MARINE ECOLOGY PROGRESS SERIES Vol. 141: 139-147, 1996 Published October 3 ' Mar Ecol Prog Ser I Abundance of gelatinous carnivores in the nekton community of the Antarctic Polar Frontal Zone in summer 1994 'Alfred-Wegener-Institut fiir Polar- und Meeresforschung. ColumbusstraBe, D-27568 Bremerhaven, Germany 'British Antarctic Survey, NERC, High Cross, Madingley Road. Cambridge CB3 OET, United Kingdom ABSTRACT The species composition, abundance, vertical distribution, biovolume and carbon content of gelatinous nekton in the Antarctic Polar Frontal Zone are described from a series of RMT25 hauls collected from a series of 200 m depth layers between 0 and 1000 m. In total. 13 species of medusa, 6 species of siphonophore, 3 species of ctenophore and 1 species of salp and nemertean were ~dentified. On average gelatinous organisms contributed 69 3% to the biovolume and 30.3% to the carbon content of the samples, although the ranges were high (0 to 98.9":) and 0 to 62 G"/;,respectively). The most important contributor to the biovolume and carbon content was the scyphomedusan Pel-iphylla peri- phylla. Some specific assoc~ationsand restricted vertlcal d~stl-ibut~ons\vcrc related to trophic interac- tions among ostracods, amphipods and cnidarians. Observations made near South Georgla showed that medusae ancl ctenophores were preyed upon by albatrosses and notothen~oidflsh respectively. The results support the premise that gelatinous organisms are a major and, at tlmes, are the main compo- nent of the oceanic macroplankton/nekton community 111 the Southern Ocean KEY WORDS: Gelatinous nekton Southern Ocean Wet biomass Carbon content. Assemblages INTRODUCTION suspension feeding pelagic tunicates (salps), become dominant (Huntley et al. 1989) The nekton communities in the Southern Ocean Recently, the distribution and biology of gelatinous comprise 4 main groups of organisms: crustaceans, organisms have received more attention in the Antarc- mesopelagic fish, squid and gelatinous forms. Their tic (Pages et al. 1994, Piatkowski et al. 1994, Rodhouse relative importance in the pelagic ecosystem is usually et al. 1994. Siege1 & Loeb 1995) as it has become evi- reported in terms of biomass (i.e. Everson & Miller dent that this previously neglected group plays a sub- 1994, Kock & Shimadzu 1994). Antarctic krill Euphau- stantial role in the Southern Ocean ecosystem. It is dif- sia superba Dana is generally considered to be the key ficult to assess the role of gelatinous organisms using organism in the marine Antarctic ecosystem. More crit- conventional sampling methods, but observations from ical studies have shown that krill abundances can vary manned submersibles have elucidated behaviours and considerably; low biomass and total absence are some- associations that are impossible to ascertain from net times reported from areas of the Southern Ocean collected material (e.g. Janssen et al. 1989). Data on where it is normally abundant (Heywood et al. 1985) the distribution and biomass of the gelatinous During these conditions other organisms, for example macroplankton and nekton from the Southern Ocean is based mainly on information about salps, indicating --p that these are strong filter-feeders and may compete with krill for food resources (Reinke 'Correspondence address: Institut de Cii?ncies del Mar (CSIC), 1987, Huntley et Plaqa del Mar s/n, E-08039 Barcelona, Spain. a1 1989).Quantitative data, based on number of spec- E-mall: fpages@~cm.csic.es imens (for medusae and ctenophores), number of poly- 0 lnter-Research 1996 Resale of full article no1 permitted 140 Mar Ecol Prog Ser 141: 139-147, 1996 gastric stages and eudoxids (for calycophoran (Netherlands) 2800 pelagic trawl (head rope 93.2 m);a siphonophores), number of colonies (for physonect 2 m2 horizontal- Antarctic multiple plankton sampler siphonophores) and sexual stages (for salps and (H-AMPS); and a 1 m2 rectangular frame neuston net doliolids), are extremely sparse. In addition, the sys- (F-net). tematic data can be in doubt when the identifications The wet biovolume of plankton and nekton was have been made by those unfamiliar with the taxa. measured by displacement soon after the samples Nonetheless, from quantitative studies made in the came onboard. The larger zooplankters were sorted Southern Ocean, it is known that gelatinous organisms into major groups to characterise the size spectra, and show high species diversity, and relatively high abun- the gelatinous components segregated for more dances and biovolumes (Pages & Kurbjeweit 1994, detailed treatment. All gelatinous forms were pooled Pages et al. 1994, Pugh et al. in press). These studies to estimate blovolume and the percentage contribution are rare and more investigations are necessary to to the whole sample. demonstrate to biologists and modellers that gelati- The medusae and siphonophores, both asexual and nous organisms have to be taken into account when sexual stages, from the RMT25 samples were identi- investigating the whole pelagic ecosystem. fied to species level. Specific protocols were adopted in The main objective during the second leg of the RRS order to estimate the actual number of specimens of 'James Clark Ross' Cruise JR06 (February-March siphonophore present. Specimens of Rosacea sp. com- 1994) was to carry out a closely integrated physical and prise an opposed pair of identical nectophores, so the biological programme in order to investigate the riu~nberof nectophores found was rounded up to an predator-prey interactions in an area shown to be fre- even number and divided by 2. The numbers of bracts quented by squid predators. Squid represent the high- and gonophores of this species were not included in est trophic level in the epipelagic zone at the Antarctic the analyses as they remain attached to the stem. In the Polar Frontal Zone (APFZ) (Rodhouse & White 1995, case of Vogtia serrata (Moser) the number of nec- Rodhouse et al. 1996) but the biological collections tophores present on each animal is approximately 8 to consisted predominantly of mesopelagic fishes and 15. Since these are of variable size, and the smallest gelatinous organisms (British Antarctic Survey 1995). are likely to be lost through the meshes of the net, we This paper describes the species composition, verti- have assumed a standard value of 10 per animal. cal distribution and abundance of the gelatinous The British Antarctic Survey down-wire net monitor macroplankton and nekton in the vicinity of the Polar (DWNM) was used to control the sampling equipment Frontal Zone in the South Georgia sector of the South- and monitor water conditions while the RMT25 and H- ern Ocean (Fig. 1).Likewise its wet biovolume and car- AMPS nets were deployed. The volume of water fil- bon content were estimated and compared with those tered by each net was calculated by averaging the flow of non-gelatinous nekton. rate data over 2 s intervals and these values were aver- MATERIAL AND METHODS The samples were collected using an open- ing-closing rectangular midwater trawl with a 25 m2 aperture (RMT25) (cf Piatkowski et al. 1994). The nekton community was sam- pled from the surface to 1000 m in 200 m depth strata during daylight and after dark. In addition, target fishing was undertaken in biomass rich layers (50-100, 300-350, 375-400 m) and at 50 and 600 m. To sample a layer, the net was towed obliquely down through the water column by slowly paying out the warp, wh~lemaintaining the net speed at -3 kt (1.5 m S-'). The RMT25 was the main net used to col- lect quantitative data but additlonal samples were collected using other gear in order to sample depth strata and different Flg. 1 Western part of the Atlantic sector, Southern Ocean. (m) Sampling size spectra. These were: a Ymuiden Stores area, north of South Georgia Pages et al.: Abundance of gelatinous carnivores in Antarct~cnekton 14 1 aged to give the mean flow for the duration of a tow. Fig. 3 shows the occurrence and depth distribution of The durat~onwas derived from the 'net-state' signal gelatinous zooplankton collected at the sampling area. from the DWNM. Volume of water filtered by each net The scyphomedusan Periphylla per-iphylla (Peron & was then calculated using the average flow data and Lesueur) (up to 35 cm in diameter), the anthomedusan duration data, assuming a 25 m2 mouth area (C. Swan- Calycopsis borchgrevinki (Browne) (maximum diame- son pers. comm.). Filtered volumes for the pelagic ter 27 mm) and the siphonophore Rosacea sp. (defini- l trawl and F-net were estimated from swept area calcu- tive nectophore up to 19 mm in length) were the only lations. species collected in all depth ranges between 1000 m and the surface. P periphylla appeared to show a die1 vertical migration, with most of the specimens occur- RESULTS AND DISCUSSION ring between 400 and 1000 m during the day, but large ones were collected in the surface 200 m at night. Hydrography Other gelatinous organisms were mainly collected below 200 m (Fig. 3). Atolla u~yvilleiHaeckel (up to The sampllng area was located in the Antal-ctic Cir- 72 mm in diameter) and A. chuni Vanhoffen (maximum cumpolar Current between the Polar Front and the diameter 61 mm) were always collected between 600 Subantarctic Front. Potential temperature decreased and 1000 m during the day, but they also occurred in from 6.0°C at the surface to 3.0°C at 250 m depth the 400-600 m depth range during the night, which (Fig. 2). Sallnity was low (34.0 psu) at the surface suggests these species also exhibit a die1 upward (Stn 334) and increased smoothly to 34.4 psu at 500 m migration. The trachymedusae Halicreas minimum indicating the presence of Antarctic Intermediate Fewkes (25 mm in diameter) and CI-ossota brunnea Water. Potential temperature was very stable between Vanhoffen (maximum width 43 mm) showed the same 300 and 1000 m depth (2.5 and 2.3"C respectively).