Fluctuations in the "Indicator" Chaetognaths Sagitta Elegans And

OCEANOLOGICA ACTA 1984- VOL. 7- W 2 ~ -----~1- Macroplankton Climate Fluctuations in the "indicator" Competition Sagitta chaetognaths Sagitta elegans Fluctuations , Macroplancton Oimat and Sagitta setosa Compétition Sagitta in the Western Channel Fluctuations A. J. Southward Marine Biological Association of the United Kingdom, The Laboratory, Citadel Hill, Plymouth, South Devon, PLl 2PB, UK. Received 22/10/82, in revised form 18/11/83, accepted 24/11/83. ABSTRACT Between 1930 and 1938 the arctic-boreal species Sagitta elegans was replaced as the dominant chaetognath off Plymouth by the temperate neritic species Sagitta setosa. The latter was dominant from 1939 to 1968. From 1969 S. elegans became abundant again and was dominant in 1979. There were corresponding changes in other zooplank ton organisms and among pelagie fish, the whole forming a cycle of events, the "Russell cycle". The fluctuation of the Sagitta species is a good example of changes in abundance of a pair of species as a result of environmental change, and provides a useful "indicator" of the general change in the ecosystem. The changes in Sagitta dominance occupied from 9 to 11 years, and the complete changes in the ecosystem took 10 to 15 years. Sorne aspects of the changes were comparatively abrupt, but the whole was quite graduai. The cycle can be correlated with a fluctuation in climate, the recent warming of the Northern hemisphere up to 1950, followed by cooling. The climatic effect may have operated through alterations in residual currents and through changes in sea temperature, but the influence on the organisms was indirect, mediated through interspecific competition and biological interactions in the whole ecosystem. Oceanol. Acta, 1984, 7, 2, 229-239 .. RÉSUMÉ Fluctuations des chaétognathes « indicateurs » Sagitta elegans et Sagitta setosa en Manche occidentale Près de Plymouth, entre 1930 et 1939, Sagitta .elegans (boréo-arctique) a été remplacée en tant qu'espèce dominante par S. setosa (néritique-tempérée). Cette dernière a dominé de 1939 à 1968. A partir de 1969, S. elegans a vu à nouveau ses effectifs augmenter, pour redevenir dominante en 1979. D'autres organismes planctoniques, ainsi que des poissons pélagiques, ont présenté des fluctuations analogues, et l'ensemble de ces changements constitue le« cycle de Russell ». Les fluctuations de Sagitta offrent un bon exemple de variations d'abondance, sous l'influence des facteurs du milieu, chez deux espèces en compétition, et fournissent un moyen commode d'appréhender les changements généraux dans un écosystème. L'inversion de dominance chez les Sagitta s'est étendue sur 9 à 11 ans et l'ensemble des changements dans l'écosystème sur 10 à 15 ans. Certains aspects de ces changements ont été relativement brusques, l'ensemble étant cependant progressif. Le cycle peut être mis en relation avec des fluctuations climatiques récentes, telles que le réchauffement de l'hémisphère Nord jusque vers 1950, et le refroidissement qui a suivi. De tels changements climatiques peuvent agir par des modifications des courants résiduels et par le biais de la tempéra ture de la mer, mais l'effet sur les organismes est probablement indirect, à travers la compétition interspécifique et les interactions biologiques dans l'écosystème entier. Oceanol. Acta, 1984, 7, 2, 229-239. 0399-1784/84/02 229 11/$ 3.1 0/© Gauthier-Villars 229 A. J. SOUTHWARD INTRODUCTION GEOGRAPHICAL AND VERTICAL DISTRIBU TION OF S. ELEGANS AND S. SETOSA Many changes have occurred in the species and ahun As Russell (1935; 1939 b) showed, following the earlier dance of zooplankton, fish and benthos in the Western work of Meek (1928), around the British Isles Sagitta Channel off Plymouth during the past 60 years (review setosa is usually restricted to coastal water, whereas by Southward, 1980; Ford, 1982). These changes forma S. elegans is more common in mixed coastaljoceanic cycle of events, the "Russell cycle" (Cushing, Dickson, water. This separation has puzzled sorne investigators 1976) which appears to be broadly linked with recent since both species are classed as neritic, in contrast to climatic fluctuations in the Northern hemisphere. Sorne the truly oceanic species of Chaetognatha (Tokioka, aspects of the changes, the apparent suddenness of 1979). Closer examination of the geographical distribu certain of them, are difficult to correlate with an envi tion shows that S. Elegans is a widespread Arctic ronmental factor such as sea-temperature which Boreal species which is neritic, and sometimes euryha changes gradually (Southward et al., 1975; Maddock, line, in the Arctic and northern Boreal parts of its Swann, 1977 and references therein). In discussing biolo range, but stenohaline and stenothermal in the North gical changes in the sea Cushing and Dickson (1976) Sea, Celtic Sea, Channel, and Northern Bay of Biscay refer to the alteration of the ecosystem in the Western (Southward, 1962; Alvarino, 1965; 1969); there is a Channel as a process of "rectification", and point out suggestion that at its southern limits it is restricted to that it occurred in stages over a decade. They appear a narrow band of salinities, around 34.0 to 35.0 in to be suggesting that a series of small changes may %, which case the variety living at lower salinities in the build up tension in the ecosystem to a level where Baltic Sea (cf. (j)resland, 1983) ought to be genetically change of state is necessary, followed by re-stabilization distinct. In contrast S. setosa is a less widely distributed at a different balancing point (Holling, 1973; May, temperate or transitional species, tolerant of changing 1977). salinity and mostly restricted to coastal waters. It has a discontinuous distribution, for in addition to the Part of the "Russell cycle" that bas attracted much main region of ~ccurrence, from the southern fjords of interest is the alternation in dominance of two common Norway and the Kattegat to Brittany, S. setosa is also species of Chaetognatha, Sagitta elegans Verrill and found along the northern side of the Eastern Mediterra Sagitta setosa J. Müller. In many previous studies these nean, in the Northern Adiatic, and in the Black Sea species have been considered from the point of view of and occurs locally in the Gulf of Naples and the Gulf their value as indicators of change in hydrographie of Gabes; this distribution suggests S. setosa may be a conditions, as originally suggested by Russell (1935). 1t relie form of previously wider range, now separating is proposed here to give details of the progress of the into local races or species, and indeed the Black Sea changes in dominance during two periods, from 1930 form is regarded as distinct by sorne authorities (Rus to 1938 when S. elegans was replaced by S. setosa, and sell, 1935; Hansen, 1951; Alvarino, 1965; 1969; Vucetié, from 1968 to 1979 when S. elegans returned again, 1961; 1970; 1973; Kahn, Williamson, 1970; GraU et al., and to include serious consideration of the effects of 1971; Jakobsen, 1971; Le Fèvre, 1971; Mironov, 1976; interspecific competition. As Darwin ( 1859) noted, com O'Brien, 1976; Furnestin, 1979; Sands, 1980; Le Fèvre petition between species is of profound importance to et al., 1981; Ibanez, 1982; (j)resland, 1983). To the geographical and vertical distribution of organisms. north of Bergen, S. setosa is replaced inshore by "Almost ali organisms can withstand more heat, cold ... S. elegans, and to the south of Brittany, as for example than they are exposed to within their natural range; in the southern Bay of Biscay and the Mediterranean, it the definitive limit to the range ~f most species, under is replaced by S agitta friderici. S. elegans and S. setosa gradually increasing unfavourable conditions being the overlap in distribution along a comparatively narrow presence of other competing forms better adapted to strip of sea extending from Southern Norway, down such conditions" ... "As species disappear other closely both sides of the British Isles, to Southern Brittany. allied or representative species, apparently filling nearly the same place in the economy of nature, take their Much of the available data on this distribution is place" (Darwin, in Stauffer, 1975, p. 264 and 267). obtained from vertical tows or oblique hauls with large These statements are particularly applicable to the pre nets. However, Russell ( 1939 b) noted that sorne exam sent problem of explaining why comparatively small ples of overlap were due to vertical differences, and changes in environmental factors appear to be asso when samples are taken at different depths in fully ciated with large biological changes. Darwin's view of stratified water, as for example in Oslo Fjord, Korsf what constituted competition was wider than that of jord, the northern North Sea and the western Channel, present-day ecologists, and today we have to take note the two species are seen to be vertically separated, with of competition for space, competition for food and also S. setosa dominant in the layer above the thermocline. the effects of direct predation of one species on another. In such circumstances the adults of S. elegans remain However, before describing the fluctuations in the two in the deeper water below the thermocline, although species and discussing the possible influence of their younger stages may mix with S. setosa in the hydrographie factors and competition it is necessary to surface layer (Furnestin, 1938; Hansen, 1951; Jakobsen, review the geographical and vertical distribution of the 1971; Southward, 1983). Data on food taken by the two species, and assess the validity of the sampling two species (Rakusa-Suszczewski, 1969) also suggest method that has provided the 55-year data series now S. setosa is more of a surface-living form than available. S. elegans (p. 16). Even in well-mixed inshore waters 230 FLUCTUATIONS IN CHAETOGNATHS there can be a separation in time, one species or the Calibrations are checked from time to time with a other appearing as a seasonal influx that can be linked benthos bathykimograph attached to the frame, and with hydrographie conditions (Kahn, Williamson, 1970; an Oceanics Instruments flow-meter placed in the O'Brien, 1976; see also Ibanez, 1982).

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