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Pelagic Community Structure of the Subtropical Convergence Region South of Africa and in the Mid-Atlantic Ocean1

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Pelagic Community Structure of the Subtropical Convergence Region South of Africa and in the Mid-Atlantic Ocean1 DEEP-SEA RESEARCH P a r t I PERGAMON Deep-Sea Research I 45 (1998) 1663-1687 Pelagic community structure of the subtropical convergence region south of Africa and in the mid-Atlantic Ocean1 M. Barange3’*, E.A. Pakhomovb, R. Perissinottob, P.W. Fronemanb, H.M. Verheye3, J. Taunton-Clark3, M.I. Lucas0 a Sea Fisheries Research Institute, Private Bag X2, Rogge Bay 8012, South Africa b Southern Ocean Group Department of Zoology and Entomology, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa c Southern Ocean Group, SANAP and Marine Biology Research Institute, Zoology Department, University o f Cape Town, Rondebosch 7700, South Africa Received 1 November 1994; in revised form 1 November 1996 A b s tra c t Cross-frontal changes in the microphytoplankton, Zooplankton and micronekton species composition and biomass were investigated in two sectors of the Subtropical Convergence region (STC) to evaluate patterns in the pelagic community in areas of contrasting hydro- dynamic structure. The first sector was south of Africa ( + 20°E, winter 1993) where the frontal zone is relatively permanent and intense. The other sector was in the mid-Atlantic ocean ( ± 2°E, summer 1994) where the STC is ephemeral and weak. Higher biological diversity and weaker zonation patterns were observed in the mid-Atlantic sector, relative to the sector south of Africa. This indicates that the boundaries of the STC were more relaxed in the former region, suggesting that the structure in the mid-Atlantic community is less controlled by hydrodynamic forcing. In both sectors, species of Antarctic and subtropical origin were present on both sides of the convergence, suggesting that cross-frontal mixing was prevalent. Changes in the relative proportion of microphytoplankton, micro- and mesozooplankton in both regions appear to reflect the seasonality of sampling, rather than regional differences in the pelagic food web structure. Despite the marked contrast in the intensity of the hydrographic front between the two sectors, higher phytoplankton, Zooplankton and mesopelagic fish abundances were *Corresponding author. Fax: 0027 21 21 7406; e-mail: [email protected] LThis paper is dedicated to the memory of the late Derek A. Krige, Master of the Sea Fisheries Research Institute’s FRS Africana since its commission in 1982, who died while this paper was in preparation. We would like to pay tribute to his seamanship and commitment to scientific excellence that firmly established Africana's reputation as a highly successful research ship. 0967-0637/98/$- see front matter /(/ 1998 Elsevier Science Ltd. All rights reserved. PII: S0967-0637(98)00037-5 1664 M. Barange et aí. / Deep-Sea Research I 45 (1998) 1663 -1687 consistently associated with the Subtropica] Convergence, reflecting the importance of this region in the pelagic production of the south Atlantic Ocean. © 1998 Elsevier Science Ltd. All rights reserved. 1. Introduction The Subtropical Convergence (STC) is a frontal zone that separates subantarc- tic waters of the west wind drift in the south from subtropical waters to the north (Lutjeharms and Valentine, 1984). It is characterized by strong horizontal tem per­ ature and salinity gradients (Deacon, 1982; Lutjeharm s and Valentine, 1984), and separates water masses of different physico-chemical properties (Allansonet ai, 1981; Lutjeharms et al., 1993). The STC also exhibits biomass and production enhance­ ments and dramatic changes in the diversity and composition of the phytoplankton (Allanson et ai., 1981; Deacon, 1982; H ara and Tanoue, 1985; Lutjeharm s and W alters, 1985; Comisoet al., 1993; Laubscher et ai, 1993; Sullivanet al., 1993; Weeks and Shillington, 1994) Zooplankton (Lomakina, 1964; Casareto and Nem oto, 1985; Pakhom ov et al., 1994), cephalopods (Robertson et al., 1978; Voss, 1985), fish (Roberts, 1980; Smith and Francis, 1982; Bekker, 1985; Bailey, 1989; Serra, 1991), and even birds (Nakamura, 1983; Abrams, 1985). It therefore constitutes a major biogeo­ graphic boundary, and because of its large geographical extent it is a significant contributor to the global ocean production. For example, Dower and Lucas (1993) estimated that the region of the STC south of Africa was responsible for between 0.5 and 0.8% of the world’s total primary production in the open ocean. Several hypotheses have been advanced to explain the features of the STC. Lut­ jeharms and Walters (1985) observed that the surface and subsurface expressions of the front seldom coincide, and suggested that the sloping front could generate a thermal stability capable of retaining and concentrating phytoplankton (as in Franks, 1992). The authors indicated that stability also may be promoted by the mixing of warm, nutrient-poor subsurface waters across the STC. Laubscheret al. (1993), on the other hand, concluded that the enhancement of phytoplankton biomass within the front cannot be solely a consequence of passive transport, because of the mono-specificity of the observed blooms. This would suggest a scenario where surface waters are enriched with macronutrients through horizontal advection (Lutjeharms and Walters, 1985) or through surface divergence (Butler et al., 1992). Recent re-analyses of Coastal Zone Colour Scanner (CZCS) data have shown an intensification in the occurrence of STC phytoplankton blooms downstream of continental masses (Comisoet ai, 1993). It has been suggested that this may be the result of an input of dissolved iron from shelf sediments (Sullivanet ai, 1993; De Baar et ai, 1995). There are also some indications suggesting that the unique and specific pelagic fauna associated with the STC (Bartle, 1976) may have developed as a result of the interaction of the different water masses meeting in this region (Frontier, 1977; Zubova and Timofeev, 1991). M. Barange et aí. / Deep-Sea Research ¡45 (1998) 1663-1687 1665 2 0 S 18 20 E / AS A94 • 7 A 103 • 9 6 • 6 A 9 7 • 98 • 99 AS A100 • 4 • 3 LONGITUDE ('E ) • 2 LONGITUDE ( ’E) Fig. 1. Multichannel Sea Surface Temperalure ( 'C, MCSST) obtained from the Advanced Very High Resolution Radiometer (AVHRR) satellite sensor during the period of the surveys, (a) July 1993, sector to the south of Africa; (b) March 1994, mid-Atlantic sector. Triangles indicate stations where CTD casts, phytoplankton collections and Bongo tows were carried out. Circles indicate stations where only phyto­ plankton collections were made. Squares indicate stations where only Bongo tows were undertaken. The meridional position of the STC and the intensity of its hydrographic gradient varies with longitude along its circumglobal extension (Shannonet al, 1990; Sullivan et al., 1993), and the thermal gradient is particularly intense in the proximity of the continents, where most observations have been recorded. Shannonet al. (1990) noted 1666 M. Barange et al. / Deep-Sea Research I 45 (1998) 1663 1687 that the average position of the STC has a narrow meridional range south of Africa, as opposed to its highly variable position in the mid-Atlantic sector. Lutjeharmset aí. (1993) concluded that in this latter region the STC may be ephemeral, rather than weak. Multi-channel Sea Surface Temperature (MCSST) charts from winter 1993 and late summer 1994 (Fig. 1) show that this hydrographic feature is consistently more intense in the sector south of Africa than in the mid-Atlantic sector. Variability in the intensity and dynamics of the frontal zone may have direct implications for its effectiveness as a boundary and for the biological productivity of the region. The objective of this study is to compare the pelagic community structure, from phyto­ plankton to fish, of the Subtropical Convergence region in the mid-Atlantic and south of Africa. To our knowledge it represents the first direct comparison between the pelagic community of the STC in the mid-ocean and in the proximity of a land mass. Special attention is given to the effects of contrasting hydrographic variability on the species composition and biomass distribution across the STC. 2. Materials and methods The sector of the STC to the south of Africa (SAS, approximately 20°E) was investigated during the third South African Antarctic Marine Ecosystem Study (SAAMES III) cruise, aboard and MV Agulhas (Voyage 72, July 1993), fromca 39.5 S to 42°S (Fig. 1). The mid-Atlantic sector (MAS, ca 2°E) was studied during the northbound leg of voyage 119 aboard the FRSAfricana to South Georgia in March 1994, from ca 43.5°S to 39.5 S (Fig. 1). In addition, acoustic and SST data also were collected during the southbound leg of this cruise, betweenca 40° and 42.5 S. Water samples for the analysis of biological and physico-chemical parameters were taken using a shipboard pump (Iwaki Magnet Pump). The pump inlet was positioned 5 m below the sea surface, and seawater was pumped to the laboratory through PVC piping. Underway sea surface temperature and salinity readings were obtained from thermosalinograph sensors. Phytoplankton biomass was estimated from chloro­ phyll-« and phaeopigment measurements. For this purpose, 11 seawater samples were filtered through GF/F Whatman filters, and pigments were then extracted in 5 ml 90% acetone for 12 h. The fluorescence of the extract was measured with a Turner Designs fluorometer, before and after acidification (Mackas and Bohrer, 1976), and pigment concentrations were calculated according to Strickland and Parsons (1968) as modified by Conover et al. (1986). For the analysis of the composition and abundance of microphytoplankton and microzooplankton, a 20-pm mesh filtration unit (Berman and Kimor, 1983) was connected to the pump outlet and a constant volume of 20 1 of sea water was filtered at each station. Plankton retained by the filter were preserved in 2% buffered formalin and enumerated and identified using a Nikon TMS inverted microscope at 400 x magnifica­ tion. A minimum of 500 cells, or 100 microscopic fields, were counted for each sample. Zooplankton samples were collected using a 300-pm mesh Bongo net towed obliquely from 300m depth to the surface at towing speeds of 1.5-2.0m s '1.
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