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Salp/Krill Interactions in the Southern Ocean:Spatial Segregation and Implications for the Carbon flux

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Salp/Krill Interactions in the Southern Ocean:Spatial Segregation and Implications for the Carbon flux Deep-Sea Research II 49 (2002) 1881–1907 Salp/krill interactions in the Southern Ocean:spatial segregation and implications for the carbon flux E.A. Pakhomova,*, P.W. Fronemanb, R. Perissinottoc a Department of Zoology, University of Fort Hare, P/Bag X1314, Alice 5700, South Africa b Southern Ocean Group, Department of Zoology and Entomology, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa c School of Life and Environmental Sciences, University of Natal, Durban 4041, South Africa Abstract Available data on the spatial distribution and feeding ecophysiology of Antarctic krill, Euphausia superba, and the tunicate, Salpa thompsoni, in the Southern Ocean are summarized in this study. Antarctic krill and salps generally display pronounced spatial segregation at all spatial scales. This appears to be the result of a clear biotopical separation of these key species in the Antarctic pelagic food web. Krill and salps are found in different water masses or water mass modifications, which are separated by primary or secondary frontal features. On the small-scale (o100 km), Antarctic krill and salps are usually restricted to the specific water parcels, or are well segregated vertically. Krill and salp grazing rates estimated using the in situ gut fluorescence technique are among the highest recorded in the Antarctic pelagic food web. Although krill and salps at times may remove the entire daily primary production, generally their grazing impact is moderate (p50% of primary production). The regional ecological consequences of years of high salp densities may be dramatic. If the warming trend, which is observed around the Antarctic Peninsula and in the Southern Ocean, continues, salps may become a more prominent player in the trophic structure of the Antarctic marine ecosystem. This likely would be coupled with a dramatic decrease in krill productivity, because of a parallel decrease in the spatial extension of the krill biotope. The high Antarctic regions, particularly the Marginal Ice Zone, have, however, effective physiological mechanisms that may provide protection against the salp invasion. r 2002 Elsevier Science Ltd. All rights reserved. Re´ sume´ Les observations disponibles sur la distribution spatiale et l’ecophysiologie! de l’alimentation du krill antractique, Euphausia superba, et du tunicier Salpa Thompsoni dans l’Ocean! Austral sont synthetis! ees! dans cette etude.! Le krill et les salpes presentent! une distribution qui se traduit en gen! eral! par une forte segr! egation! spatiale, a" toutes les echelles! d’espaces. Ceci semble etre# le resultat! d’une separation! claire des niches ecologiques! de ces deux especes" cles! du reseau! trophique antarctique. Le krill et les salpes sont observes! dans des masses d’eau differentes! qui sont separ! ees! par des frontieres" primaires et secondaires. A petite echelle! (o100 km), soit le krill antarctique et les salpes sont habituellement localisee! dans des parcelles d’eaux specifiques,! ou alors ils sont separ! es! verticalement. Les vitesses de broutage du krill et des salpes, estimees! en utilisant la technique de fluorescence in situ, sont parmi les plus elev! ees! rencontrees! au sein du reseau! trophique pelagique! antarctique. Bien que le krill et les salpes peuvent parfois consommer l’ensemble de la production primaire journaliere," l’impact de leur activite! de broutage est gen! eralement! moder! e(! p50% de la production primaire). Les consequences! ecologiques! regionales! d’annees! caracteris! ees! par des fortes densites! de salpes peuvent etre# *Corresponding author. E-mail address: [email protected] (E.A. Pakhomov). 0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S 0967-0645(02)00017-6 1882 E.A. Pakhomov et al. / Deep-Sea Research II 49 (2002) 1881–1907 dramatiques. Si la tendance au rechauffement,! qui est observee! autour de la Peninsule! Antarctique et dans l’Ocean! Austral, continue, les salpes pourraient jouer un role# plus important dans la structuration des ecosyst! emes" marins antarctiques. A cela serait probablement associe! une diminution dramatique de la productivite! du krill, en raison d’une diminution de l’extension spatiale du biotope du krill. Cependant, aux hautes latitudes, et en particulier dans la zone marginale des glaces, il existe des mecanismes! physiologiques effectifs qui pourraient offrir une certaine protection vis a" vis d’une invasion par les salpes. 1. Introduction 1993a; Park and Wormuth, 1993; Kawamura et al., 1994; Nishikawa et al., 1995). With the exception The Antarctic krill, Euphausia superba, and the of only few areas, such as Antarctic Peninsula tunicate, Salpa thompsoni, are among the most region, spatial exclusion between Antarctic krill important filter-feeding metazoans of the Southern and salps has been widely documented (Pakhomov Ocean, ranking only after copepods in terms of et al., 1994a; Hosie, 1994; Voronina, 1998). Studies total dry pelagic biomass (Pages, 1997; Voronina, on the community structure of zooplankton 1998). These two species are also recognized as conducted in the vicinity of the Greenwich microphages of key importance, as they are able to meridian have indicated that Antarctic krill and efficiently re-package small particles into large fast salps may overlap in their distribution south of the sinking feces, thereby playing a major role in Antarctic Polar Front (Fransz and Gonzalez, channeling biogenic carbon from surface waters 1997; Pakhomov et al., 2000). Feeding studies into the long-living pools and to the ocean’s conducted in this region provided evidence that interior and seafloor (Huntley et al., 1989; Fortier these species may at times consume the entire daily et al., 1994; Schnack-Schiel and Mujica, 1994; primary production (Dubischar and Bathmann, Pakhomov et al., 1997; Le Fevre" et al., 1998; 1997; Perissinotto et al., 1997; Perissinotto and Perissinotto and Pakhomov, 1998a). As a conse- Pakhomov, 1998a; Froneman et al., 2000). Unlike quence, the ecological role of these two key species for case of krill, for which there are numerous data in the Antarctic pelagic food web has recently on its feeding ecology, there are still large gaps in received much attention (e.g., Nishikawa et al., our understanding of the biology and the ecologi- 1995; Siegel and Loeb, 1995; Siegel and Harm, cal role of salps in the Southern Ocean (Le Fevre" 1996; Dubischar and Bathmann, 1997; Loeb et al., et al., 1998). The aims of this paper are to 1997; Kawaguchi et al., 1998; Perissinotto and summarize studies on the trophic ecophysiology Pakhomov, 1998b; Ross et al., 1998). It has been of Antarctic krill and of the tunicate S. thompsoni, suggested that krill and salps may be in direct with particular emphasis to the tunicates, and to competition with one another in certain areas of discuss the phenomenon of krill/salp spatial the Antarctic Peninsula (Loeb et al., 1997) and in separation. the Lazarev and Cooperation Seas (Perissinotto and Pakhomov, 1998a, b). It also was tentatively postulated that if the increase in seawater tem- 2. Materials and methods perature, already observed in the Antarctic Penin- sula region, continues (Zwally, 1991; Rott et al., Data on ingestion rates of Antarctic krill, E. 1996), salps may spread into the high Antarctic superba, and of the tunicate, S. thompsoni, regions, with important implications for the estimated using the gut fluorescence technique regional carbon flux and the Antarctic food web were obtained mainly during the three expeditions structure (Perissinotto and Pakhomov, 1998a, b). to the southern part of the Atlantic sector of the In the Southern Ocean, S. thompsoni is generally Southern Ocean:(a) the second cruise of the South restricted to the warmer water masses (Voronina, African Antarctic Marine Ecosystem Study 1984; Nast, 1986; Siegel et al., 1992; Pakhomov, (SAAMES II) conducted aboard the mv SA E.A. Pakhomov et al. / Deep-Sea Research II 49 (2002) 1881–1907 1883 Agulhas along the Greenwich Meridian (WOCE CmÀ2) and from 1 to 1200 mg C mÀ2 (mean SR2 line) in January 1993 (Perissinotto et al., 1997; 2207320 mg C mÀ2) in the regions of dense and Froneman et al., 2000; Pakhomov et al., 2000); (b) low krill concentrations, respectively (Voronina, the SAAMES IV aboard the mv SA Agulhas in the 1998). Lazarev Sea during December 1994–January 1995 Although Southern Ocean tunicates have not (Froneman et al., 1997; Perissinotto and Pakho- been targeted specifically, over the past two mov, 1998a, b); (c) the joint Scandinavian/South decades a substantial amount of data on S. African Antarctic Research Expedition aboard the thompsoni density has been accumulated in differ- mv SA Agulhas along the 61E meridian between ent sectors of the Southern Ocean (Table 1). In 491S and 601300S (Pakhomov, 2002; Pakhomov summary, these data indicate extreme variability in and Froneman, 2002a, b). salp densities across the Southern Ocean (Table 1). In addition, numerous published and unpub- Nevertheless, throughout much of the area south lished sources (mentioned throughout the text) on of 401S, S. thompsoni densities remain moderate, krill and salps ingestion rates, abundances, bio- on average varying between o0.1 and 30 mg mass and distribution in the Southern Ocean were CmÀ2, or between o0.1 and 30 ind mÀ2. However, used for this synthesis. To convert wet weight into S. thompsoni densities in the Antarctic Peninsula carbon weight, the following conversion factors region, particularly in the Bransfield Strait and were used:(a) S. thompsoni dry weight was around Elephant Island, were found to be assumed to be 4% of wet weight, and carbon consistently elevated (Table 1). Furthermore, the weight was assumed to be 4.3% of dry weight secondary frontal systems that demarcate low and (Ikeda and Mitchell, 1982; Ikeda and Bruce, 1986; high latitude water masses around the Antarctic Hagen, 1988; Huntley et al., 1989; Donnelly et al., Continent, e.g., Weddell–Scotia Confluence, 1994); (b) E. superba dry weight was assumed to be Warm Counter Weddell Current in the Lazarev 22% of wet weight, and carbon weight assumed to Sea, the northern part of the Ross Sea, also be 45% of dry weight (Ikeda and Mitchell, 1982; showed enhanced densities of S.
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