while
Tubularia made continuous cover a t [18], [I91 and [20], less a t [21]. Smaller
hydroids and bryozoa occurred in the sublittoral fringe from [16] onwards
(Boyden et al., 1977). Cordylophora caspia was the dominant growth a t [23] and
[24] but there were no hydroids a t [22].
The anemone Actinia equina (Fig. 11) was erratic in its vertical distribution but
showed a clear cut-off in its estuarine penetration a t [16]. Diadumene luciae
became common a t [ 171 and some upstream sites.
-
-6
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 11. Relative abundance o f Adinia equina at different tidal levels on 25 transect sites
(Appendix 1).
ROCKY SHORE ZONATION
135
Colonizers of primary space: Barnacles and mussels
In the sublittoral fringe, barnacles, especially Balanus crenatus (Fig. 12), also
dominated in the absence of laminarians but B. perforatus was only recorded on
marine shores (Fig. 13). Balanus crenatus and the estuarine B. improvisus were
found together at low water at [13] and [14] but B. improvisus increased its
vertical zone and general abundance in the more estuarine transects (Fig. 14),
although absent a t [2 11.
The survey pre-dates the separation of Chthamalus montagui from C. stellatus
(Southward, 1976) but it is likely that the mainland records are predominantly
C. montagui. In the west, Chthamalus (Fig. 15) extended up to 2 m or more above
MHWS although at exposed point [12] it did not reach as far as MHWS. Its
absence from the upper shore of exposed point [5] may be due to the flat profile
of that part of the transect. The lower limit of the Chthamalus zone extended
downwards from [3] to [8], reaching well below MLWN and overlapping
extensively with Semibalanus balanoides (Fig. 16). Farther east, Chthamalus was less
abundant, in a narrower vertical range and it did not extend as far along the
estuarine gradient as S. balanoides. No spatfall of C. montagui was seen at [15],
frequently inspected from 1972 to 1974.
The Bristol Channel shores were mostly barnacle-dominated in the eulittoral.
Semibalanus balanoides steadily decreased in abundance eastwards, but maintained
Ordnance
datum
0-
-)
ra - 2 -
MLWN
3
-4-
-
MLWS
--
-
-6I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
1
1
1
1
1
1
1
1
1
1
1
1
-
-6
1
1
1
1
1
1
1
1
1
1~
1 ~ 1
1
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 13. Relative abundance of Balanus perforatus at different tidal levels on 25 transect sites
(Appendix 1).
I36
C. METTAM
8
6
4
2
8
9
2
0
-2
-4
-6
l
l
l
'
l
l
l
l
l
l
l
l
l
l
l
1
1
1
1
1
1
1
1
1
1
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 14. Relative abundance of Balanus impro'ovisus at different tidal levels on 25 transect sites
(Appendix 1).
a continuous distribution across all but weed dominated shores [13], [14].
Elminius modestus, however, was the dominant barnacle of the transitional
estuarine transects [ 1 13-[ 151 (Fig. 17), overlapping the zones of both native
taxa. This barnacle was absent from the most westerly site and increased in
abundance eastwards, especially on exposed points [ 121 and [ 151. At [20] there
8
/
/MHWS
-
-6
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 15. Relative abundance of Chlhamalus spp. at different tidal levels on 25 transect sites
(Appendix I ) .
137
ROCKY SHORE ZONATION
8
-
-6
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
I
I
(
I
I
(
1
1
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 16. Relative abundance of Semibalanzu bafanoides at different tidal levels on 25 transect sites
(Appendix I ) .
was no evidence of recent successful settlement and all individuals were relatively
large.
Mussels (Mytilus edulis) had a patchy distribution along the coast (Fig. 18)
with isolated individuals on most shores visited but not always in the transects.
The upstream limit was near [I81 where individuals and small groups occurred
8
4
m
2
b
3
0
-2
-4
-
-6
1
1
1
1
1
1
1
1
1
1
1
1
1
1
I
1
I
1
I
I
I
I
I
I
I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 17. Relative abundance of Elminus modestus at different tidal levels on 25 transect sites
(Appendix 1 ) .
C. METTAM
I38
6-
-
-6
-
1
1
1
1
1
1
1
1
1
~
~
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 18. Relative abundance of Mjtilus edulis at different tidal levels on 25 transect sites
(Appendix 1).
but none in the transect itself. Extensive mussel beds were present on some shores
and their limits were detectable in aerial photographs.
Mobile herbivores
Distributional limits of the Patella species were described by Boyden et al.
(1977). Patella aspera and P. intermedia both occurred as far east as [5] but only
-
-6
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
I
I
I
I
I
I
I
I
I
I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 19. Relative abundance of Patella spp. a t different tidal levels on 25 transect sites
(Appendix 1).
~
ROCKY SHORE ZONATION
0
139
-
4 - 2 -m
-4-
-6
-
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1 2 3 4
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 20. Relative abundance of Gibbula cineraria at different tidal levels on 25 transect sites
(Appendix 1) .
P. vulgata extended farther, reaching truly estuarine conditions. I n practice,
almost all the limpets recorded were P. vulgata and the other species have not
been distinguished in Fig. 19. Limpets were common across their vertical range,
becoming less so as their eastward limit approached [ 171. Very large individuals
occurred at Sully (1 3), as previously observed by Purchon (1948).
Gibbula cineraria was sporadic, on the more marine transects at low water
(Fig. 20) but G. umbilicalis had a discontinued distribution (Fig. 21), occurring
in the west but only commonly again at [9]-[ll], especially at [lo] in the
vicinity of the heated discharge from Aberthaw power station.
The common littorines showed different degrees of eastward penetration.
Littorina littorea was unevenly distributed both within its vertical range and
between shores (Fig. 22). It was commonest in sheltered positions, such as in the
lee of rocks. Littorina obtusata and L. mariae, the flat winkles, were most abundant
6-
-6
-
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
l
I
I
I
I
I
I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17'18 19 20 21 22 23 24 25
Transects
Figure 21. Relative abundance o f Gibbula umbilicalis at different tidal levels on 25 transect sites
(Apendix 1).
C. METTAM
140
8-
6-
-
-
-61
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
I
I
I
I
I
l
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 22. Relative abundance of Litforinn lifforea at different tidal levels on 25 transect sites
(Appendix 1 ) .
in the estuarine reaches, associated with algal cover. Both species are combined
in Fig. 23 because of uncertainty in field identification, although they were
readily distinguishable at some sites (Boyden et al., 1977). At [13] for example
L. obtusata was olive coloured, thin-shelled and associated with Ascophyllum
nodosum; L. mariae was reticulated, smaller, thick-shelled and found with Fucus
8-
1
-
MHWS
6-
MHWN/
4-
-
-6
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 23. Relative abundance of flat winkles, Littorina oblusotn and L. marine combined, at different
tidal levels on 25 transect sites (Appendix 1 ) .
141
ROCKY SHORE ZONATION
-6
-
I
I
I
I
I
I
I
I
I
I
I
I
l
l
l
l
l
l
l
1
1
1
1
1
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
“ransects
Figure 24. Relative abundance of Melaraphe neritoides at different tidal levels on 25 transect sites
(Appendix 1).
serratus. All flat winkles examined from [ 161 eastwards were L. obtusata, whether
green or reticulated. Flat winkles were uncommon, even though fucoids were
plentiful, at [17] and at [21] the F. serratus-dominated lower shore lacked flat
winkles even though the upper shore had plenty.
Melaraphe neritoides extended highest into the littoral fringe on the exposed
western shores but was not confined to high levels. I t had the most restricted
estuarine penetration of all littorines, was rather erratic in its occurrence
(Fig. 24) and was absent from shores where the upper shore was either eroding
limestone cliff or a storm beach of boulders. Its distribution partly reflects the
availability of suitable, pitted, hard rock substrates on the upper shore. Records
of rough winkles pre-date recent and continuing taxonomic revision (Graham,
1988) and they were all recorded as Littorina saxatilis agg. (Fig. 25). O n the
eastern transects, specimens occurred at high levels on the shore and towards
extreme low water on bare rock below the algal-dominated zone.
Mobile carnivores
Larger mobile carnivores are not reliably counted on transects because their
appearance on shore may be transient. The starfish Asterias rubens locally
exceeded 20 individuals per square metre around mussel beds and its eastern
limit is probably around [12] (Boyden et al., 1977). The dog whelk Nucella
lapillus was the only intertidal carnivore common and regular enough in its
distribution to be included (Fig. 26). It did not penetrate far into the estuary but
exceptionally large, thick-shelled specimens were found towards its eastern limit,
often in association with Balanus crenatus and Mytilus edulis a t low shore levels.
The largest animals were incapable of withdrawing completely into their shells
and may have been confined to the sublittoral fringe because of this.
C. METTAM
142
6
4
2
0
-2
-
-6
I
I
I
I
I
I
I
I
I
I
I
I
I
1
1
I
I
I
I
1
1
I
"
I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 25. Relative abundance of Littorina saxatilis agg. at different tidal levels on 25 transect sites
(Appendix 1 ) .
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Transects
Figure 26. Relative abundance of Nucello lapillus at different tidal levels o n 25 transect sites
(Appendix I ) .
DISCUSSION
Changes along an environmental gradient
The main purpose of this paper has been to demonstrate patterns in the
distribution of species over a sequence of transects. These are illustrated in the
plots. Can the information presented here be used to identify a boundary
between the marine and estuarine environments of the Bristol Channel/Severn
Estuary? Although Boyden el al. (1977) chose the seaward limit of laminarians as
a boundary in their survey of intertidal fauna, no single biological feature signals
the transition from Bristol Channel to Severn Estuary. For convenience in the
following discussion, however, we can recognize four regions: a 'marine' section
eastwards to Swansea [ 11-[4]; a 'transitional' section between Swansea and
ROCKY SHORE ZONATION
143
Cardiff [5]-[16], an ‘estuarine’ section up to Newnham [17]-[23] and the tidal
river [24]-[25]. The boundaries between them are rather arbitrarily defined.
Despite variations due to the particular conditions of individual shores, species
distributions assembled for all the transects do reveal patterns. The causes of
these patterns are almost entirely conjecture. Turbidity is certainly a limitation
on the eastward distribution of both aquatic macroalgae, with a demand for
light, and suspension feeding animals. Hiscock (1985) related the depth reached
by sublittoral laminarian forest in the Bristol Channel to isolumes in water of
varying turbidity. Little and Smith (1980) reasonably supposed that turbidity
limited the downshore spread of fucoids and caused the bare zone on the low
intertidal rocks in the Estuary. They found little evidence for any effect of
salinity or wave action on macroalgae. Grazing, particularly by limpets,
maintains open spaces and prevents establishment of algal cover on British
shores. Nelson-Smith (1967) noted the upshore spread of F. serratus at sites in
west Wales, where the rock was moist and grazing limpets absent: these
conditions are met in the Estuary and might apply. However, Little and Smith
(1980) found that while removal of grazing gastropods from a marine shore
(Porlock) enabled macroalgae to establish, it did not happen in the Severn
Estuary at Portishead. They suggested that estuarine limpets were feeding,
‘almost without moving’, on microalgal films and were ineffective at removing
macroalgal sporelings. They also noted that loss of Ascoptzyllum nodosum and Fucus
vesiculosus from the mid-shore at Sand Point and Portishead in 1975 left a bare
zone that persisted for several years. Such anomalies made the distribution
patterns ‘difficult to explain’ (but see Crothers & Hayns, 1994).
Turbidity is presumably a major limitation on filter feeders such as Mytilus
edulis, which is abundant in docks alongside the turbid Estuary (Boyden et al.,
1977). Estuarine mussels are adapted to turbid water by enlargement of the
labial palps (pers. obs.).
While salinity is a fundamental feature of the estuarine gradient, its
importance in determining distribution limits is unclear. Salinity may determine
the penetration of hydroids into the Estuary (Mettam, 1978) and possibly the
replacement of Actinia equina by Diadumene luciae. The explanations of some
distributions are even more speculative. Unspecified factors (presumably related
to temperature) that determine distribution limits of species in an eastward
direction on the south coast of England (Crisp & Southward, 1958) may also
apply to the shore of the Bristol Channel. Gibbulu umbilicalis, for example, has a
south-westerly distribution in Britain (Lewis, 1964) and might be expected to
decline in an eastward direction; its abundance near Aberthaw may be a
response to the warm effluent of a power station.
The relative extension of the Chthamalus zone downshore into the Semibalanus
zone is an unexplained feature of the transitional transects. The limits of
penetration of the estuary by barnacle species are presumably set by the arrival
of spat which originated on distant shores, for Moyse and Knight-Jones (1967)
noted the failure of S. balanoides to release viable larvae well below its upstream
limit in the Estuary and at Penarth [16] they found many decomposing egg
masses.
Wave exposure must be a major component in the change from shores
dominated by barnacles to those dominated by algae but unfortunately the
estimation of exposure from ‘biological exposure scales’ (Ballantine, 1961) based
144
C. METTAM
on the species present, is complicated by the presence of other ‘estuarine’
influences as indicated in the previous paragraphs. The shell shape of Nucella has
also been used as an indicator of exposure to waves and there is a relatively
abrupt change in shape of the shell of Nucella on the southern coast and a
gradual one on the northern coast, consistent with the different pattern of
change in environmental features, but the link with exposure is obscure
(Crothers, 1974, 1985). Dalby et al. (1978) related the elevation of the top of the
black lichen zone above Chart Datum to a biological exposure scale: to
compensate for the effects of different tidal ranges, they recommended a
multiplication factor to be used when making comparisons with their original
sites. This calculation works in west Wales (Dalby et al., 1978) but not in the
Estuary (where an exposure grade 1 at Avonmouth would require the black
lichen zone to reach an impossible 130 m above C.D.). The intertidal biota
therefore do not provide a ready assessment of wave exposure in the Severn
Estuary by extrapolation from techniques that have been successfully applied on
other coastal sites.
Changes with time
Some long-term changes in biota have been recorded. A few species, including
Littorea littorea and Actinia equina, are known to have retreated seawards since the
1920s but comparison of recent and old records shows little change in the
penetration of most species into the estuary (Mettam, 1979b). Sabellaria alveolata
suffered in the cold winter of 1962/63 (Crisp, 1964) and the absence of living
worms in the colony on the transect at Sully [13] was probably due to its
slowness in recolonizing. Sabellaria was not found on Lundy Island in 1971 and
1974, where it had been present earlier (George, 1974). In later years, several
good settlements of S. alveolata on shores between [7] and [ I 13 have established a
strongly growing reef and S. alveolata has since returned at Sully [13]. Monodonta
lineata was lost from all the transitional estuary shores in that cold winter (Crisp,
1964) and has never recovered its former distribution on this coast.
Elminius modestus, first recorded a t Barry in 1947 (Purchon, 1948) is now the
dominant barnacle in the Estuary. Since the survey, another immigrant, the
gastropod Crepidula fornicata, has become common as far east as Sully and several
species have shown small scale extensions or contractions of range; for example,
Gibbula umbilicalis extended its range to Barry in 1977 and to Sully in later years.
Yet other species have remained remarkably constant: the location of mussel
beds, described by Boyden et al. (1977) were the same when shores were
resurveyed along the coast from Porthcawl to Cardiff (Coughlan et al., 1983).
The impression from visits to the shores over many years is that mussel beds are
enduring features in spite of their fluctuating fortunes; only the bed at Llantwit
Major has been lost in recent years. The same persistence has been shown by
Sabellaria reefs which have occupied the same localities and with similar form
(eroded, turreted or hummocked) over many years.
Changes following a tidal power barrage
The marine part of the coastline is remote from the influence of the proposed
barrage. The transitional region would change if turbidity was reduced but
ROCKY SHORE ZONATION
145
greatest change will occur within the enclosed basin, currently the estuarine
section. A barrage would realign the tidal levels within the enclosed basin.
Intertidal organisms would have to readjust to the new alignment by migration
or colonization by dispersive larvae.
Once established, the new regime promises a reduction in tidal movement and
turbidity, so alleviating some of the limiting stresses on intertidal organisms and
promoting a more productive ecosystem in the basin. These shores are already
algal-dominated and this trend is likely, if anything, to be increased since the
changes envisaged promote plant growth but are unlikely to encourage grazing
activity of limpets and winkles. At present however, recruitment of macroalgae
in the Estuary is erratic (Little & Smith, 1980) and competition for space on the
rocks from an increase in the mussel population may break the dominance that
seaweeds currently hold, perhaps in the period when new zones are being
established.
ACKNOWLEDGEMENTS
I thank many friends for their help, in particular D. B. Wildridge, who piloted
the photographic flights; Dr C. Little, who collaborated in a survey of the water
quality; students and friends, from Bristol and Cardiff Universities, who
collected water samples up and down the estuary; Sabina Thompson, who drew
the graphs from the water quality survey and Dr J. H. Crothers who coaxed this
paper from me.
REFERENCES
Abdullnh MI, Dunlop HM, Gnrdner D. 1973. Chemical and hydrographic observations in the Bristol
Channel during April and June 1971. Journal of the Marine Biological Association of the United Kingdom 53:
299-3 19.
Admiralty Hydrographic Department, 1973. Admiralp Tide Tables, volume I : European waters. HMSO
London.
Baker JM,Crothers JH. 1987. Intertidal rock. In: Biological surveys ofestuaries and coasts. Baker U M and Wolff
VJ (eds). Estuarine and brackish-water sciences handbook, Cambridge University Press, pp. 157-197.
Ballantbe WJ. 1961. A biologically-defined exposure scale for the comparative description of rocky shores.
Field Studies l ( 3 ) : 1-19.
Bassindale R. 19438. Studies on the biology of the Bristol Channel XI. The physical environment and
intertidal fauna of the southern shores of the Bristol Channel and Severn Estuary. Journal of Ecology 31:
1-29.
Bassindale R. 1943b. A comparison of the varying salinity conditions of the Tees and Severn Estuaries.
Journal of Animal Ecology 12: 1-10,
Benson-Evans K, Evans A, GrifEths HM. 1974. A survey of marine algae along the north shore of the
Severn Estuary and Bristol Channel. Aquatic Ecology and Pollution Bulletin 2( I ). 87pp. University College,
Cardiff.
Boyden CR, Crothers JH, Little C, Mettam C. 1977. The intertidal invertebrate fauna of the Severn
Estuary. Field Studies 4: 477-554.
Collins M. 1983. Supply, distribution, and transport of suspended sediment in a macrotidal environment:
Bristol Channel, U.K. Canadian Journal of Fisheries and Aquatic Sciences 40 (suppl. 1): 44-59.
Coughlan J, Whitehouse JW,Bnmber RN. 1983. Aberthaw Power Station: an assessment of the mussel
fouling risk. pp. 6 +3. CERL internal report, Central Electricity Generating Board.
Crisp DJ. 1964. The effects of the severe winter of 1962-63 on marine life in Britain. Journal of Animal Ecology
33: 165-210.
Crisp DJ, Southward AJ. 1958. The distribution of intertidal organisms along the coasts of the English
Channel. Journal of the Marine Biological Association of the United Kingdom 37: 157-208.
CrothersJH. 1974. O n variation in Nucella lapillus (L.): shell shape in populations from the Bristol Channel.
Proceedings of the Malacological Sociep of London 41: 157-170.
Crothers JH. 1976. O n the distribution of some common animals and plants on rocky shores of West
Somerset. Field Studies 4: 369-389.
I46
C. METTAM
Crothers JH. 1985. Dog-whelks: an introduction to the biology of Nucella lapillus (L.) Field Studies 6: 291-360.
Crothers JH,
Hayns S. 1994. Rocky shore distribution patterns along the Somerset coast. Biological Journal of
the Linnean Society 51: 1 15- 12 1.
Dalby DH, Cowell EB, Syratt WJ, Crothers JH. 1978. An exposure scale for marine shores in western
Norway. Journal of the Marine Biological Association of the United Kingdom 58: 975-996.
George JD.1974. The marine fauna of Lundy, Polychaeta (marine bristleworms). Report of the Lundy Field
SocieQ 25: 33-48.
Graham A. 1988. Molluscs: prosobranch and pyramellid gastropods. Ktys and notes f o r the identification of the species.
Synopses of the British Fauna (New Series), 2 (2nd edition). Leiden: EJ Brill/Dr W. Backhuys.
Hiscock K. 1981. South-west Britain sublittoral survey. Final report. Pembroke, Nature Conservancy
Council, Huntingdon/Field Studies Council Oil Pollution Research Unit.
Hughes P. 1966. The temperature and salinity of the surface of the Irish sea for the period 1947-61. Geophysical
Journal of the Royal Astronomical Society 1 0 421-435.
Lewis JB. 1964. The Ecologv of Rocky Shores. English Universities Press.
Little C, S m i t h L. 1980. Vertical zonation on rocky shores in the Severn estuary. Estuarine and Coastal Marine
Science 11: 651-669.
Mettam C. 1978. Environmental effects of tidal power generating schemes. Hydrobiological Bulletin 12: 307-32 I .
Mettam C. 1979a. Stratification of water in the Severn Estuary: photographic evidence. Proceedings of tthe
Bristol Naturalists Society 37: 99-1 03.
Mettam C. 1979b. Faunal changes in the Severn Estuary over several decades. Marine Pollution Bulletin 1 0
133-1 36.
Mettam C . 1983. A combined quadrat-level for rocky shore surveys. Hydrobiologia 99: 155-156.
Moyse JM, Knight-Jones EW. 1967. Biology of cirripede larvae. In: Proceedings of a Symposium on
Crustacea, Ernakalum 1965. Marine Biological Association of India, Symposium Series 2 , pp. 595-621.
Moyse JM, Nelson-Smith A. 1963. Zonation of animals and plants on rocky shores around Dale,
Pembrokeshire. Field Studies l ( 5 ) : 1-31.
Nelson-Smith A. 1967. The marine biology of Milford Haven: the distribution of littoral plants and animals.
Field Studies 2: 435-477.
Pingree RB,Griffiths DK. 1980. Currents driven by a steady uniform wind stress on the shelfseas around the
British Isles, Oceanologica Acta 3: 227-236.
Purchon RD. 1948. Studies on the biology of the Bristol Channel XVII: the littoral and sublittoral fauna of
the northern shores near Cardiff. Proceedings of the Bristol Naturalists Society 27 (1947): 285-310.
Smith LP. 1980. The distribution of common rocky shore algae along the south coast of the Severn Estuary.
Proceedings of the Bristol Naturalists Society 38: 69-76.
Smith L, Little C. 1980. Intertidal communities on rocky shores in the Severn Estuary. Proceedings ofthe Bristol
Naturalists Society 38: 61-67.
Southward AJ. 1976. O n the taxonomic status and distribution of Chthamalus stellatus (Cirripedia) in the
north-east Atlantic region: with a key to the common intertidal barnacles of Britain. Journal of the Marine
Biological Association of the United Kingdom 56: 1007-1028.
Stevens CV. 1986. A three dimensional model for tides and salinity in the Bristol Channel. Continental SheEf
Research 6: 531-560.
Thompson C. 1972. Surf;ng in Great Britain. London: Constable.
Williams R, Collius NR. 1985. zooplankton Atlas of the Bristol Channel and Severn Estuary. Plymouth; IMER,
169pp.
APPENDIX 1
List of transects
( 1 ) lraethyllyfyn (SM 800321), September 1973. An almost vertical stack rising from sand.
(2) Musselwick (SM 820064), September 1973. Lower shore boulders rising to a steep cliff.
(3) Kitchen Corner (SS 401875), September 1973. Lower shore boulders rising to a steep cliff.
(4) Bracelet Bay (SS 628869), April 1973. Lower shore boulders in gullies, rising to fissured cliff.
(5) Sker Point (SS 797798), March 1973. Lower shore is a fissured vertical face, upper shore an undulating
platform with boulders rising to pebble bank and sand dune.
(6) Porthcawl Point (SS 818762), March 1973. Lower shore ridges and pebble filled gullies backed by rock
slabs, rising to the sea wall.
(7) Rhych Point (SS 828765), March 1973. Regularly sloping shore. Muddy deposits on lower shore rock.
(8) Nash Point (SS 915683), April 1972. Shelving beach platform backed by cliff.
(9) Marcross (SS 917679), March 1974. Shelving beach platform backed by cliff.
(10) Limpert Bay (SS 009662), May 1972. Undulating shore with a series of protruding tilted limestone beds
facing landwards. Muddy sand in gullies and mud film on rocks, rising to storm beach of pebbles.
( 1 1 ) Porthkerry (ST 098667), April 1972. A rocky outcrop on a pebble beach, backing to a pebble ridge.
Muddy pools at low water.
(12) Friars Point (ST 111659), April 1972. An evenly sloping shore, continuing above EHWS.
ROCKY SHORE ZONATION
147
(13) Sully Bay (ST 155679), September 1973. A gently sloping shore with pools. Upper shore with flat rocks
and pebbles, well scoured and bare, rising to a low cliff.
(14) Sully Island (SS 165669). April 1972. Lower shore of flat stones and some mud.
(15) Lavernock Point (SS 187681),January 1973. A regularly sloping shore with some muddy deposits, rising
to a storm beach of boulders.
(16) Penarth Head ( S T 192721), February 1974. Lower shore red clay dotted with stones, rising to mid-shore
limestone rock and upper shore of gravel beach with concave profile.
(17) Goldcliff (ST 373819), September 1973. Lower shore consists of boulders embedded in mud and gravel. A
protruding platform of marl links the lower shore to the sea wall.
(18) Black Rock, Portskewett (ST 514881), November 1973. An outcrop of rock rising to a steep mud bank
and salt marsh.
(19) Hen and Chickens (ST 553909), April 1972. A long outcrop of rock, rising to a saltmarsh. The point of
low tide, taken on the downstream side of the promontory, was about 1 m lower than the upstream side.
(20) Inward (Pillhouse) Rocks ( S T 569953), September 1973. A steep bank of boulders.
(21) Guscar Rock (ST 597982), September 1973. Below MHWN a sloping slab of rock platform terminates in
a steep edge at low water. Higher levels are soft mud, rising to a vertical mud cliff and salt marsh at
MHWS.
(22) Wellhouse Bay (SO 670030), September 1973. A rocky outcrop on which lies the tumbled masonry of the
former rail bridge. Sand and deep mud has filled the spaces between boulders.
(23) Newnham Rocks (SO 689107), December 1973. An evenly sloping rock surface, extending below low
water.
(24) Manor Ditches (SO 803180), August 1974. Angular boulders dumped as protection from erosion on a
bend in the river. The boulders were covered with a film of mud.
(25) Old Gloucester bridge (SO 817196), December 1973. The bridge masonry provided a hard surface for
colonization but neither fauna nor macroalgae were present.
APPENDIX 2
Abundance scale used in this study
Barnacle species and Melaraphe neriloides
5. > 1 per square centimetre
4. 10-100 per square decimetre
3. 1-10 per square decimetre, individuals < 10 cm apart
2. 10-100 per square metre, few individuals < 10 cm apart
I . < 10 per square metre
Pomaloceros sp., Patella spp. and Littorina spp.
5. > 50 per square metre
4. 10-50 per square metre
3. 1-10 per square metre
2. 10-100 per square decametre (notional area)
1. < 10 per square decametre
Aclinia equina, Gibbula spp., and Nucella lapillus
5. > 10 per square metre generally
4. 1-10 per square metre, locally more
3. < I per square metre, locally more
2. 10-100 per square decametre
I . < 10 per square decametre
Mytilus edulis and Sabellaria alveolala
> 30"; cover
large patches but < 309,,, cover
many scattered individuals and small patches
scattered individuals, no patches
1. < 1 prr squarr metre
5.
4.
3.
2.
Rockweeds: Fucus spp. and Ascophyllum nodosum
5. > 30"" cover
4. 5-30?.b cover
3. < 5";, cover but zone apparent
2. scattered plants, zone indistinct
I . < 1 plant or patch per square decametre