ICES Journal of Marine Science, 53: 978–980. 1996
Short Communication
Variability and stability in benthos: twenty-two years of
monitoring off Northumberland
C. L. J. Frid, J. B. Buchanan, and P. R. Garwood
Frid, C. L. J., Buchanan, J. B., and Garwood, P. R. 1996. Variability and stability in
benthos: twenty-two years of monitoring off Northumberland. – ICES Journal of
Marine Science, 53: 978–980.
The macrobenthic fauna at a station 5.5 nautical miles off the Northumberland coast
has been monitored since 1972. During the period 1972 to 1981 the fauna exhibited
regular cycles of low abundance in spring followed by high abundance the following
autumn, suggesting a system regulated by density-dependent processes. In 1982, this
pattern broke down and the system continued to show instabilities up to at least 1991.
The considerable fluctuations in macrofauna abundance in March during this period
were positively correlated with phytoplankton abundance two years previously
(r2 =0.55). There is some indication that a stable cycle may be re-appearing in the data
from 1992.
? 1996 International Council for the Exploration of the Sea
Key words: benthos, density-dependent, macrofauna, North Sea, Northumberland,
phytoplankton, organic input.
C. L. J. Frid, J. B. Buchanan, and P. R. Garwood: Dove Marine Laboratory, University
of Newcastle upon Tyne, Cullercoats, North Shields, NE30 4PZ, England, UK.
Introduction
Nearshore macrobenthic fauna off the north-east coast
of England has been studied since the 1950s (Buchanan,
1963, 1965). Since 1972, two stations off the south
Northumberland coast have been regularly monitored
(see Buchanan (1993) and references therein). The
programme was originally initiated in order to establish
the benthic productivity of the area (Buchanan and
Warwick, 1974; Buchanan et al., 1974). Proposals to
designate part of the area as a sewage sludge dumpsite
prompted the extension of the number of stations
sampled, with the emphasis changing to one of monitoring the natural fluctuations in the benthos and trying to
understand the mechanisms driving this variation
(Buchanan et al., 1978; Buchanan and Moore, 1986).
This paper describes recent results from one inshore
station and considers the extent to which the community
is unstable and responding to fluctuating inputs of
organic material.
55 m. The sediment is predominantly medium sand, with
a median grain diameter of 500 ìm, and a silt/clay
content of about 15%. Sampling commenced in the latter
part of 1972 and, in order to establish the annual cycle
of macrofaunal abundance, was continued on a
bimonthly basis throughout 1973. This study showed
that macrofaunal abundance (animals retained on a
0.5 mm screen) reached a maximum in September and
fell to a minimum in March. From 1974 onwards, the
station has been sampled in both March and September
to characterize annual variation in the abundance
cycle. An unbroken time series of 22 years is currently
available for analysis.
The fauna
The macrofauna may be considered a variant of the
classical Petersen Amphiura filiformis community
(Petersen, 1913), but is often dominated by small-bodied
polychaetes, Prionospio fallax (Söderström) generally
being the top-ranked species.
The station
The stable decade 1973–1982
Station M1 lies 5.5 nautical miles off the south
Northumberland coast (55)07*N 01)20*W) at a depth of
March and September values (Fig. 1) show evidence of
two distinct cycles during the period 1973–1980. Firstly,
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? 1996 International Council for the Exploration of the Sea
Variability and stability in benthos
979
Figure 2. Total number of macrofaunal individuals m "2 at
station M1 in March 1982 and 1994.
Figure 1. Total number of macrofaunal individuals m "2 at
Station M1 in March (-) and September (,) 1973–1982.
there is an annual cycle of new recruits coming in
between spring and autumn, followed by mortality from
September to the following March. This annual cycle
has remained a constant feature throughout the
monitoring exercise. In addition, there appears to be a
second cycle, biennial in nature, of alternating high and
low September as well as March values, which are offset
by 1 year. Thus, relatively low March values are
followed by high September values, which are then
followed by high March and low September values. This
cycle occurred in an unbroken sequence of 7 years.
The probability of such a sequence arising by chance
(assuming a null hypothesis of p=0.5) can be set at 1 in
128 (p=0.008).
A relatively low March value was invariably followed
by a very high September value, but also high September
values were followed by higher over-wintering mortalities (Buchanan, 1993). These relationships represent
evidence of density-dependent recruitment and mortality, which kept the March figures within the stable
range of 2000–3000 individuals m "2. It is postulated
that year-to-year food input to the bottom had
stayed relatively stable and that density-dependent
mortality adjusted the numbers to a sustainable level
commensurate with the available energy.
The unstable period 1982 to the present
The stable biennial cycle collapsed in 1981 when the
March abundance showed a marked rise, instead of the
predicted fall. Numbers continued to rise and had more
than doubled by 1985. After 1985, the fauna exhibited a
series of emphatic instabilities up until 1992 (Fig. 2),
Figure 3. Regression of abundance (numbers m "2) against
CPR net colour index (Greenness) 2 years previously. r2 =55%.
including both the highest (7224 in 1991) and the lowest
values (1036 in 1986) recorded for March numbers of
individuals during the entire monitoring exercise.
Having postulated that the numbers during the stable
period were determined by density-dependent processes
in response to a stable year-to-year input of organic
matter to the bottom, the possibility has been explored
that the post-1981 rise in numbers might be explained in
terms of enhanced organic input. Since no contemporaneous data were available with regard to organic
deposition, it was necessary to rely on indirect evidence
provided by the Continuous Plankton Recorder surveys
in the area adjacent to the Northumberland coast (for
details and justification see Buchanan, 1993). These
surveys provided, for each month of each year, a
‘‘Net Colour Index’’ of the degree of greenness of the
plankton silk of the recorder, which may be equated
with phytoplankton standing stock and hence with the
eventual quantity of organic matter settling to the sea
bed.
Data show that phytoplankton abundance in the
period 1969–1978 was indeed stable at a low level but
increased markedly from 1979 onwards. However, the
break in faunal stability came 2 years later in 1981,
suggesting a 2-year lag effect between ‘‘cause’’ and
‘‘effect’’ (Fig. 3). The linear regression of total numbers
of benthic organisms against the Net Colour Index is
980
C. L. J. Frid et al.
significant (r2 =0.55), but data for individual species (e.g.
Prionospio fallax: r2 =0.84) are even more convincing.
Finally, Figure 2 suggests a possible return to the
biennial cycle from 1992, but at an increased level.
Future monitoring may test the robustness of these
patterns.
sampling. Over the years this work has been supported
by a number of funding agencies, including the Natural
Environment Research Council, the Northumbrian
Water Authority, and the University of Newcastle upon
Tyne.
References
Conclusions
During nearly a quarter of a century of monitoring there
has been no conclusive evidence of a deterioration in
the ‘‘health’’ of the benthic fauna which might be
attributable to anthropogenic influences. The pattern of
inter-annual variability does not support the idea of a
steady rise in macrobenthos abundance due to eutrophication in the area. There are a number of circumscribed
polluted areas off the Northumberland coast, due
mainly to the dumping of solid industrial waste
(Bamber, 1984; Johnson and Frid, 1995), sewage sludge
(MAFF, 1991), and harbour dredgings (Rees and
Eleftheriou, 1987). The two stations currently monitored
are the remnants of a once extensive grid. Great care has
been taken to ensure that the remaining stations were
selected on the basis of their being representative of the
benthos over a wide area (Buchanan, 1993). Therefore,
it would appear that, away from the well-defined
areas used for waste disposal, the benthic fauna off
Northumberland has changed little in the course of over
20 years.
Temporal variability in the quantity of macrobenthos
may be affected by below-average winter temperatures
(Buchanan et al., 1978; but see also Buchanan and
Moore, 1986). However, these changes are comparatively small and trivial compared with the much more
dramatic changes which have been linked to variations
in organic input to the bottom.
Acknowledgements
We thank two anonymous referees whose comments
considerably improved the manuscript, the master and
crew of the RV ‘‘Bernicia’’ for their help with the
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