Download Exceptional influx of oceanic species into the North Sea late 1997

J. Mar. Biol. Ass. U.K. (1999), 79, 737^739
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Exceptional in£ux of oceanic species into the North Sea
late 1997
M. Edwards*O, A.W.G. John*, H.G. Hunt and J.A. LindleyP
*Sir Alister Hardy Foundation for Ocean Science, The Laboratory, Citadel Hill, Plymouth, PL1 2PB. ODepartment of Biological
Sciences, University of Plymouth, Drake Circus, Plymouth, PL4 8AA. PNatural Environmental Research Council, Plymouth Marine
Laboratory, Prospect Place, Plymouth, PL1 3DH
Continuous Plankton Recorder records from the North Sea and north-east Atlantic from September 1997 to March 1998
indicate an exceptional in£ux of oceanic indicator species into the North Sea. These in£ow events, according to historical
evidence, have only occurred sporadically during this century. This exceptional in£ow and previous in£ow events are
discussed in relation to their similarity in terms of their physical and climatic conditions.
Continuous Plankton Recorder (CPR) samples have been
taken at a depth of 10 m in the North Atlantic from 1932 to 1939
and 1946 to the present. The CPR survey samples at monthly
intervals over major regions of the North Atlantic, e.g. North
Sea, Celtic Sea, Irish Sea, Iceland Basin, Irminger Basin,
Rockall Channel, and Labrador Basin and routinely identi¢es
approximately 400 taxa of phytoplankton and zooplankton.
Sampling by the CPR survey in the North Sea has revealed the
presence of an unprecedented number of oceanic indicator
species since September 1997 to the early part of 1998. The
presence of so many indicator species, and previously unrecorded oceanic species, in this region suggests an exceptional
in£ow of oceanic water into the North Sea.
Doliolids were ¢rst found on CPR samples in the North Sea
in September 1997 on the `LG' route (Rotterdam to Gothenburg), and the `M' route (Aberdeen to Stavanger) between
53824'N 04841'E and 55809'N 06819'E and 57820'N 00840'W
respectively. By October high numbers of doliolids were found in
the German Bight, and for the ¢rst time in the Skagerrak and
Kattegat, on the `C' route (Hanstholm to the Humber) and the
`LG' route from 55859'N 05833'E to 57801'N 07854'E and from
55823'N 06840'E to 57840'N 11810'E, respectively. The population in the German Bight persisted into November in approximately the same area and was recorded on the `C' route
(Figure 1). Subsamples were taken from the central North Sea
routes and re-analysed to speciate the doliolids which were
subsequently identi¢ed as Doliolum nationalis.
The importance of the appearance of doliolids in the North
Sea as indicators of exceptional oceanic in£ow and above average
sea-surface temperatures has been stressed in the past (Delsman,
1933; Lucas, 1933; Lindley et al., 1990). The species D. nationalis
and Dolioletta gegenbauri are usually found in oceanic waters to the
west and south-west of the British Isles (Hunt, 1968), and have
only occurred sporadically in the North Sea during the last
century. Many of the individuals recorded have been from the
northern North Sea (Fraser, 1952). Dolioletta gegenbauri has only
been recorded twice in the central (54^578N) North Sea at 578N
in 1933 (Lucas, 1933) and at 568N in 1972 (Harper, 1974). Doliolum
nationalis was recorded in 1911 o¡ the Dutch coast and in 1989 in
the German Bight (Lindley et al., 1990).
Although the mesozooplanktonic copepods Metridia lucens
and Candacia armata are capable of successful reproduction in the
Journal of the Marine Biological Association of the United Kingdom (1999)
North Sea when conditions are favourable, their long-term
variations in abundance are associated with the in£ow of
oceanic water (Glover, 1957; Bainbridge & Forsyth, 1972) and
are typical for Atlantic waters to the west of Scotland (Lee &
Ramster, 1977). Both these species have been present on CPR
tows in the northern and central North Sea since September
1997. By December 1997, C. armata and M. lucens were found on
southern North Sea routes. The presence of these species in the
North Sea is usually associated with in£ow from the north,
entering the North Sea via the Fair Isle Current and the East
Shetland Atlantic in£ow. However, C. armata was also found on
the `R' route in the southern North Sea. The `R' route tows from
Felixstowe to Scheveningen (Netherlands) in the southern North
Sea, suggesting that oceanic water has also entered the North
Sea through the English Channel. The copepods Rhincalanus
nasutus and Eucalanus elongatus, which are also associated with
oceanic in£ow (Fransz et al., 1991), and Euchaeta hebes, which is
associated with mixed oceanic/neritic waters to the west of
Ireland, were recorded in the North Sea during late 1997/early
1998. Although R. nasutus and E. hebes are intermittently found
in the northern North Sea, it is extremely rare to ¢nd them in
the central North Sea on the `LR' (November 1997) and `C'
routes (March 1998). Colonies of the peritrich ciliate Zoothamnium pelagicum, which is normally associated with oceanic water
in the Bay of Biscay area, was found for the ¢rst time in the
central North Sea in October and December 1997. The oceanic
tintinnid, Dadayiella ganymedes, which is of similar biogeographical origin to Z. pelagicum, was also found for the ¢rst time in
the southern North Sea in January 1998 (Table 1).
A similar episodic event of oceanic water entering the North
Sea at this scale (evident from the CPR survey) occurred in
1988^1990 (Lindley et al., 1990). In the area around the British
Isles the winter of 1988^1989 was exceptionally mild with
average North Sea surface temperatures ranging between 1 and
28C above normal and air temperatures up ‡ 2.58C above the
anomaly with respect to 1961^1990 (Hadley Centre, UK
climate database). Positive anomalies in air temperatures over
the British Isles and sea-surface temperature in the North Sea
persisted through the rest of the year (Northcott, 1989, 1990).
The winter of 1997/1998 was also very mild, with all months
having positive temperature anomalies. Air temperatures for
February 1998 were a remarkable ‡ 3.48C above the long-term
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Figure 1. Map of standard monthly Continuous Plankton Recorder (CPR) tow routes in the North Sea showing the distribution
of doliolids on CPR tows in September and October 1997. Arrows refer to in£ow of Atlantic waters into the North Sea: North
Atlantic Drift (NAD), Shelf Edge Current (SEC), Fair Isle Current (FIC), East Shetland Atlantic In£ow (ESAI), Norwegian
Trench Current (NTC). After Svendsen et al. (1995). Symbols indicate samples with doliolids in September (&) and October (*).
Table 1. Occurrence of doliolids (D), Candacia armata (Ca), Metridia lucens (Ml), Euchaeta hebes (Eh),
Rhincalanus nasutus (Rn), Eucalanus elongatus (Ee), Zoothamnium pelagicum (Zp) and Dadayiella ganymedes
(Dg) on North Sea CPR routes between September 1997 and March 1998. For positions of routes see Figure 1.
CPR route
Sept 1997
Oct 1997
Nov 1997
Dec 1997
Jan 1998
Feb 1998
Mar 1998
Northern
North Sea
Northern
North Sea
Central
North Sea
Central
North Sea
Central
North Sea
Southern
North Sea
Southern
North Sea
`A'
ö
Ml, Eh, Ca,
Rn, Ee
Eh, Ml
Eh, Ml
Eh
ö
Ml
`M'
D
No tow
`LR'
Ca
Ca, Ml
`LG'
D
D, Ca
`C'
ö
D, Ca, Zp
`HE'
ö
ö
`R'
ö
ö
No tow
Rn, Eh, Ml
No tow
Ml, Eh
Ml, Ca
Rn, Ca, Ml
Ml
No tow
ö
Ml
Ca, Ml
ö
Ca
ö
ML, Ca
D, Ca, Ml
Ml, Ca, Zp
Ml, Ca
Ml, Ca
Ca, Ml, Eh
ö
Ca
Ca, Ml, Dg
Ml, Ca
Ca
ö
ö
Ca
ö
ö
average, with respect to 1961^1990 (Hadley Centre, UK
climate database).
Becker & Dooley (1995) reported that during the late 1980s
and early 1990s salinity levels in the English Channel, the
southern North Sea and western German Bight were extremely
Journal of the Marine Biological Association of the United Kingdom (1999)
high, with values exceeding 35.5 psu. The magnitude of this
anomaly approached that of the Great Salinity Anomaly event
which was observed in the northern and central North Sea
during 1977^1979. Heath et al. (1991) also reported an in£ow of
water with exceptionally high salinity in January 1990 o¡ the
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Norwegian coast (58^598N 3.5^048E). These values exceeded
anything previously recorded by the Marine Laboratory,
Aberdeen, since records began in 1920. Preliminary physical
evidence recorded by Dutch scienti¢c research vessels have indicated high salinities during the early part of 1998 in the
southern Bight and the English Channel. In March 1998
recorded salinities were substantially higher than long-term
averages (ICES data), with 35.44 psu recorded at 518N and
35.29 psu recorded at 538N (J. van Bennekom, personal communication).
The similarity between the climatic conditions of the late
1980s/early 1990s and the present, and the appearance of doliolids in the North Sea is not restricted to these time periods.
Lucas (1933) noted that when doliolids appeared in the North
Sea in 1911 and 1933, the summers were dry and warm with seasurface temperatures well above average. The appearance of
Dolioletta gegenbauri in 1972 coincided with sea-surface temperatures above the long-term mean and exceptionally high salinities
on a par with the late 1980s/early 1990s event (Prahm, 1974;
ICES data).
It is possible that oceanic indicator species recorded in the
North Sea in the late 1980s/early 1990s signalled subtle changes
in current patterns in the North Atlantic related to the changing
weather conditions, bringing water from a more southerly origin
into the north-east Atlantic. Evidence that changes in the circulation in the north-east Atlantic is bringing more southerly
water into the north-east Atlantic derives from the presence of
the copepod species E. hebes and R. nasutus and the ciliates
Z. pelagicum and Dadayiella ganymedes in the North Sea. These
species derive from a more southerly origin and represent the
Lusitanian and south-western Atlantic fauna that is carried
north by the Shelf Edge Current. Results from the CPR survey
also show that doliolids ¢rst appeared prior to September 1997
in the Bay of Biscay area in June 1997. During the following
months doliolids spread northwards and were found in the
Celtic Sea, the western approaches of the English Channel, west
of Orkney and Shetland and as far north-west as 62844'N
17829'W (just south of Iceland). Corten & van de Kamp (1992)
and Reid et al. (in press) have recently shown an increase in the
abundance of more southerly pelagic ¢sh (mackerel and horse
mackerel) which travel with the Shelf Edge Current, suggesting
an increase in £ow along the Continental Shelf from a more
southerly origin. What mechanisms drive these changes in circulation are, however, little understood.
The changes in circulation in the North Atlantic and oceanic
in£ow into the North Sea have important chemical/biological
consequences. The increased oceanic in£ow into the North Sea
during the late 1980s and early 1990s coincides with marked
biological changes in the ecosystem of the North Sea from
phytoplankton to ¢sh suggesting an important regime shift
through the trophic levels (Reid et al., in press; Reid et al.,
1998). It is not yet known whether the observed changes in
oceanic in£ow into the North Sea in late 1997 will herald
similar biological changes, following so closely behind the
previous event, and whether we are seeing a more persistent
alteration to the physical environment of the north-east Atlantic.
The CPR survey is supported by a funding consortium comprising: IOC, UNIDO, the European Commission and agencies
from Canada, Denmark, Netherlands, UK, and USA. The
survey depends on the voluntary cooperation of owners, masters
and crews of merchant vessels which tow the CPRs on regular
Journal of the Marine Biological Association of the United Kingdom (1999)
739
routes. We wish to thank the CPR survey team for contributions
to this paper and Johan van Bennekom (Netherlands Institute
for Sea Research) for his correspondence and for kindly supplying salinity data.
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Submitted 30 July 1998. Accepted 6 October 1998.