Pelagic community structure of the subtropical convergence region south of Africa and in the mid-Atlantic ocean

Deep-Sea Research I 45 (1998) 1663—1687

Pelagic community structure of the subtropical convergence region south of Africa and in the mid-Atlantic Ocean M. Barange *, E.A. Pakhomov
, R. Perissinotto
, P.W. Froneman
, H.M. Verheye , J. Taunton-Clark , M.I. Lucas Sea Fisheries Research Institute, Private Bag X2, Rogge Bay 8012, South Africa
Southern Ocean Group Department of Zoology and Entomology, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa  Southern Ocean Group, SANAP and Marine Biology Research Institute, Zoology Department, University of Cape Town, Rondebosch 7700, South Africa Received 1 November 1994; in revised form 1 November 1996

Abstract 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 hydrodynamic 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 ($2°E, summer 1994) where the STC is ephemeral and weak. Higher biological diversity and weaker zonation patterns were observed in the mid-Atlantic sector, relative to the sector south of Africa. This indicates that the boundaries of the STC were more relaxed in the former region, suggesting that the structure in the mid-Atlantic community is less controlled by hydrodynamic forcing. In both sectors, species of Antarctic and subtropical origin were present on both sides of the convergence, suggesting that cross-frontal mixing was prevalent. Changes in the relative proportion of microphytoplankton, micro- and mesozooplankton in both regions appear to reflect the seasonality of sampling, rather than regional differences in the pelagic food web structure. Despite the marked contrast in the intensity of the hydrographic front between the two sectors, higher phytoplankton, zooplankton and mesopelagic fish abundances were —————— *Corresponding author. Fax: 00 27 21 21 7406; e-mail: [email protected] This paper is dedicated to the memory of the late Derek A. Krige, Master of the Sea Fisheries Research Institute’s FRS Africana since its commission in 1982, who died while this paper was in preparation. We would like to pay tribute to his seamanship and commitment to scientific excellence that firmly established Africana’s reputation as a highly successful research ship. 0967-0637/98/$—see front matter  1998 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 9 8 ) 0 0 0 3 7 - 5

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consistently associated with the Subtropical 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 subantarctic 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 temperature and salinity gradients (Deacon, 1982; Lutjeharms and Valentine, 1984), and separates water masses of different physico-chemical properties (Allanson et al., 1981; Lutjeharms et al., 1993). The STC also exhibits biomass and production enhancements and dramatic changes in the diversity and composition of the phytoplankton (Allanson et al., 1981; Deacon, 1982; Hara and Tanoue, 1985; Lutjeharms and Walters, 1985; Comiso et al., 1993; Laubscher et al., 1993; Sullivan et al., 1993; Weeks and Shillington, 1994) zooplankton (Lomakina, 1964; Casareto and Nemoto, 1985; Pakhomov 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 biogeographic 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. Lutjeharms 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. Laubscher et 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 (Comiso et al., 1993). It has been suggested that this may be the result of an input of dissolved iron from shelf sediments (Sullivan et al., 1993; De Baar et al., 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).

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Fig. 1. Multichannel Sea Surface Temperature (°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 phytoplankton 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 (Shannon et 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. Shannon et al. (1990) noted

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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. Lutjeharms et al. (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 phytoplankton 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), from ca 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 FRS Africana 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, between ca 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 chlorophyll-a and phaeopigment measurements. For this purpose, 1 l 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-lm mesh filtration unit (Berman and Kimor, 1983) was connected to the pump outlet and a constant volume of 20 l 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;magnification. A minimum of 500 cells, or 100 microscopic fields, were counted for each sample. Zooplankton samples were collected using a 300-lm mesh Bongo net towed obliquely from 300 m depth to the surface at towing speeds of 1.5—2.0 m s\. An electronic flowmeter mounted off-centre in the mouth of the net provided

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information on the volume of water filtered per tow. Samples were preserved in 5% buffered formalin. Aliquots of 1/10 of the original samples were obtained with a Wiborg’s modified whirling apparatus (Kott, 1953), and the major zooplankton taxa were identified and counted. Animals larger than 20 mm (e.g. tunicates, euphausiids and fish) were counted from entire samples and identified to species level. Acoustic data were obtained with an EK500 echo-sounder, operated at 120 kHz on MV Agulhas and at both 38 and 120 kHz on FRS Africana. These frequencies are suitable for the detection of individual scatterers of ca 25 and 8 mm, respectively, as well as of aggregations of smaller organisms. The echo-sounders were calibrated using standard calibration spheres (Foote et al., 1987) in shelf waters off Cape Town (MV Agulhas) and in waters around South Georgia (FRS Africana). Acoustic backscattering strength was integrated at intervals of 5 nautical miles (MV Agulhas) or 2 nautical miles (FRS Africana), and in several depth strata, from the surface to 250 m. Acoustic targets were identified from Bongo net collections. In addition, Polish Krill Trawl 1641 collections were taken in the region of maximum scattering, during the MAS cruise. However, in the absence of accurate estimates of target strength for the various components of the complex scattering community, data are expressed in units of acoustic reflectivity (m nm\), providing a continuous record of the fine structure of the macrozooplankton and fish populations across the STC. The distribution of backscattering energy in the vertical (depth) and horizontal (distance) scales was obtained using a SURFER graphic package, employing isotropic linear kriging routines for interpolations. To compare the plankton communities sampled at the different stations, cluster analyses were carried out using the Shorigin similarity index K (Shorigin, 1952) with 1 pair-group single linkage (nearest neighbour clustering), L K " min r(i, j ), 1  where min r(i, j ) is the minimal percent abundance of a species r in two samples i and j, and n is the number of species common to both samples i and j. 3. Results Along the transect in the sector south of Africa (SAS), the STC was characterized by a strong temperature gradient (ca 8°C change in SST in 14 nm), extending from the surface to below 300 m depth (Figs. 2a and 3a). In contrast, there was little evidence of a thermal front in the mid-Atlantic sector (MAS, Fig. 2b, see also Fig. 8a), where the temperature gradient was less pronounced (ca 4°C change in SST in 200 nm), and exhibited only in surface waters ((100 m). 3.1. Sector south of Africa During July 1993, chlorophyll-a concentrations increased across the convergence, peaking in the southern (cold) side (Fig. 3b). Phaeopigment levels were relatively high

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Fig. 2. Cross-section of sea surface temperature profiles of the transects sampled in the sector to the south of Africa (a) and the mid-Atlantic sector (b). Dots denote station positions.

(0.25—0.35 lg l\) on both sides of the STC and lower near its centre (Fig. 3b). A general trend of decreasing microphytoplankton abundance from north to south was observed across the frontal zone (Fig. 3c). The contrasting patterns between the distribution of microphytoplankton and chlorophyll-a reflected the predominance of the nano- and pico-size fractions in the phytoplankton assemblage (Lutjeharms et al., 1994). A substantial increase in microzooplankton was recorded at the southern edge of the front, although densities were generally low along the transect (Fig. 3c).

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Fig. 3. Sea surface temperature and salinity (A), surface chlorophyll-a and phaeopigment concentrations (B), surface micro-phytoplankton and zooplankton densities (C), and mesozooplankton densities (D), along the transect to the south of Africa. Surface temperature and salinity were obtained from the ship’s underway sensor. Also indicated is the distance from the start of the acoustic data logging.

Mesozooplankton abundance increased markedly within the region of the frontal zone (Fig. 3d). Cluster analysis of the numerical distribution of microphytoplankton taxa (Fig. 4) showed a similar composition between stations located south of ca 40°S. These stations were dominated by the chain-forming diatoms Pseudoeunotia doliolus and ¹halassiosira spp., which accounted for 51.7—93.1% of total cells counted (Table 1). Stations north of 40°S (St. 113 and 114, Fig. 4) were dominated by Chaetoceros constrictus, which comprised 40.8—51.9% of the total abundance. Dinoflagellates (including species of the genera Dinophysis, Ceratium and Peridinium) and tintinnids were poorly represented along the transect, while aloricate ciliates accounted for ca 60% of all microzooplankton cells. A clear microplanktonic latitudinal boundary appeared therefore to be located at the northern edge of the STC. The two main groups within the mesozooplankton community also were identified by cluster analysis, the main separation occurring in the region of maximum

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Fig. 4. Dendrogram of similarities between stations in the sector south of Africa obtained from microphytoplankton (left panel) and mesozooplankton (right panel) species composition. Also shown are stations numbers and cluster groupings (see also Fig. 1). Table 1 Microphytoplankton species composition and abundance across the STC in the sector South of Africa Only species contributing *5% to the total abundance are listed. cf"chain-forming

St.

No. Spp

Abundance (cells l\)

114

31

1256

113

26

1525

86 108 94 103 96 97 98

12 22 18 15 21 15 11

276 677 725 702 739 471 59

99 100

16 18

598 696

Species composition and precentage contribution Chaetoceros constrictus (40.8%), Chaetoceros spp. (10.2%), Chaetoceros compressus (7.3%), Rhizosolenia stolterfothii (6.9%), Chaetoceros messanensis (5.8%) Chaetoceros constrictus (51.9%), Pseudoeunotia doliolus (c.f.) (11.2%), Climacodium frauenfeldianum (5.9%) Pseudoeunotia doliolus (c.f.) (79.3%), ¹halassiosira spp. (12.2%) Pseudoeunotia doliolus (c.f.) (76.7%), ¹halassiosira spp. (12.0%) Pseudoeunotia doliolus (c.f.) (86.6%), ¹halassiosira spp. (5.2%) Pseudoeunotia doliolus (c.f.) (86.0%), ¹halassiosira spp. (7.2%) Pseudoeunotia doliolus (c.f.) (82.2%), ¹halassiosira spp. (6.1%) Pseudoeunotia doliolus (c.f.) (85.1%), ¹halassiosira spp. (7.6%) Pseudoeunotia doliolus (c.f.) (40.8%), Nitzschia spp. (c.f.) (29.9%), ¹halassiosira spp. (10.9%), Rhizosolenia bergonii (5.4%) Pseudoeunotia doliolus (c.f.) (83.4%), ¹halassiosira spp. (11.5%) Pseudoeunotia doliolus (c.f.) (86.7%), ¹halassiosira spp. (6.0%)

hydrographical gradient (Fig. 4). The small calanoid copepod Clausocalanus spp., and to a lesser extent Pleuromamma spp. (mostly P. abdominalis), formed the bulk of the mesozooplankton in the region north of the STC (Table 2). Several pelagic

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Fig. 5. Macrozooplankton composition and abundance along the transect to the south of Africa during July 1993. A: euphausiids, B: tunicates and fish. Arrow indicates the position of the STC frontal zone.

crustaceans and tunicates of subtropical origin (e.g. Eucalanus longipes, E. sewellii, E. pileatus, Iasis zonaria and Stomatopoda larvae) also were found at these stations, enriching the diversity of this area. South of the front the zooplankton was dominated by Pleuromamma abdominalis and Metridia lucens (Table 2), and within the convergence, where the highest zooplankton abundance was recorded, Metridia lucens comprised almost 50% of the total number. The macrozooplankton communities on either side of the STC were markedly different (Fig. 5). Euphausia recurva dominated the euphausiid community to the north of the convergence, while subtropical species, with a well-known affinity for the STC region (Lomakina, 1964), such as Euphausia similis, E. spinifera and Nematoscelis megalops, were prevalent in the south (Fig. 5a). Pyrosoma sp., salps and mesopelagic fish were also most abundant in the northern region, including the edge of the STC (Fig. 5b), although about 50% of the fish species were of subantarctic origin (Table 3). Myctophiids formed the dominant component of this community.

No. 1000 m\

67 265.3 — — 3591.8 4408.2 24 816.3 3755.1 5877.5 — 2122.4 9469.4 1632.6 1306.1 6693.9 7349.9 14 040.8 11 591.8

Species composition

Copepoda Rhincalanus nasutus Eucalanus sewellii Calanus australis Calocalanus spp. Clausocalanus spp. Euchaeta spp. ¸ucicutia spp. Metridia lucens Scolecithrecella spp. Pleuromamma abdominalis Corycaeidoe gen. spp. Oithona spp. Oncaea spp. Euphausiacea Ostracoda Chaetognatha

39°36S—St. 114

58.3 — — 3.1 3.8 21.6 3.3 5.1 — 1.8 8.2 1.4 1.1 5.8 6.3 9.7 10.4

% 43343.7 771.7 900.3 2958.2 2443.7 14 147.9 1800.6 3601.3 771.7 1543.4 7331.2 1157.5 1672.0 — 9774.9 6945.3 2572.3

No. 1000 m\

40°03S—St. 91

61.5 1.1 1.3 4.2 3.5 20.0 2.5 5.1 1.1 2.2 10.4 1.6 2.4 — 13.8 9.8 3.6

% 41 494.6 44.6 — 1963.2 1115.4 6514.2 2186.3 5354.1 89.2 5755.7 8298.9 624.6 2453.9 2543.2 4998.3 7451.2 5443.4

No. 1000 m\

40°08S—St. 86

62.7 (0.1 — 3.0 1.7 9.9 3.3 8.1 0.1 8.7 12.6 0.9 3.7 3.9 7.6 11.3 8.3

%

245 345.0 — — — 876.2 45 856.0 2044.5 — 115370.6 — 71 187.7 — 876.2 4965.3 5943.8 2336.6 2920.7

No. 1000 m\

40°20S—St. 108

95.2 — — — 0.3 17.8 0.8 — 44.8 — 28.8 — 0.3 1.9 2.4 0.9 1.1

%

Table 2 Species composition and abundance of the major zooplanktonic taxa collected in the STC sector South of Africa using 300 lm Bongo nets (integration over the upper 300 m of the water column)

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No. 1000 m\

39 003.5 — — 187.9 187.9 5733.1 563.9 93.9 25 939.8 187.9 5827.1 — 93.9 93.9 164.5 187.9 949.2

Species composition

Copepoda Rhincalanus nasutus Eucalanus sewellii Calanus australis Calocalanus spp. Clausocalanus spp. Euchaeta spp. ¸ucicutia spp. Metridia lucens Scolecithrecella spp. Pleuromamma abdominalis Carycaeidae gen. spp. Oithona spp. Oncaea spp. Euphausiacea Ostracoda Chaetognatha

40°35S—St. 94

96.3 — — 0.5 0.5 14.2 1.4 0.2 64.0 0.5 14.4 — 0.2 0.2 0.3 0.5 2.4

% 28 043.6 — — 167.9 — 1847.2 923.6 251.9 7052.9 755.7 14 441.6 — 83.9 335.8 356.8 1763.2 1011.8

No. 1000 m\

40°47S—St. 103

88.6 — — 0.5 — 5.8 2.9 0.8 22.3 2.3 45.7 — 0.3 1.1 1.1 5.6 3.3

% 9895.4 — — 146.4 460.2 2761.5 146.4 20.9 41.8 20.9 5230.1 20.9 1004.2 — 58.5 41.8 944.6

No. 1000 m\

41°13S—St. 97

87.6 — — 1.3 4.1 24.4 1.3 0.2 0.4 0.2 46.2 0.2 8.9 — 0.5 0.4 8.4

% 39 043.5 — — 313.6 — 7369.7 313.6 — 6115.2 — 24 304.2 — 156.8 — 493.9 1097.6 470.4

No. 1000 m\

41°50S—St. 100

94.0 — — 0.8 — 17.7 0.8 — 14.7 — 58.4 — 0.4 — 1.2 2.6 1.1

%

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Table 3 Species composition, relative abundance and origin of the fish collected with Bongo tows during the STC cruise in the sector South of Africa SAZ"Subantarctic Zone, STZ"Subtropical Zone, STC"Subtropical Convergence Taxa

Individuals

Percentage

Origin

Muraenidae Leptocephal

2

2.7

?

Notosudidae ¸uciosudis normani Scopelosaurus sp.

1 1

1.3 1.3

? (SAZ)

Melanostomiidae Bathophilus sp.

1

1.3

STZ

Myctophidae Hygophum hygomii Diogenichthys atlanticus ¸obianchia dolfeni Scopelopsis multipunctatus Benthosema suborbitale Diaphus taaningi Diaphus meadi Diaphus hudsoni Diaphus sp. (of D.indicus?) Diaphus spp. Gymnoscopelus bolini Protomyctophum normani Protomyctophum (normani?) sp. Ceratoscopelus warmingi ¸ampanyctus australis ¸ampanyctus alatus ¸ampanyctus pusillus ¸ampanyctus sp. ¸ampadena pontifex Notolychnus valdiviae Symbolophorus evermanni Symbolophorus boops

6 3 1 2 4 1 2 1 1 2 2 18 2 10 2 3 1 2 2 2 1 1

8.1 4.1 1.3 2.7 5.4 1.3 2.7 1.3 1.3 2.7 2.7 24.3 2.7 13.5 2.7 4.1 1.3 2.7 2.7 2.7 1.3 1.3

Total

74

100.0

STZ, STC 50°N—48°S 50°N—40°S 25°S—STC 50°N—50°S STZ STC SAZ ? ? SAZ SAZ SAZ—STC 42°N—45°S SAZ—STC STZ—STC STZ ? STZ 56°N—40°S STZ SAZ—STC

The continuous record of acoustic backscattering along the transect (Fig. 6) reflects the zooplankton distribution, showing high backscattering strength in the northern edge of the STC. It is likely that the copepod Metridia lucens, the euphausiids E. recurva and E. spinifera and several myctophiid species were the main scatterers responsible for the strong acoustic signal recorded in this area. 3.2. Mid-Atlantic sector The continuous acoustic record of the southbound leg of the mid-Atlantic cruise (February 1994) across the STC (Fig. 7a) indicated a marked increase in pelagic

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Fig. 6. Three-dimensional and contour plots of acoustic backscattering strength at 120 kHz along the transect to the south of Africa. Echoes were integrated over 5 nm intervals and the following layers: 25—50, 50—100, 100—150, 150—200 and 200—250 m depth. A noise reduction of 6 dB was applied. The layer 7—25 m was discarded due to heavy aeration caused by bad weather conditions. The arrow across the contour plot indicates the position of the maximum SST gradient. Black areas on the x-axis denote periods of darkness.

biomass in the region of the convergence. The acoustic backscattering integrated over horizontal and vertical intervals of 2 n.miles and 10 m depth, respectively, shows high backscattering strengths in the upper 100 m of the water column just south of the zone of maximum SST decrease (Fig. 7b). Some deep scattering also was observed at depths in excess of 150 m (around mile 100, Fig. 7a), which is indicative of the vertical ascent of migratory scatterers early in the evening. Although no target identification was effected, the strong signature recorded during this leg prompted a more detailed study of the area during the northbound leg of the cruise, approximately four weeks later. During the northbound leg, temperature and salinity increased with decreasing latitude at a much slower rate than was observed along the SAS transect (Fig. 8a). This increase was limited to surface waters, while below 100 m depth a constant temperature of ca 10°C was observed throughout the transect (Fig. 2b). The hydrographical

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Fig. 7. Three-dimensional and contour plots of acoustic backscattering strength of 38 kHz along the southbound transect in the mid-Atlantic sector (A). Echoes were integrated over 2 nm intervals and 10-m depth layers from 50 to 250 m depth. A noise reduction of 12 dB was applied. The arrow across the contour plot indicates the position of the maximum SST gradient. Black areas on the x-axis denote periods of darkness. Also shown are sea surface temperature (SST) and sigma-t along the transect (B), to identify the position of the STC.

expression of the convergence was therefore less distinctive and the spatial gradient extended much further in the MAS than in the SAS, at least in terms of temperature (see also Fig. 1).

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Fig. 8. Sea surface temperature and salinity (A), surface chlorophyll-a and phaeopigment concentrations (B), surface micro- phytoplankton and zooplankton densities (C), and mesozooplankton densities (D), along the northbound transect in the mid-Atlantic sector. Also shown is the distance from the start of the acoustic data logging.

Surface chlorophyll-a, measured at intervals of ca 30 min, showed a trend similar to that recorded along the SAS transect, with concentrations increasing across the front and peaking at its southern edge (Fig. 8b). Phaeopigment concentrations, however, showed an opposite trend to that observed for chlorophyll-a (Fig. 8b). Levels for both pigments exhibited large variability in the region of the front. Microphytoplankton and microzooplankton biomass largely covaried with the distribution of chlorophyll-a, and were most abundant at the southern edge of the front (Fig. 8c, Table 4). However, while absolute chlorophyll-a concentrations were close to those measured along the SAS transect, microphytoplankton and microzooplankton abundances were approximately twice as high. Mesozooplankton abundances showed a general decrease southwards (Fig. 8d), contrasting with the increase observed in the microphytoplankton and microzooplankton communities. Both mesoand macrozooplankton densities were substantially lower than those along the SAS transect (Table 5).

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Table 4 Microphytoplankton species composition and abundance across the STC in the Mid-Atlantic sector. Only species contributing *5% to the total abundance are listed c.f."chain-forming

St.

No. Spp

Abundance (cells l\)

1

19

2628

2

15

2293

3

24

412

4

22

291

5

18

294

6

17

140

7

23

59

8

17

66

9

14

68

Species composition and precentage contribution Chaetoceros peruvianum (21.5%), Chaetoceros spp. (15.6%), Chaetoceros dichaeta (13.9%), Pseudoeunotia doliolus (c.f.) (13.9%), Odontella weissflogii (6.9%), Nitzschia closterium (5.8%) Chaetoceros peruvianum (50.4%), Pseudoeunotia doliolus (c.f.) (22.9%), Odontella weissflogii (6.3%), Chaetoceros dichaeta (6.1%), Nitzschia closterium (4.9%) Chaetoceros peruvianum (15.6%), Odontella weissflogii (14.9%), Chaetoceros dichaeta (14.2%), Nitzschia spp. (c.f.) (12.9%), Chaetoceros spp. (8.5%), Rhizosolenia bergonii (6.8%), Nitzschia closterium (6.4%) Chaetoceros peruvianum (19.4%), Odontella weissflogii (14.6%), Rhizosolenia bergonii (13.2%), Nitzschia spp. (c.f.) (12.8%), Chaetoceros dichaeta (6.3%), Nitzschia closterium (5.9%), Pseudoeunotia doliolus (c.f.) (5.3%), Ecampia antarctica (4.8%), Nitzschia spp. (c.f.) (18.0%), Chaetoceros peruvianum (16.8%), Chaetoceros dichaeta (15.6%), Rhizosolenia bergonii (12.9%), Odontella weissflogii (8.7%), ¹halassiosira spp. (5.3%) Nitzschia spp. (c.f.) (42.9%), Odontella weissflogii (15.0%), Pseudoeunotia doliolus (c.f.) (7.1%), Eucampia antarctica (6.8%), Rhizosolenia bergonii (6.4%), ¹halassiosira spp. (5.4%), Chaetoceros peruvianum (5.0%) Nitzschia spp. (c.f.) (27.1%), Odontella weissflogii (21.9%), ¹halassiosira spp. (7.3%), Pseudoeunotia doliolus (c.f.) (6.3%) Nitzschia spp. (c.f.) (25.9%), Pseudoeunotia doliolus (c.f.) (20.2%), ¹halassiosira spp. (11.7%), Odontella weissflogii (6.4%), Chaetoceros peruvianum (6.4%), Nitzschia closterium (5.3%) Nitzschia spp. (c.f.) (65.3%), Odontella weissflogii (8.7%), ¹halassiosira spp. (6.0%)

The results of the cluster analyses performed on the phytoplankton composition across the MAS transect indicate a weak segregation between the stations south of the STC (Sts. 1 and 2, Fig. 9) and the rest (with the exception of St. 8). The community composition was more homogeneous and diverse than in the SAS, and the middle and northern regions of the STC were dominated by the small chain-forming diatoms Nitzschia spp. and Pseudoeunotia doliolus. as well as the free diatom Odontella weissflogii (Table 4). Chaetoceros peruvianum, Pseudoeunotia doliolus and Chaetoceros dichaeta dominated the diatom community at stations south of the STC (Table 4). The taxonomic composition of the microzooplankton assemblage was similar throughout

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Table 5 Species composition and abundance of the major zooplanktonic groups collected in the mid-Atlantic sector of the STC using 300 lm Bongo nets (integration over the upper 300 m of the water column) Species composition

Copepoda Calanus australis Pleuromamma abdominalis Metridia lucens Euphaeta spp. Clausocalanus spp. Oithona spp. Oncaea spp. Euphausiacea Ostracoda Chaetognatha Eukrohnia spp. Sagitta spp.

43°32S—St.C1

41°31S—St.C5

No. 1000 m\

%

No. 1000 m\

13 394.7 380.4

77.4 2.2

131 471.5 1545.4

440.5 5746.3 200.2 1741.9 4344.8 560.6 262.3 240.3 2785.1 1982.2 802.9

2.5 33.2 1.1 10.1 25.1 3.2 1.5 1.4 16.1 11.5 4.6

18 103.5 41 064.1 25 389.1 40 512.2 1655.8 1103.9 2340.2 3753.2 2475.4 2428.5 46.9

39°59S—St.C8 %

No. 1000 m\

%

92.9 1.1

138 925.8 2381.1

95.9 1.7

12.8 29.0 17.9 28.6 1.2 0.8 1.7 2.6 1.7 1.7 (0.1

13 095.8 55 497.1 824.2 63 189.7 1648.4 549.5 597.5 915.8 3480.0 3480.0 —

9.0 38.2 0.6 43.5 1.2 0.4 0.4 0.6 2.4 2.4 —

Fig. 9. Dendrogram of similarities between stations in the mid-Atlantic sector obtained from microphytoplankton (left panel) and mesozooplankton (right panel) species composition. Also shown are station number and cluster groupings (see also Fig. 1).

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the frontal transect (data not shown). Heterotrophic dinoflagellates (comprising species of the genera Dinophysis, Ceratium and Peridinium) were dominant, followed by aloricate ciliates, while tintinnids comprised only 15% of all cell counts. Cluster analysis revealed two different mesozooplankton communities in the frontal region (Fig. 9). The northernmost stations were dominated by Pleuromamma spp. (mostly P. abdominalis), Metridia lucens and Clausocalanus spp. (Table 5), which was very similar to the composition found in the SAS, where P. abdominalis and M. lucens dominated the central and southern regions of the STC. Macrozooplankton of subantarctic origin appeared to dominate the central part of the convergence (Table 5), comprising mainly the euphausiid ¹hysanoessa gregaria, the amphipod ¹hemisto gaudichaudi, the chaetognath Sagitta gazellae and the polychaete »anadis longissima. The fish community at the centre of the STC was dominated by myctophiids of either subantarctic origin or those more typical of the STC transitional fauna. These included Protomyctophum normani and Ceratoscopelus warmingi (Table 6). Macrozooplankton and micronekton acoustic backscattering strengths recorded at both 120 and 38 kHz were most intense at the northern edge of the STC (Fig. 10, miles 70—130), especially in the upper 100—120 m of the water column. This supports the conclusion that a general enhancement in pelagic biomass was associated with the STC at that time. This enhancement in biomass mirrors the distributional patterns in the microphytoplankton and zooplankton (Fig. 8), indicating that a strong biological signal of the Subtropical Convergence persists in this region, despite the weak and meandering structure of this sector of the front.

4. Discussion 4.1. Biomass enhancement at the STC From the results presented it can be concluded that pelagic biomass enhancements, from microphytoplankton through to micronekton, are associated with the Subtropical Convergence zone. This pattern is consistent in both regions investigated, despite the marked differences in terms of their hydrographic structure and seasonality of the sampling. There is some evidence to suggest that this pelagic biomass enhancement is due to localized increases in phytoplankton production, which ultimately provides the major source of carbon and energy for the entire food web (Lutjeharms et al., 1985; Dower and Lucas, 1993; Laubscher et al., 1993; Weeks and Shillington, 1994). A number of mechanisms have been proposed to explain the in situ increase in primary production within the STC. These can be summarized as the iron hypothesis and the macronutrients/vertical stability hypothesis. The iron hypothesis for the Southern Ocean was introduced by Martin et al. (1990) to explain the occurrence of phytoplankton blooms in neritic waters around the Antarctic Continent (see De Baar et al., 1995). In the proximity of land masses, iron concentrations are generally high relative to the open-ocean because of run-off from land and sediment resuspension. Recent analyses of pigment data from the Nimbus-7

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Table 6 Species compositiion, mean size, relative abundance and origin of the fish collected in the Polish Krill Trawl during the STC cruise in the mid-Atlantic sector. SAZ"Subantarctic zone. STC"Subtropical Convergence Taxa Lampiridae ¸ampris immaculatus Gempilidae Paradipluspinus gracilis Nemichthyidae Nemichthys curvirostris Stomidae Stomias boa Melanostomiidae Opostomias micripnus Bathophilus sp. Idiacanthidae Idiacanthus atlanticus Notosuoidae Scopelosaurus hamiltoni ¸uciosudis normani Paralepididae ¸estidiops similis Melanocetidae Melanocetus johnsoni Bathylagidae Bathylagus sp.

Individuals

Percentage

Origin

1

(0.1

South of 30°S

2

(0.1

Circumantarctic

7

0.1

1

(0.1

20°—45°S

1 1

(0.1 (0.1

South of 33°S ?

1

(0.1

26°S—STC

1 2

(0.1 (0.1

30°—60°S STC

3

(0.1

45°N—45°S

1

(0.1

Widely distributed

3 4

(0.1 0.1

? ?

12

0.2

Circumtropical Widely distributed

Scopelarchidae Scopelarchoides danae Sternoptychidae Maurolicus muelleri Myctophidae Metelectrona herwigi Electrona pancirastra Electrona subaspera Diaphus hudsoni Protomyctophum normani Symbolophorus boops Ceratoscopelus warmingi

6

0.1

72 48 6 102 2472 186 1920

1.5 1.0 0.1 2.1 50.9 3.8 39.6

Total

4852

100.0

Widely distributed

STC STC SAZ SAZ SAZ—STC SAZ—STC 42°N—45°S

Coastal Zone Colour Scanner (CZCS) reveal an asymmetric distribution of phytoplankton pigments in the region of the STC (Comiso et al., 1993; Sullivan et al., 1993), with bloom concentrations consistently clustered downstream of the continental masses of Africa, South America, Australia and New Zealand. Sullivan et al. (1993) have pointed out that this pattern is consistent with the iron hypothesis.

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Fig. 10. Three-dimensional and contour plots of acoustic backscattering strength at 38 (top) and 120 kHz (bottom) along the northbound transect in the mid-atlantic sector. Echoes were integrated over 2 nm intervals and 10-m depth layers, from 10 to 250 m depth (38 kHz) and from 10 to 150 m depth (120 kHz). No noise reduction was applied. Black areas on the x-axis denote periods of darkness.

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It has been recognized that the subsurface expression of the STC is located predominantly north of the surface expression (Lutjeharms and Valentine, 1984). Several studies have shown that mixing of warm, nutrient-poor subtropical waters with colder Subantarctic waters leads to an increase in density stratification, the resulting enhanced vertical stability causing retention of phytoplankton and promoting conditions favourable to increased primary production (Lutjeharms et al., 1985; Dower and Lucas, 1993; Weeks and Shillington, 1994). The asymmetric distribution of phytoplankton blooms within the STC, which is spatially related to the occurrence of the major western boundary currents in the southern hemisphere (the Agulhas Current and Return Current System, the East Australian Current and the Falklands/ Brazil Current), supports this hypothesis. In these systems, a steady flux of macronutrients to the warm, nutrient-depleted waters of the northern boundary of the convergence, combined with the retention of phytoplankton cells within the euphotic zone, would result in the high occurrence of blooms in these regions. The regular formation of baroclinic eddies along the STC south of Africa (Lutjeharms et al., 1993) may promote production even further in this sector, through boundary-induced stability (Dower and Lucas, 1993). In the mid-Atlantic, on the other hand, Lutjeharms et al. (1993) indicated that fronts do not retain their identity for more than 2—3 weeks. The variation of the surface thermal structure observed in this study, with waters (12°C shifting from 41.5°S in February (Fig. 7) to 43°S in March (Fig. 8), agrees with their observation. The biomass enhancement in the frontal zone in the MAS was less intense than south of Africa. This difference between the two sectors may be due to either different in situ primary production levels or different energy transfer efficiencies to higher trophic levels. To address this problem, a comparison between the biological patterns in both areas is required. 4.2. The mid-Atlantic versus the south of Africa sectors In the SAS high chlorophyll-a values were recorded south of the frontal zone while microphytoplankton were more abundant in the north, indicating that the nano- and pico-plankton fractions dominated the region during the study (Lutjeharms et al., 1994). The low ratio between chlorophyll-a and phaeopigments in the northern side suggests that the phytoplankton communities on the subtropical side of the STC either were aged or heavily grazed upon (Laubscher et al., 1993). A similar situation was evident in the MAS. However, low microzooplankton densities were recorded in the SAS, which were an order of magnitude lower than in the MAS. It may be that carnivory by meso- and macrozooplankton was substantial, in the absence of microphytoplankton, their preferred food (Hansen et al., 1994). The copepod Metridia lucens, as well as some euphausiid and tunicate species that were dominant at the frontal zone, are likely to have been the organisms responsible for this apparent heavy grazing. Macrozooplankton and fish were also more abundant in the northerly edge of the front. Such patterns agree with the position of the biogeographical boundary (Fig. 4), probably reflecting the strong influence of the Agulhas Retroflection and Return Current in accumulating biomass and promoting pelagic production in the SAS. In support of this hypothesis, the microphytoplankton community differed on

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both sides of the STC mainly as a result of the tropical/temperate (Boden and Reid, 1989) chain-forming diatom Pseudoeunotia doliolus, which could have been transported southwards through cross-frontal mixing, blooming in the nutrient-rich zone. This transport could be mediated by eddies and meanders, which are common in this region (Lutjeharms, 1985; Lutjeharms and Van Ballegooyen, 1988). The high concentration of subantarctic myctophiids in this sector of the convergence will result in a fraction of the enhanced biomass of primary and secondary producers being exported to deep oligotrophic areas via their diel vertical migration. In the MAS both chlorophyll-a and microphytoplankton abundances peaked in the nutrient-rich southern side of the STC, indicating that this size-class dominated the phytoplanktonic community. This assemblage was also more diverse there than in the SAS, including both subantarctic (Odontella weissflogii, Table 4) as well as subtropical (P. doliolus, Chaetoceros peruvianum, Table 4) species (Boden and Ried, 1989). This dramatic change in the phytoplankton assemblage, from a microphytoplankton-dominated MAS (sampled in summer) to the nano- and picophytoplanktondominated SAS (sampled in winter) is more likely to be the result of the seasonality of the sampling (Laubscher et al., 1993), rather than reflecting regional differences. The higher microzooplankton densities recorded in the MAS are also probably due to seasonal cycles of abundance. Mesozooplankton densities were lowest in subantarctic waters in both sectors. In the MAS the composition was dominated by mixed-water species on both sides of the STC, particularly Metridia lucens and Clausocalanus spp., as reflected in the results of the cluster analysis (Fig. 9). This suggests that cross-frontal transport was higher in this sector, as expected from the less intense hydrographic gradients there, compared to the SAS. Similarly, the fish community was more diverse and less dominated by myctophiids than in the SAS, although also formed by species of both subtropical and subantarctic origin (Table 6). The higher diversity and complexity of the pelagic community in the MAS strongly suggest that its dynamics differ from that in the SAS. Satellite thermal infra-red sensor data have demonstrated that the meridional position of the frontal zone in the mid-Atlantic basin is highly variable. This is presumably due to the lack of interaction with any western boundary currents (Shannon et al., 1990). Lutjeharms et al. (1993) concluded that the surface expression of the STC there may be intermittent and diffuse, with no clear seasonal cycle. This suggests that the more relaxed zonation patterns in the pelagic community are a consequence of the less intense environmental forcing in the mid-Atlantic sector. More attention to this sector of the STC is required to understand its dynamics as well as its role in the exchange of biota between the Atlantic and Southern Oceans. In summary, the biological activity of the Subtropical Convergence appears to be intense in both sectors, perhaps responding to the long-term stability of the STC as a zone of convergence and high density stratification. The community composition suggests regional variability in the intensity of the boundaries of the STC, being more relaxed or ephemeral in the mid-Atlantic, and more permanent and intense in the region south of Africa. We suggest that this is a direct consequence of the role that both the African continent and the Agulhas Return Current play in enhancing hydrographical gradients, thereby accumulating biomas and promoting cross-frontal mixing and pelagic production. In contrast, the higher diversity and lesser zonation

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patterns of the pelagic community in the mid-Atlantic may be a direct reflection of the intermittent and diffuse condition of the Subtropical Convergence in the open ocean. The results of this study also confirm the hypothesis of Frontier (1977) and Zubova and Timofeev (1989) that there is a unique intermediate pelagic community, which is associated to the STC, and promoted by the specific hydrological features of this frontal zone. The possible influence of increased iron levels in the proximity of land masses superimposed to the herein suggested environmentally-driven scenario may play an important role and needs to be investigated. The more diffuse and variable character of the convergence in the mid-Atlantic sector, combined with the weaker zonation patterns of biota, suggests that a concept of Subtropical Convergence Zone (STCZ) may be more appropriate in this region (Lutjeharms et al., 1993).

Acknowledgements We are grateful to the Sea Fisheries Research Institute, the South African Department of Environmental Affairs and Tourism and Rhodes University for providing funds and facilities for this study. Thanks also to W.B. Campbell (NOAA, National Ocean Service, Camp Springs, USA) for providing the multi-channel sea surface temperature charts in Fig. 1, and to the Ocean-Climatology Group of the Oceanography Department, University of Cape Town, for supplying data for Fig. 2a. We also like to thank O. Gon and E. Anderson (J.L.B. Smith Institute of Ichthyology, Grahamstown, South Africa) for the identification of the fish collected during both surveys. The officers, crew and scientific personnel of the MV Agulhas and the FRS Africana are acknowledged for their help during the surveys.

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