Jncc coastal Directories Project Region 11 The Western Approaches

4.3.1  Introduction
Plankton include the bacteria (bacterio-), plant (phyto-) and
animal (zoo-) plankton.  In temperate continental shelf seas,
as in this region, the phytoplankton assemblage is
dominated by diatoms and dinoflagellates, and the
zooplankton, although containing representatives of most
marine animal phyla at some stage, is dominated by
crustaceans, principally copepods.  The plankton’s
abundance is strongly influenced by factors such as depth,
tidal mixing and temperature stratification, which
determine the vertical stability of the water column.  The
distribution of species, here and elsewhere, is influenced
directly by salinity and temperature, by water flows into the
area and by the presence of local benthic (bottom-dwelling)
and littoral (shoreline) communities.  Many of the species of
these communities, including commercially important fish
and shellfish (see 
sections 5.5
and 
5.7
), have temporary
planktonic larval forms (meroplankton).  Tidal fronts
(boundary zones between stratified and well mixed water
masses) in this region are likely to be of significant
biological importance, since they are usually rich in
plankton, which attracts other marine life. Phytoplankton
blooms (transient, unsustainable growths, usually of a
single species and often associated with a visible
discolouration of the water) are a normal feature in the
seasonal development of plankton.  Some blooms may reach
exceptional proportions (>10
6
cells/l) or contain species
(principally dinoflagellates) that can be toxic to humans and
possibly have an important economic impact on
mariculture, fisheries and tourism.  
In Region 11, as elsewhere, the plankton has a
fundamental role in the food chain of both benthic (sea-bed)
organisms (see 
sections 4.2
, 
5.4 
and 
5.5
) and fish (see
sections 5.7 - 5.9
).  For both ecosystems, the availability of
food and nutrients, larval survival, maintaining populations
and timing of egg production are highly dependent on the
amount of phyto/zooplankton available.  Any
environmental stress imposed on the plankton will have
consequences throughout the food chain and may affect the
amount of food available to fish, birds, marine mammals
etc.  In coastal management, plankton can also give early
warning of adverse human impacts (for example the effects
of eutrophication) and highlight different water masses.
Section 2.3
describes the physical environment of the
seas in Region 11.  In the Celtic Sea, thermal stratification
becomes well established by late spring and spreads
eastward along the north Cornish coast, establishing frontal
boundaries at Land’s End and the eastern part of the Celtic
Sea (
Map 4.3.1
).  In the Bristol Channel, however, vertical
mixing is sufficient to maintain vertical homogeneity of the
water column throughout the year.
Owing to high levels of turbidity in the inner Bristol
Channel, estimated annual primary production is low
(6.8 g C m
-2
) compared with the outer channel (164.9 g C m
-2
)
and the Celtic Sea (Joint & Pomroy 1981).  
Figure 4.3.1
shows the seasonal cycles of phytoplankton production for
Region 11, based on a visual estimate of chlorophyll and the
numbers of copepods per sample (Warner & Hays 1994).
Region 11  Chapter 4  Marine and estuarine environments
76
4.3  Plankton
M. Edwards & A.W.G. John
Well mixed
Stratified
CP
R:
‘1B
’
CPR: ‘B’
5°W
Region 10
Regi
on
12
Phaeocystis pouchetii blooms
(Williams & Collins 1985)
Approximate position of front
Map 4.3.1  
Plankton surveys, ‘fronts’ and areas of well mixed and
transitional water.  See 
Table 4.3.1 
for symbols and
details of surveys.  Source: Sir Alister Hardy Foundation
for Ocean Science (SAHFOS).
2.0
1.5
1.0
0.5
0.0
200
400
600
800
1,000
1,200
1,400
0
J
F
M
A
M
J
J
A
S
O
N
D
T
otal copepods (nos. per sample)
Months
Phytoplankton Colour Index (arbitrary units)
Colour Index
Total copepods
Figure 4.3.1  
Average seasonal cycles of an index of phytoplankton
colour (a visual estimate of chlorophyll) and the
numbers of copepods per sample (approximately 3 m
3
of water filtered), derived from Continuous Plankton
Recorder data for 1958-1992.  Source: SAHFOS.
4.3.2  Important locations and species
Evidence from the Continuous Plankton Recorder (CPR)
and other surveys indicates that the phytoplankton of this
region is fairly typical of shallow and enclosed waters
around the British Isles.  The dominant phytoplankton
associated with the spring bloom in the Celtic Sea are
Skeletonema costatum and Thalassiosira spp., while in the
Bristol Channel diatoms such as Rhizosolenia hebetata and
Coscinodiscus spp. dominate.  The onset of the spring bloom
is associated with the development of the thermocline (a

---

temperature gradient), which usually begins in the Celtic
Sea and spreads eastward.  By summer the highest levels of
chlorophyll a are found along frontal boundaries on the
stratified side, reaching 8 mg m
-3
(Pingree et al. 1976).  High
levels of chlorophyll a may persist along frontal boundaries
right through the summer until late September, until the
thermocline disintegrates.  Although the Bristol Channel has
higher levels of nutrients than the Celtic Sea, phytoplankton
development is delayed and chlorophyll a levels are low
(1-2 mg m
-3
) because of the high levels of turbidity
encountered in the Channel (Joint & Pomroy 1981).  The
zooplankton of this region is typically dominated by
copepods such as Acartia clausi, Pseudocalanus elongatus and
Temora longicornis.  Studies by Collins & Williams (1981)
found that seasonal variations in distribution and
abundance of the most numerically dominant copepods
found in the Bristol Channel - Eurytemora affinis, Acartia
bifilosa and Centropages hamatus - are related to different
salinity regimes (<30 g/kg, 27-33.5 g/kg and 31-35 g/kg
respectively).  The copepod Calanus helgolandicus (important
food for fish), more commonly found in the Celtic Sea,
penetrates into the Bristol Channel when higher salinities
(>33 g/kg) intrude up the channel.  Copepod abundance
begins to increase in March and reaches maximum densities
in July in the Bristol Channel (nearly 100 times winter
levels).  Other commonly found zooplankton include small
hydromedusae, amphipods and meroplanktonic larvae of
echinoderms, polychaetes, decapods, molluscs and
cirripeds.  Studies by Collins & Williams (1981) and
Southward (1962) have shown that the biodiversity of
zooplankton in the Bristol Channel is relatively low
compared with other shelf seas.  
4.3.3  Human activities
In June 1974 a bloom of Phaeocystis pouchetii was observed in
the central Bristol Channel, producing high levels of
chlorophyll a (see 
Map 4.3.1
).  These blooms occur in most
years in the Bristol Channel and make a considerable
contribution to the phytoplankton standing crop (Williams
& Collins 1985).  However, it is not known why Phaeocystis
blooms develop in some years and not in others (for a
comparison of chlorophyll a concentrations for a bloom year
and a non-bloom year, see Joint & Pomroy (1981)).  These
blooms are of importance to the coastal manager because in
Dutch coastal waters they have been associated with
eutrophication, and once they collapse they may result in
the accumulation on beaches of large banks of foam, which
look and smell unpleasant.  
A bloom of Alexandrium tamarense in the Fal Estuary in
July 1995 caused the first recorded instance of Paralytic
Shellfish Poisoning (PSP) in the UK south of the Humber
(Reid et al. 1995) (see also 
section 5.5
).
4.3.4  Information sources used
During the 1970s and early 1980s the planktonic assemblage
of the Bristol Channel was extensively studied (
Table 4.3.1
).
Details of zooplankton distributions and a taxonomic list of
phytoplankton and zooplankton can be found in Williams &
Collins (1985).  Joint & Pomroy (1981, 1982) studied levels of
primary production within the Bristol Channel between
4.3  Plankton
77
Table 4.3.1  
Details of surveys
Identification
Frequency
Period
Reference
in 
Map 4.3.1
CPR: ‘B’ route
Monthly
1967-1993
Warner & Hays 1994
CPR: ‘IB’ route
Monthly
1986-present Warner & Hays 1994
Bristol Channel Monthly
1973-1975
Williams & Collins
1985
Bristol Channel Various
1975-1978
Joint & Pomroy 1982
Bristol Channel Monthly
1973-1977
Joint & Pomroy 1981
Bristol Channel Seasonal
1977-1978
Burkill & Kendall
1982
Celtic Sea
Occasional 1975
Pingree et al. 1976
Celtic Sea
Occasional 1958-1960
Southward 1962
Coastal regions Occasional 1970-1985
Riley et al. 1986
PS (
s
s
)
Occasional 1980-1982
Williams et al. 1987
PS (
q
q
)
Occasional 1976
Holligan et al. 1980
Key: CPR: Continuous Plankton Recorder; PS: Plankton samples.
1973-1977, and the importance of bacterioplankton within
the estuary.  Apart from the CPR survey, which includes
long-term data, studies of the eastern Celtic Sea tend to be
limited to occasional surveys (Southward 1962; Pingree et al.
1976).
4.3.5  Further sources of information
A.  References cited
Burkill, P.H., & Kendall, T.F.  1982.  Production of the copepod
Eurytemora affinis in the Bristol Channel.  Marine Ecology Progress
Series, 7: 21-31.
Collins, N.R., & Williams, R.  1981.  Zooplankton of the Bristol
Channel and Severn Estuary.  The distribution of four copepods
in relation to salinity.  Marine Biology, 64: 273-283.
Holligan, P.M., Maddock, L., & Dodge, J.D.  1980.  The distribution
of dinoflagellates around the British Isles in July 1977: a
multivariate analysis.  Journal of the Marine Biological Association
of the United Kingdom, 60: 851-867.
Joint, I.R., & Pomroy, A.J.  1981.  Primary production in a turbid
estuary.  Estuarine, Coastal and Shelf Science, 13: 303-316.
Joint, I.R., & Pomroy, A.J.  1982.  Aspects of microbial heterotrophic
production in a highly turbid estuary.  Journal of Experimental
Marine Biology and Ecology, 58: 33-46.
Pingree, R.D., Holligan, P.M., Mardell, G.T., & Head, R.N.  1976.
The influence of physical stability on spring, summer and
autumn phytoplankton blooms in the Celtic Sea.  Journal of the
Marine Biological Association of the United Kingdom, 56: 845-873.
Reid, P.C., Pratt, S., & Harbour, D.  1995.  A red tide event in the Fal
Estuary.  Plymouth, Sir Alister Hardy Foundation for Ocean
Science.  (Unpublished report for NRA.)
Riley, J.D., Symonds, D.J., & Woolner, L.E.  1986.  Determination of
the distribution of the planktonic and small demersal stages of
fish in the coastal waters of England, Wales and the adjacent
area between 1970 and 1984.  MAFF Fisheries Research Technical
Report, 84: 1-23.
Southward, A.J.  1962.  The distribution of some plankton animals
in the English Channel and approaches.  II.  Surveys with the
Gulf III high speed sampler, 1958-1960.  Journal of the Marine
Biological Association of the United Kingdom, 42: 275-375.
Warner, A.J., & Hays, G.C.  1994.  Sampling by the Continuous
Plankton Recorder survey.  Progress in Oceanography, 34: 237-256.
Williams, R., & Collins, N.R.  1985.  Zooplankton atlas of the Bristol
Channel and Severn Estuary.  Plymouth, Institute for Marine
Environmental Research.

---