Planktonic Foraminifera from south-western Atlantic

J. Mar. Biol. Ass. U.K. (2000), 79, 203^213 Printed in the United Kingdom

Planktonic Foraminifera from south-western Atlantic epipelagic waters: abundance, distribution and year-to-year variations Esteban Boltovskoy*, Demetrio BoltovskoyOP$, and Frederico Brandini½ *Deceased 4 September 1997. ODepartamento de Ciencias Biolo¨gicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, 1428 Buenos Aires, Argentina. E-mail: [email protected]. PConsejo Nacional de Investigaciones Cient|¨ ¢cas y Te¨cnicas, Argentina. $Museo Argentino de Ciencias Naturales ``Bernardino Rivadavia''. ½Centro de Estudos do Mar, Universidade Federal do Parana¨, Av. Beira Mar s/n, Pontal do Sul, PR 83255-000, Brazil

The abundance and latitudinal and vertical (0^100 m) distribution of planktonic Foraminifera was investigated on the basis of 38 samples collected in November 1994 in the south-western Atlantic Ocean (34^608S, along 51^568W). Mean foraminiferal densities were 1.5 ind l 1 (range: 0.15.9 ind l 1), with highest concentrations in subsurface waters (20^50 m). Couplings between the distribution of chlorophyll-a and foraminiferal abundances were very loose. Distribution patterns of the 15 species recorded allowed six distinct areas to be de¢ned along the transect surveyed. From north to south these are: Subtropical (dominated by Globigerinoides ruber and G. trilobus), Cold intrusion (Globigerinita uvula), Transitional-Subtropical and Transitional (Globigerina bulloides, G. quinqueloba), Subantarctic (G. quinqueloba), and Antarctic (Neogloboquadrina pachyderma, left coiling). Close comparisons with the yields of a similar data set collected in November 1993 show very good agreement. Foraminiferal thermic re¨gimes were also similar in 1993 and in 1994, but for some species signi¢cant di¡erences with previous data were detected. While the southwards extensions of the ranges of warm water species are fairly well circumscribed by the Brazil current-in£uenced waters, several foraminifers widely used as indicators and palaeoindicators of cool waters (in particular Globigerina bulloides, G. quinqueloba and Globigerinita uvula) were recorded in very signi¢cant numbers at temperatures as high as 20^248C. The implication of these ¢ndings for hydrological, ecological, and palaeoecological interpretations is discussed.

INTRODUCTION The distribution of planktonic foraminifers from the south-western Atlantic has been investigated since the early 1960s, de¢ning the horizontal ranges of the *40 species present in the area, their relative abundance relationships, and their seasonal variations (see reviews in E. Boltovskoy, 1981b; Hemleben & Kemmle von Mucke, in press). Particular attention has been paid to the use of these protists as indicators of currents, fronts, and watermasses, thus contributing signi¢cantly to describe the main features of the surface-layer circulation between 208 and 608S (E. Boltovskoy, 1970; 1981a). However, vertical distribution patterns, absolute abundances, and interannual variations in the foraminiferal assemblages are still very poorly understood. These data are necessary not only for the adequate assessment of biogeographic ranges of the living populations, but also for sound palaeoenvironmental interpretations based on surveys of foraminiferal deposits on and below the sea-£oor (e.g. D. Boltovskoy, 1994; E. Boltovskoy et al., 1996). This work, based on a plankton sampling performed in November 1994 between 348 and 608S, along 51^568W, contributes information to help ¢ll in the above-mentioned gaps. New data are presented on the latitudinal distribution of foraminiferal species across two major frontal zones in the south-western Atlantic: the Transition Zone (or Journal of the Marine Biological Association of the United Kingdom (2000)

Subantarctic ^ Subtropical Convergence), and the Polar Front (or Antarctic Convergence), thus encompassing typically subtropical taxa, which dwell best in the Brazil Current, to Antarctic species which peak in the Southern Ocean. Comparisons with a similar collection carried out the previous year (E. Boltovskoy et al., 1996) allow assessment of year-to-year variations in the composition and distribution of the foraminiferal assemblages, while those with other previous reports suggest important di¡erences in the thermic re¨gimes of some of the foraminifers recorded.

ENVIRONMENTAL SETTING (Figure 1) The surface layer of the southernmost part of the area surveyed is occupied by cold (up to *48C) waters of the West Wind Drift (=Antarctic Circumpolar Current). At the Polar Front (50^608S, depending on season and longitude), these waters sink under the less dense Subantarctic Surface waters and move north (Antarctic Intermediate Waters) probably as far as 20^258S (Thomsen, 1962; Lenz, 1975; Tsuchiya et al., 1994). Around the tip of South America part of the West Wind Drift branches o¡ as the Malvinas (=Falkland) Current moving north to northeast over the continental slope. At the surface this current typically reaches around 358S, but Subantarctic waters

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South-western Atlantic planktonic Foraminifera

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MATERIALS AND METHODS

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(Falkland) Current (see Figure 2; right panel), or a local upwelling cell (as was suggested previously by E. Boltovskoy et al., 1996).

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Figure 1. Location of Transition Zone in the south-western Atlantic according to foraminiferal data (based on E. Boltovskoy, 1970), main currents and fronts (modi¢ed from Peterson & Stramma, 1991), and station locations.

are regularly present in subsurface layers as far north as 208S (E. Boltovskoy, 1970; Valentin et al., 1987; Brandini, 1990). East of *558W the Malvinas (Falkland) Current turns east and south entering the large South Atlantic Central gyre where subantarctic waters mix with subtropical ones from the Brazil Current, and with waters from the central Atlantic. This Transition Zone, which o¡ Argentina stretches roughly between 30 and 49³S, hosts mixed waters as well as isolated cells and tongues of pure Subantarctic and subtropical origin. The northernmost stations occupied in this survey are located in the Subtropical waters of the Brazil Current (Table 1 and Figure 1). Surface temperatures along the transect covered were between 28 and 228C, increasing smoothly toward the tropics. A notable exception in this gradual increase was a sharp drop around 368S (Figure 2, left panel). Interestingly, a similar drop was also observed the previous year at a nearby location (approximately 378S; E. Boltovskoy et al., 1996); satellite imagery suggests that the source of these low-temperature waters (Figure 2, left panel) is an eastward excursion of the core of the Malvinas Journal of the Marine Biological Association of the United Kingdom (2000)

Planktonic foraminifers were collected from the upper 100 m between 348 and 608S, along 51^568W, between 4 and 16 November, 1994, from the Brazilian Antarctic RSV Àry Rongel' (TABIA II cruise, Table 1 and Figure 1). Flowmetered, closing nets 30 cm in diameter (30-mm mesh) were towed vertically covering the following layers: 0^5, 5^15, 15^30, 30^50, and 50^100 m (Table 1). Materials were preserved with 3% bu¡ered formaldehyde. Sixteen to 1421 (mean: 257) foraminiferal tests were hand-picked at random from each wet, unsieved sample and identi¢ed under a compound microscope. In the present report species-speci¢c data transformed into percentages are used; these values exclude the unidenti¢ed specimens (almost exclusively juveniles of planktonic taxa), which averaged 24% of all shells recorded (Table 1). Absolute foraminiferal abundances were assessed on the basis of the individuals picked from the wet samples, plus subsequent counting of the shells left in aliquots of the remaining material in 10 or 25 ml settling chambers under the inverted microscope. No attempt was made to di¡erentiate living vs dead specimens in the samples. The widely used technique of staining the protoplasm (with Bengal rose, Sudan black B, or eosin; E. Boltovskoy, 1981b) does not provide unequivocal information because of uncertainties associated with the speed of decomposition of the protists' cytoplasm (D. Boltovskoy, 1998). E. Boltovskoy & Lena (1970), for example, concluded that specimens of several planktonic Foraminifera still contained protoplasm in their shell 98 days after death. Bernhard (1988) compared estimates of the proportions of presumably live benthic Foraminifera as indicated by Bengal rose and Sudan black B staining and by adenosine triphosphate assay, concluding that stained protoplasm was present in individuals up to four weeks after actual death of the cell. At any rate, given the fast sedimentation rates of foraminiferal shells after death of the organism (*300 to over 1000 m day 1, cf. Takahashi & Be¨, 1984), we contend that the greatest majority of the yields of our rather shallow tows (maximum: 100 m) must be represented by live specimens at the time of capture. Water-temperature pro¢les were based on underway bucket surface measurements and 25 expendable bathythermograph launchings (mostly to 460 m) along the transect occupied. Because one of the main goals of the present report is to assess year-to-year changes in the structure of foraminiferal planktonic assemblages in the area surveyed, most data for the November 1994 cruise are presented in a comparative way with those of a similar sampling programme carried out along the same transect during November 1993 (E. Boltovskoy et al., 1996). Information on the classi¢cation system used for this work, as well as short remarks on the criteria adopted are presented in E. Boltovskoy et al. (1996). Detailed accounts of the morphology and classi¢cation of South Atlantic foraminifers can be found in E. Boltovskoy (1981b) and Hemleben & Kemmle von Mucke (in press).


E. Boltovskoy et al. 205

Table 1. Sample data. Foraminiferal data Sta. 2 2 2 3 3 3 3 4 5 6 6 7 7 8 9 10 10 10 10 10 11 12 12 12 12 13 13 15 16 16 16 16 17 18 18 21 21 22

Date

Surface temp. (8C)

Lat. S

Long.W

08/11/94 '' '' 09/11/94 '' '' '' '' '' '' '' '' '' 10/11/94 '' 11/11/94 '' '' '' '' '' 12/11/94 '' '' '' '' '' 13/11/94 14/11/94 '' '' '' '' '' '' 15/11/94 '' 16/11/94

22.1 '' '' 22.5 '' '' '' 19.0 17.5 19.3 '' 20.7 '' 21.4 18.3 16.5 '' '' '' '' 16.8 13.1 '' '' '' 8.7 '' 7.1 6.9 '' '' '' 7.3 5.4 '' 1.7 '' 1.7

34800.0' '' '' 35800.0' '' '' '' 36808.0' 36836.0' 37805.0' '' 37832.0' '' 38804.0' 39844.0' 42800.0' '' '' '' '' 43858.9' 46800.0' '' '' '' 48800.0' '' 51859.8' 54800.0' '' '' '' 55800.0' 55800.0' '' 58803.3' '' 59836.5'

51800.0' '' '' 51812.0' '' '' '' 51825.0' 51829.0' 51840.0' '' 51846.0' '' 51854.0' 52814.0' 52850.0' '' '' '' '' 53817.0' 53848.0' '' '' '' 54816.0' '' 55808.2' 55823.2' '' '' '' 55836.4' 55825.8' '' 56809.0' '' 56833.5'

Maximum Minimum Mean

1421 16 257

81.3 0.0 23.8

RESULTS AND DISCUSSION Foraminiferal densities

Juvenile specimens were largely dominant in the taxocoenoses regardless of latitude and depth, averaging over 85% of all individuals. Mean foraminiferal densities over the area sampled were 1.5 ind l 1, ranging between 0.1 and 5.9 ind l 1, with highest concentrations in subsurface waters (20^50 m), in the Transitional and Subantarctic areas (Figure 3C). These concentrations are higher than most previous data reported for the south-western Atlantic (cf. E. Boltovskoy & D. Boltovskoy, 1970; E. Boltovskoy, 1971; Alder & Boltovskoy, 1993), yet discrepancies are most probably chie£y due to our coverage of the smallest specimens, as well as to dissimilarities in the Journal of the Marine Biological Association of the United Kingdom (2000)

specimens % % warm % cold Tow depths(m) recorded unident. water water 5^0 15^5 30^15 5^0 30^15 50^30 100^50 15^0 15^5 5^0 15^5 30^0 15^5 5^0 15^5 5^0 15^5 30^15 50^30 100^50 30^15 5^0 30^15 50^30 100^50 5^0 50^30 100^50 5^0 30^15 50^30 100^50 50^30 15^5 30^15 5^0 30^15 100^50 100.0 0.0 16.5

16 132 115 54 115 158 291 174 92 122 95 269 181 67 164 110 257 376 344 323 238 218 1421 533 967 196 587 286 211 189 149 303 353 96 216 44 111 178

81.3 74.2 47.8 0.0 29.6 19.6 67.4 63.2 76.1 53.3 45.3 21.6 18.2 37.3 23.2 13.6 13.2 20.5 7.0 0.0 13.9 9.6 12.9 13.9 11.6 7.7 7.3 3.8 4.7 7.9 6. 5.9 35.7 15. 14.4 6.8 7.2 7.9

96.7 94.1 100.0 28.4 41.7 33.3 31.5 3.1 0.0 21.2 1.8 79.6 53.4 38.1 4.0 0.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

1.7 0.0 0.0 69.5 56.7 66.7 68.5 93.8 100.0 78.8 87.7 20.4 45.3 61.9 75.4 62.8 73.8 72.6 76.2 85.3 82.4 82.8 91.9 90.6 94.9 98.3 94.5 95.3 90.5 96.4 94.8 93.5 93.8 95.7 100.0 99.0 100.0 100.0

100.0 0.0 76.1

mesh-sizes used in the di¡erent surveys. On the other hand, collections from more fertile areas of the World Ocean reported much higher foraminiferal densities (e.g. Bishop et al., 1978; Be¨ et al., 1985), suggesting that foraminiferal densities in these waters are generally low. Quantitative data for the November 1993 cruise (TABIA I) are only available for the 5^15 m layer. Comparison of these values with 0^5 m densities in November 1994 (this study) shows good overall agreement (Figure 3D). Numbers retrieved in 1994 were slightly higher than in 1993, but absolute densities and their spatial pattern compared favourably. As in November 1993, couplings between the distribution of chlorophyll-a and foraminiferal densities were very loose (cf. E. Boltovskoy et al., 1996, and Figure 3B,C). We

206

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Figure 2. Geographic distribution of the stations surveyed superimposed on SST satellite-based data (right panel), and vertical temperature pro¢le (0^400 m) during the cruise (left panel). See Figure 5 for validation of biogeographic divisions indicated.

contend that lack of spatial association between the foraminifers and their main source of food (phytoplankton) is chie£y due to the fact that over much of the area surveyed phytoplankton enhancements are too short-lived to boost densities of these rather slow-reproducing protists. The south-western Atlantic is a highly dynamic area where hydrographic conditions change rapidly. Build-up of plant biomass in the vast Transition Zone is probably due to vertical interleaving of cold (Malvinas) and warm (Brazil) waters, which engenders circumscribed cells of enhanced stability of the upper layers, thus contributing to the retention of plant cells in the photic layer and consequent autotrophic biomass build-up. These phytoplankton-rich patches may be a recurrent feature (Brandini et al., in preparation), but judging from satellite images of the area they are short-lived. Short-lived bloom conditions would selectively favour organisms with fast turnover rates, which can react swiftly to enhanced feeding conditions, while more permanent ones allow the development of slowergrowing animals, partly at the expense of the phytoplankton, and partly using the fastest-growing consumers as food. This assumption is supported by the fact that while the abundances of foraminifers, radiolarians, crustaceans and pteropods (whose doubling rates are of the order of weeks to months; Caron & Swanberg, 1990; Wells, 1976) do not seem to be spatially associated with phytoplanktonic enhancements, those of the tintinnid ciliates (whose duplication times are of the order of hours; Heinbokel, 1987, 1988), are closely coupled with chlorophyll-a (Thompson et al., in Journal of the Marine Biological Association of the United Kingdom (2000)

press). Interestingly, these dissimilar spatial relationships with plant biomass are also re£ected in time-series analyses, whereby tintinnid abundances (as measured by £ux rates of their loricae in sediment traps) react swiftly to phytoplanktonic blooms, whereas those of the other protist and metazoan groups dealt with do not (D. Boltovskoy et al., 1993). Foraminiferal distribution patterns

Fifteen foraminiferal species were recorded. Tropical ^ Subtropical taxa were represented by Globigerinoides ruber, G. trilobus, G. conglobatus, Globigerina rubescens, Globorotalia menardii and Globigerinella aequilateralis. At SST (sea surface temperature) above 208C these species accounted for 27^100% of all identi¢ed foraminifers (mean: 58%), yet only Globigerinoides ruber and G. trilobus were abundant (maximum percentage contributions: 78 and 67%, respectively; see Figure 4). Globigerina rubescens reached 10^12% exceptionally (at station 2, between 5 and 30 m). Globorotalia menardii and Globigerinoides conglobatus were only present in very low proportions (maximum percentage: 6%; Figure 4). At northernmost station 2 (Figure 1) these six taxa accounted for almost all the foraminifers recorded, their share dropping to 20^30% at station 3, and further decreasing to almost zero values at stations 4^6 (36^378S); between 37 and 388S (station 7) they again reached 50^80% of the fauna, and disappeared from the samples altogether shortly thereafter, at station 10 (528S).



Figure 3. Pro¢les of temperature (0^100 m; A), chlorophyll-a (B), and foraminiferal abundances (C). Lower panel (D) compares foraminiferal densities in November 1993 and November 1994. See Figure 5 for validation of biogeographic divisions indicated.

The cosmopolitan species Globigerinita glutinata was present in about half the samples investigated in low numbers (exceptionally up to 6%, mean for the entire collection: 1%), from the northernmost locale surveyed to 558S. Spatial distribution of the occurrences of this taxon did not suggest any discernible geographic pattern (Figure 4). Globorotalia in£ata, a species typical of Subtropical ^ Subantarctic transitional waters, was recorded in 2/3 of the samples analysed. It was present throughout the transect in moderate to high numbers (mean: 7%, maximum: 34%), clearly peaking between 408 and 458S (Figure 4). Taxa generally associated with colder Subantarctic and Antarctic waters included the following (in order of decreasing mean percentage contribution to the entire collection): Globigerina bulloides, G. quinqueloba, Journal of the Marine Biological Association of the United Kingdom (2000)

Neogloboquadrina pachyderma (left coiling), Globigerinita uvula, Neogloboquadrina pachyderma (right coiling), Globorotalia truncatulinoides, and G. scitula (Figure 4).The ¢rst two accounted for *70% of all cold-water specimens recorded. With the exception of the 0^15 m layer at station 2 (348S), these cold water species were present in all the samples retrieved, including those de¢ned herewith as Subtropical and Transitional-Subtropical (see below). As a whole, these cold water species accounted for less than half of all forams at only two stations: 2 and 7. Most clearly circumscribed latitudinal and temperature ranges were those of N. pachyderma (left coiling), which clearly peaked south of the Polar Front, at temperatures below 58C, comprising 450% and up to 96% of all the specimens identi¢ed. This is the only typically Antarctic foraminifer whose abundances decrease gradually toward the north, disappearing from the surface

208



Figure 4. Latitudinal distribution of foraminiferal percentage contributions to the overall assemblage (all depths pooled), and SST. See Figure 5 for validation of biogeographic divisions indicated.

between 358 and 408S, at temperatures around 158C. Globigerina bulloides was dominant between 37^468S, at temperatures of 13^218C, where it accounted for 30^40% of the foraminifers; below 58C and above 218C it dropped to 510^15%. Globigerina quinqueloba was abundant throughout the area covered (mean: 23%), with low densities (around 15%) in Antarctic waters, and peaking in the Subantarctic area, at temperatures around 7^158C. Imprints of regional hydrology on biogeographic patterns

Figure 5 shows the overall geographic pattern de¢ned on the basis of the foraminifers recorded. There are six rather well di¡erentiated areas, each characterized by a distinct combination of species. Station 2 (348S, Subtropical), located north of the northernmost extension of Malvinas-in£uenced waters, was strongly dominated by the warm water species Globigerinoides ruber and G. trilobus (Figure 4). At station 3 (358S, Transitional-Subtropical) a mixed assemblage was present, with warm water representatives accounting for somewhat less than half of all foraminifers (30^40%). Between approximately 36^388S the share of the Subtropical foraminifers decreased drastically, the cold water forms Globigerinita uvula, Globigerina bulloides and G. quinqueloba dominating the assemblage conspicuously. Notably, at the stations involved the temperature dropped about 28C (from a mean of 21.88C to 19.68C, Figure 4). Interestingly, a similar isolated colder-water sector, with enhanced proportions of Subantarctic foraminifers, was also recorded the previous year (November 1993, cf. E. Boltovskoy et al., 1996) at roughly the same site (378S). This phenomenon was Journal of the Marine Biological Association of the United Kingdom (2000)

tentatively interpreted as a local upwelling cell (E. Boltovskoy et al., 1996). It is also conceivable that the stations in question were located within a north-eastward extension of the Malvinas Current, as suggested by November 1994 satellite images of the Brazil ^ Malvinas Con£uence area (Figure 2, right panel). Judging from satellite data, between 378 and 388S the cruise track re-entered Brazil-dominated waters (Figure 2, right panel). The temperature increased to around 228C, and a mixed Subtropical/Transitional ^ Subtropical foraminiferal assemblage reappeared in the samples. While these two types of assemblages intermingle both geographically and vertically, single-sample inventories always matched either type rather clearly (Figure 5). At stations 9^11 (40^448S, Transitional), characterized by a mean temperature of *158C, warm water foraminifers were practically absent (only traces of Globorotalia menardii and Gobigerina rubescens were present at station 9). Globorotalia in£ata reached its highest relative abundances in this area (18^34%), which was dominated by Globigerina bulloides and G. quinqueloba (Figures 4 & 5). The Subantarctic assemblage, dominated by G. quinqueloba, G. bulloides and N. pachyderma, stretched roughly between 488 and 548S. Warm water species were absent from these samples altogether, while the transitional Globorotalia in£ata accounted for up to 9.5% of the fauna (mean: 3.4%). At southernmost stations 17, 21 and 22 (55^608S) N. pachyderma (left coiling) accounted for 50^96% of the foraminifers recorded, followed by moderate numbers of Globigerina bulloides and G. quinqueloba (Figures 4 & 5). Because of its implications for biogeographic and palaeoenvironmental analyses, the area of mixing of


A

B

SAMPLE GROUPS


C

SAMPLES ENCOMPASSED BY EACH GROUP

DOMINANT FORAMINIFERS WITHIN EACH GROUP

Bray-Curtis

1.0

0.5

0.0

Subtropical (St) Cold intrusion (C)

2 St St St T/S T/S T/S

3 T/S C

4

C

5

6 C T/S Transitional/ Subtropical (T/S)

7 St T/S St 8 T/S 9

T

10 T T T T T Transitional (T)

11

T

12 Sa

T

13 Sa 14 Subantarctic (Sa) 16 Sa

Sa Sa Sa Sa T Sa Sa

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T T

G. ruber G. trilobus G. inflata G. uvula G. bulloides G. quinqueloba N. pachyderma (right+left) Others

Tow depths (m)

Figure 5. Faunal breaks along the transect surveyed, as revealed by a cluster analysis (A) using all samples and all species identi¢ed (Bray^Curtis between-sample similarity index and UPGMA clustering method; cf. Sneath & Sokal, 1973). Location of samples grouped in the dendrogram is illustrated in B, while C indicates the dominant foraminiferal species in each sample group.

Brazil ^ Malvinas waters is of special interest (e.g. D. Boltovskoy, 1981, 1986, 1994). Insofar as planktonic organisms can be used as conservative indicators of the origins of a given water mass (E. Boltovskoy, 1970, 1981a,b), comparison of the latitudinal extensions of these two currents, as derived from physical evidence (SST), with those based on foraminiferal distribution patterns can yield useful insights into the use of these protists as indicators and palaeoindicators of oceanographic settings. Figure 6 illustrates the results of such an analysis by comparing the modal positions of the Brazil and Malvinas extensions along 40^558W (based on satellite data for the period July 1984^ June 1987), with the distributions of several foraminiferal species and species groups (data for November 1994 and November 1993 are included). Both in 1993 and in 1994 warm water foraminifers practically disappear from the samples around the southernmost modal position of the Brazil current, with only traces of a few species (e.g. Globorotalia menardii, Journal of the Marine Biological Association of the United Kingdom (2000)

Globigerinella aequilateralis, Globigerina rubescens) extending south of 41^438S. In contrast, cold water taxa range consistently north of the assumed northern modal extension of the Malvinas Current. According to Olson et al. (1988), the maximum northern extension of the Malvinas current (July 1984^ June 1987) along TABIA I and II cruise tracks is located around 378S; however, our data show that most cold water species extended far beyond this latitude (Figure 4). North of 378S G. bulloides reached up to 40^50% of all foraminifers (mean: *18%), N. pachyderma (right-coiling) 6%, Globorotalia quinqueloba 36%, Globorotalia scitula 1%, G. truncatulinoides 4%, and Globigerinita uvula 80%. It should be noticed that both cruises concerned were carried out during the southern spring ^ summer, when the northward extension of the Malvinas Current is at its lowest. These results pose questions as to the correct identi¢cation of the Malvinas extensions based on SST data or, alternatively, to the thermic tolerance, geographic

210



Figure 6. Modal positions of the Brazil and Malvinas (=Falkland) currents extensions as deduced from monthly mean SST images derived from High Resolution Picture Transmission digital data of the (US) National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (based on Olson et al., 1988; left panel), and the percentages of selected foraminifers along the cruise tracks of TABIA I and II.

origins, and concomitant environmental and palaeoenvironmental interpretation of the species discussed as water-mass indicators. As pointed out by Olson et al. (1988), the choice of the temperature cut-o¡ values for Malvinas water extensions (168C) is not unique; it is therefore probable that a higher value would more realistically re£ect the distribution of Malvinas-in£uenced areas along this frontal zone. E. Boltovskoy (1970), in his review of the proposed locations of the Brazil ^ Malvinas front showed that, depending on the temperature and salinity threshold values used by di¡erent authors, the assumed position of this con£uence varied up to 14³ in latitude. Such disagreements greatly exceed the known seasonal and interannual variability of the front. However, even assuming that Malvinas în£uenced waters are widespread in areas where SST is over 168C, some of our presumably cold water species are very abundant well beyond what can be reasonably accepted as Malvinas-in£uenced zones. Indeed, Globigerina bulloides, for example, was present in very high proportions (up to 48%) in waters ranging between 218 and 248C. Within this same temperature range G. quinqueloba reached up to 20%, Globigerinita uvula 21%, N. pachyderma (right-coiling) 2%, and Globorotalia truncatulinoides 4% (combined TABIA I and TABIA II data). Maximum water temperatures at which cold water foraminifers were recorded in the south-western Atlantic listed by E. Boltovskoy (1970) include values even higher than these, but the author stressed that his ¢ndings in such warm waters were restricted to isolated and poorly developed individuals. TABIA III materials, on the other hand, included very abundant and well developed representatives of at least three cold water species (Globigerina bulloides, G. quinqueloba and Gobigerinita uvula) at these high water-temperatures. As noticed by Beklemishev (1969), the distribution range of planktonic species can be subdivided into several o¡sets of decreasing ¢tness. From the point of view of the usage of a given species as a hydrological and/or palaeoenvironmental indicator, of major importance is di¡erentiating its core distributional area (where the species can reproduce), from its sterile expatriation range Journal of the Marine Biological Association of the United Kingdom (2000)

(where it can survive, but does not reproduce). Knowledge of the boundary between these two areas is necessary for the correct interpretation of the implications of a given distributional pattern for water-mass, current and palaeoenvironmental analyses. In e¡ect, because in these surveys planktonic organisms (or their remains on the sea-£oor) are used as tracers of a particular ecological setting (as de¢ned, for example, by a current), unless the geographic origin (and, therefore, the ecological requirements) of the taxon concerned is known, its value as an indicator is limited. In this context, `geographic origin' implies the area where the organism was born; assessment of the boundaries of this area and information supplied by the location of its recovery site furnish reliable evidence of the extension of a given current. Regarding our particular data set, the above considerations pose the question whether Globigerina bulloides, G. quinqueloba and Globigerinita uvula are indeed reliable indicators of cold water environments, or if their ecological tolerance is too high for qualifying as adequate markers of cool temperatures. As noted above, unless unequivocal evidence on the ecological boundaries of their `core distributional areas' (i.e. those where they reproduce) is available, no ¢rm answer can be o¡ered. In reality, because the number of planktonic species whose thermic reproductive ranges are e¡ectively known is probably less than a fraction of a per cent of the taxa used for environmental and palaeoenvironmental analyses, indirect evidence, such as changes in morphology or, more often, areas of maximum abundance are used for de¢ning these core distributional areas (e.g. Be¨ et al., 1973; Hecht, 1974; Vincent & Berger, 1981). The fact that the above three foraminifers are very abundant in the plankton at water temperatures above 208C (Figure 4) suggests that they indeed may be capable of maintaining these high concentrations by local reproduction, rather than by immigration alone. It should be stressed that while the (probable) capacity of these foraminifers to reproduce beyond the Malvinas Current inhibits their use as tracers of this current, it does not preclude their use as sensors of the


20

40 0

5

10

15

Tabia I

10

38

Tabia II 42 46

50

Latitude South

0

0

G. bulloides G. quinqueloba N. pachyderma (left) G. uvula N. pachyderma (right) G. truncatulinoides G. scitula

34

50

54

G. inflata G. glutinata

0 30

Tabia I (6-21 Nov. 1993) Tabia II (4-16 Nov. 1994) 20

Cold water

Cold water Warm water

Percent of all forams (0-50 m only)

90

G. ruber G. rubescens G. trilobus G. aequilateralis G. menardii G. conglobatus G. dutertrei G. falconensis H. pelagica

Percent of all forams

30


Warm water

58

G. ruber 0

10

20

G. rubescens 0

4

8

0

30

60 5

20

40

0

40

80 0

25

50

30

34 38 42 46 50

54 58

G. inflata

G. glutinata

G. uvula

G. quinqueloba

N.pachyderma (left-coiling)

G. bulloides

Figure 7. Comparison of relative foraminiferal abundances in November 1993 (TABIA I, E. Boltovsoy et al., 1996), and in November 1994 (TABIA II, this work).

environmental conditions of the area they dwell in (see D. Boltovskoy, 1986, for a discussion of the di¡erences between biological tracers and sensors). This area, however, does not seem to be well circumscribed by Subantarctic waters, but centres on the transition zone, for which reason they might be ecologically closer to Globorotalia in£ata, than to N. pachyderma. Interannual consistency of distributional trends

Comparison of the present data set (TABIA II) with the collection retrieved along a similar transect in November 1993 (TABIA I, E. Boltovskoy et al., 1996) shows remarkable consistency. Figure 3D indicates that overall foraminiferal densities in the upper layers (5^15 m for TABIA I, 0^5 m for TABIA II) are similar, although in 1994 (5^15 m overall mean: 2.4 ind l 1 ) most samples yielded slightly higher foraminiferal abundances than in 1993 (0^5 m mean: 1.1ind l 1 ). Chlorophyll-a concentrations were somewhat higher in 1994 (overall mean: 0.48 mg l 1), as compared with those in 1993 (mean: 0.39 mg l 1 ), and also particulate organic carbon (POC) and particulate organic nitrogen (PON) levels were higher in 1994 (POC 1993: 0.16 mg l 1, 1994: 0.28 mg l 1 , PON 1993: 0.02 mg l 1, 1994: 0.04 mg l 1). The 1994 peak in foraminiferal numbers at *56^578S was also absent in 1993 (Figure 3D). Interestingly, di¡erences in chlorophyll-a values between 1993 and 1994 were especially noticeable south of approximately 50³S: 1993 values in this area hardly reached 0.4 mg l 1 , whereas in 1994 Journal of the Marine Biological Association of the United Kingdom (2000)

patches with over 0.8^1.2mg l 1 were common (Figure 3C). This may indicate better feeding conditions during TABIA II, thus contributing to the maintenance of denser protist populations. Although the two datasets contrasted are derived from somewhat di¡erent depth ranges, (quantitative information for the TABIA I cruise is limited to the data presented herein) foraminiferal concentrations in the upper 15 m did not show major vertical discontinuities, which suggests that interannual absolute abundances are comparable in general terms. Figure 7 illustrates the proportions of the most abundant foraminifers along both transects (curves), as well as overall contributions to whole foraminiferal assemblages (bars), suggesting that distributional patterns were very similar in November 1993 and in November 1994. Similarity between the distributional compared patterns reinforces the assumption that the data derived are not the product of short-lived `non-normal' conditions, but indeed re£ect the abundance relationships usual for this time of the year. Foraminiferal distribution ranges as a function of SST: comparison with previous data

Be¨ & Tolderlund (1971) produced the most comprehensive account so far of the biogeography and thermic re¨gimes of the planktonic foraminifers of the Atlantic Ocean.While the patterns found in our survey generally agree with their data, there are some discrepancies worth pointing out. Our

212



records indicate that at temperatures above 17^188C in the area concerned Globigerinoides ruber is less abundant (around 25% of all foraminifers in TABIA I and II) than reported by Be¨ & Tolderlund (1971) (*60^70%). In the southwestern Atlantic, Globigerina bulloides, and G. quinqueloba show considerably greater northward extensions and reach maximum percentage values at noticeably higher SSTs than those found by Be¨ & Tolderlund (1971). Neogloboquadrina pachyderma (left right) also spreads further north in our datasets, but its peak relative abundances are rather well circumscribed to waters colder than 78C, and the peak at 13^158C reported by Be¨ & Tolderlund (1971) was absent from our two cruises. We are indebted to Victor Zlotnicki for supplying the satellite SST data presented in Figure 2. This work was supported by grants from the University of Buenos Aires, Argentina (UBA EX-040), from the Consejo Nacional de Investigaciones Cient|¨ ¢cas y Te¨cnicas, Argentina (CONICET PID-BID 366), and from the Conselho Nacional de Desenvolvimento Cienti¢co e Tecnologico, Brazil (CNPq PROC no. 670001/93-7). Logistic and technical assistance was provided by the Diretoria de Hidrogra¢a e Navegac°a¬o (Brazil), by crew members aboard Àry Rongel' (Brazil), and by A.I. Matsubara (Argentina).

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Submitted 26 November 1998. Accepted 28 April 1999.