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J. Mar. Biol. Ass. U.K. (1999), 79, 737^739 Printed in the United Kingdom SHORT COMMUNICATIONS 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 738 SHORT COMMUNICATIONS 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 SHORT COMMUNICATIONS 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. REFERENCES Bainbridge,V. & Forsyth, D.C.T., 1972. An ecological survey of the Scottish herring ¢shery. Part V. 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