Major nutrients and dissolved oxygen as indicators of the ...klinck/Reprints/PDF/dafnerJGR2003.pdfMajor nutrients and dissolved oxygen as indicators of the frontal zones in the Atlantic

Major nutrients and dissolved oxygen as indicators of the frontal zones

in the Atlantic sector of the Southern Ocean

E. DafnerDepartment of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa,Honolulu, Hawaii, USA

N. Mordasova, N. Arzhanova, V. Maslennikov, Y. Mikhailovsky, I. Naletova,

V. Sapozhnikov, P. Selin, and V. ZubarevichRussian Federal Research Institute of Fisheries and Oceanography (VNIRO), Department of Marine Ecology, Moscow,Russia

Received 1 December 1998; revised 14 June 2000; accepted 23 April 2003; published 15 July 2003.

[1] The chemical tracers of the main frontal zones of the Atlantic sector of the SouthernOcean are considered. Before the beginning of the spring bloom, frontal zones aredistinguished by lateral gradients of dissolved oxygen, phosphate, nitrate, and silicate.During the spring bloom, the smoothing of nutrient concentrations on both sides of thefronts weakens lateral gradients of chemical properties. The position of surface gradientsof nutrients within the Subtropical Frontal Zone (STFZ) does not coincide with thelocation of temperature and salinity gradients. As a result, fronts in this region have astepped character. The best chemical indicator of the Northern STFZ front is the dissolvedoxygen gradient, which coincides with the temperature and salinity gradients. Thesouthern boundary of the STFZ is distinguished by the gradient of nitrate. The chemicalcriterion for identifying the Subantarctic Front is the gradient of oxygen, which rangesfrom 0.5 to 4.0 mmol kg�1 per km; the Polar Front is identified by the gradient of silicate(0.56 to 2.78 mM per km). At the surface, the Weddell-Scotia Confluence (WSC) isdistinguished not by the temperature and salinity, but by chemical parameters: The bestyear-round criterion is the lateral gradient of silicate-to-phosphate atomic ratio, whichranges from 25 to 35. Other markers of the WSC are the gradients of silicate at the surface,oxygen at the upper boundary of the Circumpolar Deep Water, and the depths of itslocation. INDEX TERMS: 4845 Oceanography: Biological and Chemical: Nutrients and nutrient cycling;

4808 Oceanography: Biological and Chemical: Chemical tracers; 4283 Oceanography: General: Water masses;

KEYWORDS: nutrients, dissolved oxygen, chemical indicators, Atlantic sector of the Southern Ocean

Citation: Dafner, E., N. Mordasova, N. Arzhanova, V. Maslennikov, Y. Mikhailovsky, I. Naletova, V. Sapozhnikov, P. Selin, and

V. Zubarevich, Major nutrients and dissolved oxygen as indicators of the frontal zones in the Atlantic sector of the Southern Ocean,

J. Geophys. Res., 108(C7), 3227, doi:10.1029/1999JC000288, 2003.

1. Introduction

[2] Research over the past 3 decades has identifiedseveral frontal zones associated with the Antarctic Circum-polar Current (ACC). These include the Subtropical FrontalZone (STFZ), the Polar Frontal Zone (PFZ) [Nowlin et al.,1977; Joyce et al., 1978], and the frontal zone between thesouthern boundary of the ACC and waters in the polarregime [Orsi et al., 1995]. Each of these frontal zones(Figure 1) is bordered by two fronts (for more details, seeBelkin and Gordon [1996]), is convergent in nature [Hof-mann, 1985], consists of continuous circumpolar circulation(except the STFZ) [Orsi et al., 1995], and borders geo-graphical and biological zones of the Southern Ocean[Gordon et al., 1977; Deacon, 1982; Makarov et al.,

1990; Fedulov and Shnar, 1990]. From their spatial andtemporal scales, these features may be considered as plane-tary frontal zones [Fedorov, 1983].[3] A frontal zone between the southern boundary of the

ACC and waters of the polar regime in the Atlantic sector ofthe Southern Ocean is known as the Weddell-Scotia Con-fluence (WSC). This confluence divides waters of theWeddell and Scotia Seas [Gordon, 1967; Bogdanov et al.,1969]. Along the Antarctic coast, the Continental SlopeFront separates the Coastal Continental Shelf Zone from theAntarctic Zone [Stanley, 1992]. Foster and Carmack [1976]have described the frontal mixing zone during the formationof the Antarctic Bottom Water in the Southern Weddell Sea.Stramma and Lutjeharms [1997] have also considered theAgulhas Front as a component of the Southern Ocean.[4] The first maps of dissolved oxygen distribution in the

Atlantic Ocean were published after the expedition of theRV Meteor, 1925–1927 [Wattenberg, 1932]. These maps

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C7, 3227, doi:10.1029/1999JC000288, 2003

Copyright 2003 by the American Geophysical Union.0148-0227/03/1999JC000288$09.00

20 - 1

Figure 1. Atlantic sector of the Southern Ocean. (a) Principal bathymetric features; depths less than3000 m are shaded [Whitworth and Nowlin, 1987]. (b) Upper layer circulation pattern and geographicnomenclature from historical data; shaded squares and vertical lines represent areas discussed in thisstudy: 1, Falkland Islands; 2, South Georgia Island; 3, Ewing Bank; 4, South Sandwich Islands; 5, SouthOrkney Islands; 6, South Shetland Islands.

20 - 2 DAFNER ET AL.: MAJOR NUTRIENTS AND DISSOLVED OXYGEN IN FRONTAL ZONES

were the first detailed description of the oxygen distribution.Measurements during this cruise were done as far south as thePFZ area. Observations from this expedition are perhapsthe one reason why it was possible to define the position ofthe Subtropical Convergence based only on its oxygendistribution. Long-term chemical data were also obtainedduring the International Geophysical Year in 1957–1958[Department of Geodesy and Cartography of USSR, 1966].[5] It is common to use chemical properties as tracers of

deep water masses [Sievers and Nowlin, 1984; Broecker etal., 1985; Whitworth and Nowlin, 1987; Peterson andWhitworth, 1989; Read and Pollard, 1993] because theseproperties do not display seasonal variability, as do thesurface layers. Over the past decade, several studies haveshown the distribution of chemical properties at differentsurfaces in all sectors of the Southern Ocean. Most ofthese studies, however, reflect only large-scale distributionof dissolved oxygen and some nutrients [Department ofGeodesy and Cartography of USSR, 1966; Department ofNavigation and Oceanography, 1977, 1974; Gordon andMolinelli, 1982]. When using large-scale analyses it isvery difficult to trace some features of the circulation andregional variations (e.g., the position of frontal zones)because these atlases present only averaged or smoothedpictures of property distributions. Patterson and Sievers[1980] noted, ‘‘In spite of the non-synoptic nature andsometimes questionable quality of the data, the distribu-tions show that in the Southern Ocean, water massproperties are relatively constant within bands parallel tothe direction of flow of the Antarctic Circumpolar Current,which is nominally zonal, but these properties change inthe cross-stream or meridional direction.’’ In some areas,these changes occur abruptly at the sharp zonal fronts.Park et al. [1993] have also stressed that the other majordifficulty arising from the various criteria proposed in theliterature for characterization of the fronts is that somefronts are defined based on particular surface or subsur-face property values, while others use phenomenologicaldefinitions.[6] The present study focuses specifically on the synoptic

and mesoscale nature of chemical tracers in the STFZ, PFZ,and WSC. Our specific aim is to avoid smoothing throughthese narrow fronts. The main objectives of our researchare (1) summarizing knowledge of the dissolved oxygenand nutrient distributions along planetary frontal zones;

(2) defining the optimal chemical tracers for identifyingeach frontal zone; and (3) studying the variability ofchemical properties at the PFZ and determining factorsresponsible for this variability.

2. Data

[7] The data described here were collected in the Atlanticsector of the Southern Ocean during several expeditions ofthe RV Academic Knipovich and the RV Vozrozhdeniye(Table 1 and shaded squares in Figure 1). Samples weretaken between surface and 1000 m at extreme points(maximum or minimum) of the vertical temperature,salinity, oxygen, and chlorophyll a (Chl a) distributionsusing a General Oceanics water bottle rosette system,mounted on a Neil Brown Mark 3 CTD. The CTD wastank-calibrated for temperature and pressure before andafter each cruise.[8] Chemical constituents were measured on board im-

mediately after sampling. Oxygen was measured using theWinkler method [Sapozhnikov et al., 1988]; nitrate, nitrite,phosphate, and silicate were analyzed using an AutoAna-lyzer Technicon

1

IIAC system [Technicon, 1973a, 1973b,1977]. During cruises of the RV Academic Knipovich,nutrient measurement methodologies described by Sapozh-nikov et al. [1988] were used. Specification of the chemicalanalyses, including precision, accuracy, and detection limits,are provided in Table 2. For example, for cruise XIII ofthe RV Vozrozhdeniye the relationship of nitrate to phos-phate in deep waters (>175 m), is 14.76 ± 0.56 (n = 470),which is similar to the value of 14.29 ± 0.32 reportedpreviously by De Baar et al. [1997] for the Atlantic sector.This ratio is not significantly different from the global valueof 15 published by Fanning [1992]. The slope and interceptof the regression,

NO�3 ¼ 12:86HPO2�

4 þ 4:02 r2 ¼ 0:87; n ¼ 470� �

;

agree with previous measurements from the Southern Ocean[De Baar et al., 1997; Bainbridge, 1981], indicating that ourdata is close to that obtained from nutrient measurementsconducted in the Southern Ocean.

3. Results and Discussion

[9] Criteria for identifying fronts in the Southern Oceanmay be divided into two classes. These include quantitativecriteria, which distinguish fronts based on gradients oftemperature, salinity, and other indices, and qualitative, orphenomenological criteria, which distinguish fronts byisopleth patterns. As a rule, phenomenological criteria are

Table 1. Data Used During the Study of the Frontal Zones in the

Atlantic Sector of the Southern Ocean Collected by the Department

of Marine Ecology of Russian Federal Research Institute of

Fisheries and Oceanography (VNIRO)

Research Vessel Cruise Date Area

Academic Knipovich XXIII January to April 1979 WSCVozrozhdeniye XII September 1987 to

February 1988PFZa

Vozrozhdeniye XIII March to June 1988 PFZVozrozhdeniye XIV August 1988 to

February 1989PFZ

Vozrozhdeniye XV March to April 1990 PFZ, STFZAcademic Knipovich XXV December 1989 to

May 1990PFZ, WSC

aHereafter the area of research in the PFZ was bordered by 25�300S–40�000S, and 46�300W–52�000W.

Table 2. Overall Performance of Oxygen (mmol kg�1) and

Nutrients (mM) Analysis We Used in the Atlantic Sector of the

Southern Ocean

NutrientDetectionLimit Precision

Accuracy atTypical Concentration

Oxygen 2.0 0.6 0.6 mmol kg�1 at 7.8 mmol kg�1

Silicate 0.5 0.5 1.5 mM at 100 mMPhosphate 0.02 0.02 0.05 mM at 2.0 mMNitrate 0.5 0.5 0.5 mM at 30.0 mMNitrite 0.05 0.05 0.05 mM at 0.5 mM

DAFNER ET AL.: MAJOR NUTRIENTS AND DISSOLVED OXYGEN IN FRONTAL ZONES 20 - 3

used for the analysis of water mass structure, while quan-titative indices are used to identify fronts at the surface.

3.1. Phenomenological Identification of Fronts

[10] One of the main advantages of using phenomeno-logical criteria for identifying of fronts is the quasi-perma-nent nature of frontal properties of the South Ocean frontalzones. The PFZ has been studied most extensively usingphenomenological criteria and has been defined as thelocation where the temperature minimum deepened below200 m [Deacon, 1933; Mackintosh, 1946; Lutjeharms andEmery, 1983], as the northern limit of the subsurfacetemperature inversion [Garner, 1958], as the location ofthe minimum salinity axis at 200 m [Ostapoff, 1962;Gordon et al., 1978; Taylor et al., 1978], and as the locationof the temperature minimum [Gordon, 1967]. Proposeddefinitions for the SAF include the rapid deepening of thesubsurface salinity minimum [Whitworth and Nowlin,1987], the origin of the intermediate salinity minimumnorth of the SAF [Peterson and Whitworth, 1989], andthe location where the salinity minimum layer crosses the400 to 600 m depth for tracing the SAF [Patterson andWhitworth, 1990]. However, even these definitions cannotbe applied at all locations. In the southwestern Atlantic,Peterson and Whitworth [1989] could not define the bound-aries of the PFZ at the surface by gradients of temperatureand salinity. Instead they used the location of enhancedvertical displacements of property isopleths.[11] Recently, numerous sections which provide a de-

tailed view of the Southern Ocean structure were completed.These sections differ in both spatial resolution and in thechemical properties being analyzed. Figure 2 shows thevertical distribution of physical and chemical properties atthe composite section along the Greenwich meridian con-structed from data by Whitworth and Nowlin [1987]. Thedistributions of temperature, salinity, density, dissolvedoxygen, phosphate, nitrate, and silicate show that the mostprominent features along vertical sections are related to thefronts. The ACC fronts in Figure 2 correspond to regions ofsharp latitudinal gradients of all parameters from the surfaceto the deep waters. In the vertical plane, the fronts clearlydistinguish themselves as regions of increased isoplethslope.[12] When the phenomenological approach is used, fronts

along this section are clearly depicted as a series of stepswhich gradually sink from the surface. These steps areassociated with the subduction of Antarctic IntermediateWater (AAIW), which forms in the APF region. Analysis ofphysical and chemical properties demonstrates that AAIWsinks from about 100 m near the APF down to 800–1200 mnear the SAF. Along the section shown in Figure 2, the SAFis observed between the surface salinity minimum and adepth of 450 m. The step of the temperature minimumbetween 300 m and 850 m corresponds to the APF [Whit-worth and Nowlin, 1987]. In addition, there is anotherfrontal zone between the ACC and Weddell Sea near55�S. Differences of physical and chemical properties oneither side of this zone reflect the separation between waterson the southern periphery of the ACC and waters in theWeddell Gyre.[13] Figure 3 presents the large-scale distribution of

dissolved oxygen and nutrients along 49�300W based on

data from Gordon and Molinelli [1982] and data collectedby VNIRO [Arzhanova, 1990]. This figure shows that thefronts are easily separated from each other in the surfacelayer (0–100 m), where they serve as the boundariesbetween different water masses. Note that, in this area, theSTFZ occupies its most southern position in the Atlanticsector. In this same region, the SAF meanders as far north as43�S. The locations of frontal zones are distinguishable inthe vertical distribution of chemical properties, although it isvery difficult to identify the individual fronts which borderthem. The STFZ along this section can be identified in thedistributions of dissolved oxygen, phosphate and nitratebetween 43�S and 45�S (Figures 3a, 3b, and 3c). ThePFZ can be distinguished by its distribution of oxygen,nitrate and silicate (Figures 3a, 3c, and 3d) between 53�Sand 55�S. The WSC (59�S–60�S) is most easily identifiedfrom its silicate distribution (Figure 3d). The positions ofthese frontal zones, inferred from chemical properties, are ingood agreement with those described in the literature [e.g.,Peterson and Whitworth, 1989].

3.2. Chemical Characteristics of the Surface Fronts

3.2.1. Subtropical Frontal Zone[14] The Subtropical Frontal Zone separates waters of the

South Atlantic Current (a term introduced by McCartney[1977]) and the ACC. Stramma and Peterson [1990]pointed out that fronts are water mass boundaries that needto be associated with currents. Thus the STFZ separatessubtropical from subpolar waters and associates with theSouth Atlantic Current [Stramma and Peterson, 1990]. Thiszone is the least studied frontal zone of the Southern Ocean.Previously, the STFZ was known as Southern SubtropicalConvergence [Meyer, 1923], then as the Subtropical Con-vergence [Deacon, 1933, 1937], and later as the SubtropicalFront [Whitworth and Nowlin, 1987; Hofmann, 1985]). It iswell known that the STFZ has a complex two-steppedcharacter; that is, the STFZ is limited by the northern andsouthern fronts [Lutjeharms, 1985; Lutjeharms and Valen-tine, 1988], and that it is the northernmost frontal zonewithin ACC system [Stramma and Peterson, 1990]. Belkinand Gordon [1996] have proposed the use of terms Northand South Subtropical Fronts for the fronts which border theSTFZ from the north and south, respectively.[15] Many researchers proposed different criteria for dis-

tinguishing the STFZ, for example, by the temperaturegradient on the surface or the position of the 34.9 isohalineon the surface. Commonly, the northern boundary of theSTFZ is identified by gradients of temperature and salinity,which vary across the front from 4� to 5�C [Deacon, 1933,1960; Stramma and Peterson, 1990] and from 0.4 to 0.5 insalinity [Deacon, 1982; Stramma, 1992], respectively. Atpresent, the southern boundary of the STFZ is identified bythe gradient of temperature [Zemsky and Zozulya, 1990],which could reach up to 1�C per km south of Africa, asreported by Lucas et al. [1995].[16] Figure 4 depicts results of an investigation of

mesoscale structure at a site within the STFZ (39�300–43�S and 17�300–16�W) in early autumn 1990 during theXVth cruise of RV Vozrozhdeniye. This figure illustrates thecoincidence between the dissolved oxygen gradient, withgradients of temperature and salinity, and the axes oftwo high-speed flows corresponding to the northern and


southern borders of the STFZ. In this area, the STFZ hasbeen found over the Mid-Atlantic Ridge, which causesmeandering of the STFZ to the south. Moreover, a decreaseof oxygen concentration, down to 288 mmol kg�1, wasobserved in crossing the STFZ from the west to the east. Achange in phosphate concentration (0.4–0.7 mM) was alsoobserved between 40�S and 41�S near the North Front ofthe STFZ. Large variability in nitrate content (0.5–6.5 mM)was found between 41�S and 43�S at the South Front of theSTFZ.[17] The STFZ is a good example of the stepped character

of the frontal zones, which results from a lack of coinci-dence of thermohaline and chemical properties (Figure 4).

As a rule, good coincidence of gradients of thermohalineparameters and dissolved oxygen characterize both thenorthern and southern boundaries (Figure 4). The gradientof dissolved oxygen at 40�S reached about 5 mmol kg�1 perkm. The gradient of phosphate is commonly observedfurther south than the gradient of dissolved oxygen (at adistance of about 60 km). The gradient of nitrate wasregistered at the same distance (60 km) from the gradientof phosphate.[18] A similarly complex multistepped structure was

observed in the STFZ in April–May 1987 along a sectionbetween Africa and the Prince Edward Islands. At that time,the STFZ, distinguished by the decrease of temperature

Figure 2. Location of core layers along the Greenwich meridian (0�) corresponding to the frontsobtained according to the data of Whitworth and Nowlin [1987].


from 16 to 9.2�C, was located in the latitudinal range of43�050–43�440S. The increase of phosphate and nitrateconcentrations up to 0.91 and 16.2 mM, respectively, wasmost remarkable in the region between 42�250 and 43�590S[Duncombe Rae, 1989]. Similarly, Van Bennekom et al.[1988] and Lutjeharms et al. [1985] found a steep increasein phosphate and nitrate contents in practically the samearea of the Atlantic sector (14�E–4�E, 42�S–�45�S).[19] Chemical studies in the Southern Ocean have dem-

onstrated that nitrate concentration is the best chemical

marker of the STFZ. For example, while crossing the STFZin the Atlantic sector of the Southern Ocean, one can see anaverage increase from 0.5 to 6.5 mM; data obtained byDuncombe Rae [1989] show an increase of nitrate contentby a factor of 16.5. Laubscher et al. [1993] have also foundthat nitrate rose 3.5 times (from 0.70 to 2.44 mM) and thatphosphate levels doubled (from 0.25 to 0.50 mM) south ofthe STFZ in the Atlantic sector. In the Pacific sector, nitratecontent in the STFZ increased by a factor of 100, from 0.1to 10.0 mM [Stunzhas, 1988; Metreveli et al., 1993]. This

Figure 3. Large-scale distribution of (a) oxygen (mmol kg�1), (b) phosphate, (c) nitrate, and (d) silicate(mM) along 49�300W. This section was designed using our observations reported here and database fromGordon and Molinelli [1982], after Arzhanova [1990].


range is equal to the maximum variability of nitrate con-centration observed for the whole Southern Ocean.3.2.2. Polar Frontal Zone3.2.2.1. Chemical Signatures of the Subantarctic andAntarctic Polar Fronts[20] There are many reviews describing the history of

research and definition of the PFZ and the fronts bordering it(the most recent are by Park et al. [1993], Orsi et al. [1995],and Belkin and Gordon [1996]). We adopted a definition andterms used in these papers and describe data which were

collected in the region of the PFZ to the north of SouthGeorgia Island during the period from October 1987 to April1990 (Table 1).[21] Figure 5 shows that the distribution of oxygen and

silicate was patchy and complex. There is a good agreementin the distribution of thermohaline and chemical propertiesalong fronts which are strongly affected by bottom topog-raphy (Figure 1a). Patchiness in the distribution of waterproperties was due partly to spatial and temporal variabilityof the flow configuration of the PFZ and partly to the

Figure 3. (continued)


injection of the Subantarctic and Antarctic waters into thePFZ. According to Peterson and Whitworth [1989], thisrepresents a recurring mechanism for exchange of heat andsalt across the two fronts. There is also some evidence foran interaction between Subantarctic and Antarctic waterssouth of the APF. For example, Trathan et al. [1997] havefound a warm patch of Subantarctic water located approx-imately 150 km south of the APF to the north of SouthGeorgia Island. Advection of an admixture of Subantarcticwaters along 6�W as far south as 52� was reported by DeBaar et al. [1997].[22] As illustrated by Figures 5a, 5b, 5c, 5e, and 5f, the

gradient of dissolved oxygen is a good chemical indicator ofthe SAF at the surface, while the latitudinal gradient of silicateis a good tracer of the APF (Figures 5a, 5c, 5d, and 5e).The oxygen gradient at the surface increased from 0.5 to4 mmol kg�1 per km and the gradient of silicate increased from0.6 to 2.8 mM per km, depending on the season and hydro-logical conditions. Maximum values occurred over thenorthern side of Ewing Bank (48�S–49�S, 38�W–40�W)where the PFZ narrows to form a strong continuous gradient(Figure 5).[23] Peterson and Whitworth [1989] described the posi-

tion of fronts bordering the PFZ in the southwesternAtlantic observed from the RV Melville (without nutrientdata) during early 1986 and 1987. Over the northern side ofEwing Bank, they found that the Subantarctic and PolarFronts appeared to merge, forming a single, intense currentcore. They concluded that the SAF and APF were adjacentand indistinguishable from one another; that is, ‘‘there is noPFZ.’’ As suggested by Figure 5, the general spatialpatterns of the Subantarctic and Antarctic Polar Fronts,which were identified by Zozulya et al. [1990] on the basisof the complexity of the thermohaline criteria [Antipov etal., 1987], were similar to those observed from the RVMelville.

[24] As the merged front follows the narrow ridge, itsposition is very variable (Figure 5). During austral summerand autumn of 1990, the Subantarctic and Antarctic regionwere characterized by anomalously high heat content due tothe less intensive northward advection of Weddell Seawater[Dafner and Selin, 1993]. At this time, the Weddell-ScotiaConfluence was shifted 160–180 km south of its meanposition [Maslennikov, 1979; Maslennikov and Solyankin,1988]. As a result, the fronts bounding the PFZ at 30�Wwere located 180 km south of the positions observed duringthe previous 2 years [Mordasova et al., 1990]. For theperiod from January to April, we traced the shift of the SAFand APF at 40�W by 35–55 km to the south and north,respectively. These shifts reduced the width of the PFZ toabout 55 km. This is a factor of 2–4 smaller than the PFZwidth observed in Drake Passage [Sievers and Nowlin,1984] and along the Greenwich meridian [Whitworth andNowlin, 1987].[25] As our observations showed, the Antarctic and

Subantarctic waters differed not only in the absolute valuesof their chemical properties, but also in the verticaldistribution of their chemical constituents. In Subantarcticwaters, the relatively homogeneous upper layer was ratherthick (�200–300 m), while in most cases the thickness ofthe homogenous layer in Antarctic waters did not exceed50–75 m (Figure 3). The depths of the oxygen minimumand phosphate maximum also differed. These depthsdecreased from 1000 m and more in Subantarctic watersto less than 500 m in Antarctic waters.[26] The identification of the SAF by gradients of dis-

solved oxygen is possible due to the interaction of waterswith different oxygen content on either side of the PFZ. Thecold Antarctic Surface Water (AASW), with its high oxygencontent, sinks under the warmer Subantarctic Surface Water,which has considerably lower dissolved oxygen content.The APF is distinguished by the gradient of silicate which

Figure 4. Distributions of (a) temperature (�C), (b) salinity, (c) dynamic topography of surface watersrelative to 1000 m (dyn. m), (d) dissolved oxygen (mmol kg�1), (e) phosphate, and (f) nitrate (mM)concentrations at mesoscale site in the Subtropical Frontal Zone above the Atlantic Mid-Ridge (data fromthe XV cruise of RV Vozrozhdeniye, 20 March to 10 April 1989).


differs in Subantarctic and Antarctic waters. During thespring phytoplankton bloom, this difference is smooth andthe latitudinal silicate gradient in the APF is reduced(Figures 5b and 5f). Laubscher et al. [1993] used the smallincrease of silicate concentration (from 6.8 to a maximum of11.7 mM) to identify the position of the APF (for compari-son, also see the location of the PFZ in Figure 3).[27] Peterson and Whitworth [1989] studied the structure

of the PFZ in the northern Drake Passage, where the SAFmeanders to the north. They found that the PFZ waters inthis area were under the influence of Subantarctic ModeWater (SAMW), which forms in the southeastern Pacific as

a result of late winter deep convection. SAMW can beidentified by its low temperature and salinity and highoxygen content [McCartney, 1977]. In the northern DrakePassage the distribution of oxygen and nutrients could notbe used to identify the SAF and APF positions.[28] According to Peterson and Whitworth [1989], it was

possible to identify gradient zones of nitrate and phosphateonly within the PFZ, i.e., from the opposite side of theinteraction zone between SAMW and SAF. Concentrationsof these nutrients decrease from 22.3 (in the PFZ waters) to16.1 mM for nitrate in the Subantarctic Zone (SAZ), andfrom 1.62 mM (in waters close to the PFZ) to 1.24 mM for

Figure 5. Position of the Subantarctic Front and Antarctic Polar Front and distribution of oxygen (mmolkg�1) and silicate (mM) according to the data obtained in the cruises of the RV Vozrozhdeniye. (a, b) XIIcruise; (c, d) XIII; (e) XIV; (f) XV. Thin line marks the location of the SAF, thick line marks the positionof the APF; positions of both fronts were identified by phenomenological criteria of the temperature(APF) and salinity (SAF) vertical distributions [Zozulya et al., 1990].


phosphate in the SAZ [Peterson and Whitworth, 1989]. Onthe basis of these values, we can estimate gradients of about0.14 mM per km for nitrate and 0.0038 mM per km forphosphate. The SAMW is isothermal, isohaline, well-oxy-genated, and homogenous in nutrients. The injection ofSAMW from the west into the PFZ causes the formation ofnitrate and phosphate gradients, which are restricted to theeastern side of the SAF meander.[29] At the Greenwich meridian, Whitworth and Nowlin

[1987] also observed that ‘‘the Subantarctic Front showsdramatic increases in surface nitrate and phosphate concen-trations. Within the Polar Frontal Zone, surface character-istics are nearly constant until the abrupt gradients associatedwith the Polar Front are encountered.’’ In contrast, we couldnot observe the sharp gradient zones of nitrate and phosphateto the north of South Georgia Island. While crossing thePFZ from the SAF to APF, pronounced lateral gradients ofnitrate and phosphate were, however, observed. Concen-trations of nitrate and phosphate increased by 8.0 and0.42–0.64 mM, respectively, along this section. Theseincreases are of the same order as those reported byPeterson and Whitworth [1989] in the northern DrakePassage and by Whitworth and Nowlin [1987] along theGreenwich meridian.[30] In summary, the dissolved oxygen content increased

at the SAF, the nitrate and phosphate concentrations in-creased within the PFZ, and the silicate concentrationincreased across the APF. In addition to these mentionedgradients, zonal gradients of chemical properties were alsoobserved. These zonal variations were consistent withprevious analyses of nutrient distributions [e.g., Gordonand Molinelli, 1982].3.2.2.2. Seasonal Variability of Chemical Properties inthe PFZ Surface Waters[31] Data demonstrating variability of the chemical struc-

ture of the PFZ north of South Georgia Island is presentedin Table 3 and in Figure 5. In this area of the PFZ,mesoscale variability associated with the activity of phy-toplankton in the upper 50 m affects variability associatedwith the advection of the Weddell Sea modified water inthe 200-m layer. The influence of phytoplankton on thechemical structure of Antarctic waters adjacent to the APFwas patchy during all periods of our observations. Char-acteristic length scale of this variability was on the order of20–30 km [Bondarenko, 1990; Dafner and Mordasova,1994]. As shown in Figure 6, good agreement wasobserved between regions of high Chl a and oxygenvalues, and low silicate, phosphate, and nitrate concentra-

tions. This figure demonstrates that the shift in phosphateconcentrations is about 0.5 mM and about 10 mmol kg�1 inoxygen. If only primary production was the cause of thisshift, the oxygen shift should be around 80 mmol kg�1.Hence the primary production is not the only explanation.We cannot also preclude physical processes, such as aninteraction and mixing of different water masses and air-seaoxygen exchange. Similarly, in the area of the APF southof Tasmania, Yamamoto [1986] found maximum Chl avalues in a zone 10 km wide, although in most cases, thedevelopment of phytoplankton in the Southern Ocean wasobserved at spatial scales of about 100 km [Poisson et al.,1994].[32] Observations showed that in October 1987, spring

had already begun in the region of the APF, according toall environmental indices (see Table 3). In areas of strongstratification, photosynthesis was particularly active. Thehigh concentration of nutrients illustrated that this periodcoincided with the beginning of the phytoplankton bloom.Observations of atomic Si/P ratios equal to 25 indicatedthat the southern periphery of the PFZ was influenced bythe modified Weddell Sea water (this ratio is discussed ingreater detail in section 3.2.3).[33] In December 1987, when according to the tempera-

ture data, the seasonal warming of surface layer wasobserved, the seasonal thermocline developed everywhere.At this time, the chemical data indicated a peak of phyto-plankton, saturation of oxygen in the surface waters over thewhole area was 110–118%, and the silicate concentrationdecreased to 1.1–3.3 mM. A similar situation was observedin December 1988 (Figures 6 and 7). South of the APF thesummer season was at its peak, and almost completeconsumption of silicate was observed in the surface layer,with concentrations ranging from undetectable to 1.1–2.3 mM. Laubscher et al. [1993] reported an analogoussituation for the late summer when the nutrient concentra-tions decreased across the APF. Nitrate concentrationsdecreased about threefold (from 20.9 to 7.4 mM), andsilicate concentrations decreased from 15.9 to 4.6 mM, butphosphate showed a modest increase from 1.19 to 1.62 mMat the southern edge of the front. All nutrients were presentat reduced concentrations in the proximity of the Chl amaximum at the APF.[34] Fukuchi [1980] and Banse [1996] demonstrated that

the pigment concentrations for the Antarctic and Subant-arctic waters were independent of the mixed-layer depth,although during period of observation in the southern PFZarea (October 1987 to April 1990), peaks of phytoplankton

Table 3. Seasonal Variability of Chemical Properties in the PFZ Surface Waters for the Period From October 1987 to

April 1990a

Period of Observations Oxygen, mmol kg�1 Silicate, mM Phosphate, mM Nitrate, mM Si/P

October 1987 326–394 10–50 1.40–2.20 20–32 5–25December 1987 326–403 <1–25 1.00–2.00 10–25 <2.5–15March–April 1988 312–360 5–40 0.80–1.50 10–25 5–15June, 1988 312–374 5–45 0.80–1.75 10–25 5–25June 1989 336–403 <2–35 0.80–2.00 10–25 3–15October 1988 326–403 10–50 1.6–2.2 24–32 5–25January 1990 312–374 <1–<1 N.D. 10–20 N.D.April 1990 298–365 <2.5–10 <1.0–1.60 <5–>20 2–8aThe first values correspond to the Subantarctic waters near the SAF; the second ones correspond to the Antarctic waters near the APF.

For January and April 1990 data are referred only to the section along 40�W. N.D. means no data.


were always found after development of water stability. Inthis area of the Southern Ocean, APF waters were under theinfluence of modified Weddell Seawaters. This caused astratification of the surface layer and subsequent favorableconditions for intensive spring blooms.

[35] In June 1988, because of a warm and late autumn,the phytoplankton bloom continued north of 50�S, althoughthe intensity of photosynthesis was reduced as comparedwith the period between March and April. Large concen-trations of nitrite, especially in the summer-autumn period,indicated the high intensity of organic material accumula-tion and its mineralization in the PFZ region. In April 1988,increased concentrations of nitrite, of up to 1.50 mM at300 m and from 0.35 to 0.42 mM at 1000 m, were observedin the eastern part of the study area at the 25�W sectionacross the PFZ. In June 1989, patches of high nitriteconcentration, up to 0.40–0.60 mM, were noted over thewhole area; average values in waters deeper than 100 mexceeded 0.30 mM.[36] In June 1989, surface waters of the PFZ were

characterized almost everywhere by supersaturation ofoxygen by 5–7%. Values of supersaturation in the regionwere 4–8% higher than those observed during the sameseason of 1988. The silicate content for the whole regiondecreased by a factor of 1.5–2.0, on average, as comparedwith the same season of 1988 (Table 3). This could be eitherthe result of the pronounced influence of Subantarcticwaters in December 1987 or the result of a delay in thedevelopment of winter convection.[37] The period between January and April of 1990 was

anomalously warm. Low nutrient concentrations in thisperiod indicated an earlier phytoplankton bloom in compari-son with the other spring and summer data. As shown inTable 3, the lowest silicate concentrations were measuredduring this time, and concentrations close to the detectionlimit were observed. Similarly, Whitehouse et al. [1996]reported that anomalously high temperature and low silicateconcentrations were found around the South Georgia Islandduring January–February 1990. They also suggested thatthe plankton community had already passed its period ofmaximum growth by January 1990, and that maximumgrowth may here occurred up to two months earlier.[38] At the same time, the organic carbon and nitrogen

concentration data from the PFZ provide evidence oforganic matter accumulation at the vertical boundaries ofAAIW [Dafner and Selin, 1993]. At the surface sectionfrom the Falkland Islands to the South Orkney Islands analmost complete consumption of nutrients by phytoplanktonwas observed (data not shown). As a result, it was impos-sible to identify the PFZ by nutrient gradients. A slightincrease of silicate concentration, up to 10 mM, was noted

Figure 6. (a) Dynamic topography at surface watersrelative to 400 m (dyn. m), and distribution of (b) oxygen(mmol kg�1), (c) phosphate, (d) nitrate, (e) silicate (mM), and(f) Chl a (mg L�1) at the surface of the mesoscale site (9–11December 1988) [Dafner and Mordasova, 1994].

Figure 7. Mesoscale vertical distribution of (a) silicate, (b) phosphate, and (c) nitrate (mM) along52�300S [Dafner and Mordasova, 1994].


only in the Weddell-Scotia Confluence region. Much highersurface silicate concentrations were reported in this area forJanuary–March 1975 by Gordon et al. [1977], who notedthat ‘‘The surface silicate distribution shows a spectacularincrease within the Weddell-Scotia Confluence, attainingvalues above 80 mM (an order of magnitude above the PolarFront Zone values).’’ The high silicate concentrations acrossthe WSC are also clear from the large-scale silicate distri-bution plotted in Figure 3.[39] Taken together, these observations in the surface

waters of the PFZ and adjacent AASW suggested thatalthough the minimum nitrate and phosphate concentrationswere high (15.3 and 0.85 mM, respectively), silicate deple-tion limited the diatom growth. Zentara and Kamykowsky[1981] found a similar situation in the region of the southeastPacific between 140�W–71�W and 30�S–60�S. They ob-served silicate depletion, while concentrations of nitrateremained high (about 15 mM). Jacques [1983, 1989] dem-onstrated that, in some Antarctic diatoms, the Michaelis-Menten half-saturation constant (Km) for silicate transportranged between 12 and 22 mM. At lower concentrations, thegrowth rate decreased quickly. De Baar et al. [1999] havealso recently reported much lower Km (from 0.4 to 5 mM) forlarge diatoms.[40] From the above-mentioned work, it is possible to

assume that the development of phytoplankton in this areaof the PFZ is severely limited by silicate concentration. Ironlimitation is not a primary factor, as was hypothesized forthe section across PFZ along 7�W by De Baar et al. [1995].The conclusion of silicate limitation along the northernborder of the Permanent Open Ocean Zone (i.e., thecircumpolar belt between 50�S and 60�S), was also reachedin a more detailed and extensive study by Treguer andJacques [1992].3.2.2.3. Gradient Zones of Chemical ParametersAssociated With the Activity of Phytoplankton[41] Chemical properties of the bloom which developed

within the APF during the spring of 1988 were evaluatedusing the method of superposition of surveys with differentspace and timescales. Four surveys, each containing 25–36stations, sampled with spatial scales of about 46 by 55 kmand temporal scales of 36–48 hours. These surveys weresuperposed on the main survey, to obtain samples over theentire region of interest. An example of the distribution ofchemical properties at one such mesoscale site is shown inFigures 6 and 7. For comparison, in the main survey(Figure 5e), the distance between stations was about54 km in latitude and about 80 km in longitude [Dafnerand Mordasova, 1994].[42] Before the spring phytoplankton bloom, the main

survey indicated that the lateral gradients of chemical andthermohaline properties coincided. Phytoplankton growthwas accompanied by a gradual change in the isolines ofchemical parameters. As a result, lateral gradients of silicate,phosphate, and nitrate developed, generally following thepatchy distribution of phytoplankton. The concentration ofChl a in the patches of increased phytoplankton biomassreached 4.5 mg L�1 (Figure 6f). Minimum nutrient concen-trations were observed in the cores of these patches at thesurface. Phosphate levels were less than 1.4 mM (Figures 6cand 7b) and silicate levels were less than 1.0 mM (Figures 6dand 7a). In contrast, at a section across the PFZ along 7�W,

De Baar et al. [1995] reported incomplete consumption ofnutrients during the bloom (nitrate about 24.3 mM, phosphateabout 1.82 mM, silicate about 21.4 mM), although values ofChl a reached approximately 1.2–2.4 mg L�1 and rates ofprimary production were about 1200–3000 mg C m�2 d�1.[43] The further development of phytoplankton makes the

lateral gradients of physical and chemical properties per-pendicular to each other, and gradients of chemical param-eters border the ‘‘patchy’’ phytoplankton distribution. In themost distinct of these ‘‘biological’’ fronts, the lateral silicategradient reached 7.22 mM per km. The chemical gradientzones associated with the phytoplankton activity were lessdistinct by the spring-summer period, i.e., because of theduration of the bloom season, and the intensity of windmixing.[44] As was already noted, the Antarctic waters adjacent

to the APF were the most productive waters in the area ofthe PFZ. Studies by Nurnberg et al. [1995] on barium in theAtlantic sector have shown that the export production ratejust south of the APF (3.2–5.6 g C m�2 yr�1) was similar tothe values from the coastal areas of the Bransfield Strait(0.1–6.8 g C m�2 yr�1), which is one of the mostproductive area of the Southern Ocean. Therefore, to theconclusion of Sullivan et al. [1993] that the highest phyto-plankton crops occur in the coldest waters of the SouthernOcean, it is necessary to add that the most intensive bloomsof the Open Southern Ocean occur in waters adjacent to thePFZ from the south.3.2.3. Weddell-Scotia Confluence[45] The first references regarding the Weddell-Scotia

Confluence can be found in Deacon’s [1933, 1937] workon the frontal zones of the southwestern Atlantic. Gordon[1967] gave the name ‘‘Weddell-Scotia Confluence’’ to thezone of intermixing of the Weddell and Scotia Seas waters.At present, it is known that the WSC does not separatewater bodies of different origin but that it separates amodification of AASW along the southern periphery ofthe ACC from a modification in the Weddell Sea. This is themain reason why the WSC is known as the SecondaryFrontal Zone in the Russian scientific literature [Oradovskyet al., 1969; Bogdanov et al., 1969; Maslennikov andSolyankin, 1979]. This term has not, however, found wide-spread use outside of Russia, although sometimes it ispossible to find it in the international literature [e.g., Fosterand Middleton, 1984]. At present, there is no clear ideaabout the location of the WSC in the eastern part of theregion, near the eastern termination of the Weddell Sea drift[Gordon, 1967; Patterson and Sievers, 1980; Fedulov,1986].[46] Analysis of thermal structure is the primary method

for identification of the WSC [Bogdanov et al., 1969;Solyankin, 1969]. The depth of the temperature maximum(Tmax) is the best index for this purpose. Gordon [1967]pointed out that Tmax corresponds to the upper layer ofCDW which is unaffected by mixing with waters above;that is, in this case, the temperature maximum is induced bythe overlying cold waters and does not characterize theCDW core. In the upper 100–150 m, the front is not welldefined, apparently because of mixing due to winds andwinter convection. Near the surface, the zone of maximumtemperature and salinity gradients is shifted to the north, asa rule, relative to the location of these features at depth.


[47] On the shelf of the Antarctic Peninsula, anothermodification of the WSC forms due to the interaction ofshelf waters and waters of the ACC southern periphery. Theshelf waters arise from high-latitude modification and arecharacterized by very low temperatures from the surface tothe bottom, high salinity, high oxygen and nutrient contents,and by the absence of signs of warmer deep water.[48] As a result of interaction between the shelf waters of

the Antarctic Peninsula and water of the southern peripheryof the ACC, a gradient zone forms due to the difference oftheir physical and chemical properties. Different authorsgave it different names. For example, Model [1958] distin-guished this front as the eastern boundary of the WeddellGyre and named it ‘‘The Bellingshausen Front’’; Makarovet al. [1987] considered it as a ‘‘Regional or LocalSecondary Frontal Zone’’; Guretsky et al. [1990] used theterm ‘‘High-Latitude Off-Shelf Frontal Zone’’ for its des-ignation. Chemical tracers of this frontal zone coincide withchemical criteria for identifying the WSC in the southernScotia Sea.[49] Surface waters of the Weddell Sea and ACC differ

considerably in silicate content in the upper 150–200 m(Figures 3 and 8). In the waters of the ACC, silicate contentranges from 5 to 40 mM, while in the Weddell Sea water,silicate content of 60–120 mM is observed. Hence it is likely

that the silicate gradient is sharpest in the zone where thesewaters interact [Oradovsky et al., 1969; Arzhanova, 1974;Gordon et al., 1977; Michel, 1984; Jones et al., 1990].[50] Table 4 displays chemical indices of the WSC in the

southwestern part of the Atlantic sector of the SouthernOcean. In this table, the changes of chemical propertieswithin the WSC are listed for the mixed frontal waters alongthe northern periphery to the mixed frontal waters along thesouthern periphery. Figure 8 depicts the distribution ofsilicate in the surface layer, values of the atomic ratio ofsilicate to phosphate, the absolute content of dissolvedoxygen at depth of its minimum on the upper boundary ofCDW, and the depth of its location in water column. Thesecharacteristics are good indicators of the WSC [Arzhanovaand Mikhailovsky, 1980; Arzhanova, 1982].[51] Owing to the difference in oxygen concentrations in

the layer of its minimum (between the ACC and WeddellSea waters in the region of WSC south of the Scotia Sea), azone in which the dissolved oxygen content changes from192–204 mmol kg�1 is formed. The area near the SouthernShetland Islands is characterized by considerably greatervariability in the distribution of dissolved oxygen. Valuesranging from 188 to 282 mmol kg�1 are observed at theboundary between the ACC waters and waters of theBransfield Strait. The other important characteristic of

60

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a) b)

c) d)

Figure 8. Distributions of (a) silicate (mM), (b) atomic relationship Si/P, (c) dissolved oxygen (mmolkg�1) on the upper boundary of the CDW, and (d) the depth of minimum oxygen location (m) in summer1979, the cruise XXIII of the RV Academic Knipovich [Arzhanova and Mikhailovsky, 1980].

Table 4. Main Chemical Characteristics of the Weddell-Scotia Confluence [Arzhanova et al., 1987]a

Chemical Indices Scotia Sea Bransfield Strait Shelf of the Antarctic Peninsula

Range of atomic Si/P ratio at the surface 10(15)–35(40) 20–40(45) 10–35Minimal oxygen concentration, mmol kg�1 188–228(240) 192–282 absentDepth of oxygen minimum, m less than 500 less than 400 absentOxygen concentration in the 200 m layer, mmol kg�1 216–252 216–252 216–240Width of gradient zone, km 90–180 60–150 90–180

aIn parentheses are shown maximum values of Si/P ratio and oxygen observed form the southern periphery of the WSC.


oxygen minimum distribution in the WSC is its depth,which corresponds to the upper boundary of the CDW.From 70� to 40�S it ranges from 250 to 1000 m and deeper.[52] Figure 9 depicts a spring bloom situation (January–

February 1976) within the WSC when oxygen saturationhas exceeded 120% and silicate concentration is reduced toless than 5 mM. At this time it was impossible to trace thelocation of the WSC from the surface silicate distribution,and Arzhanova [1982] introduced the atomic ratio Si/P asindicator of the location of the WSC in the surface layer(Figure 9d). She has pointed out that the atomic ratio ofsilicate to phosphate is the most conservative index of thefrontal zone between Weddell and Scotia Seas, taking intoconsideration physiological features of diatom phytoplank-ton. Silicate and phosphate are consumed by diatom phy-

toplankton in the ratio corresponding to that in its cells; thatis, to a large extent, this is determined by the speciescomposition of phytoplankton. On the basis of the chemicalcomposition of diatom phytoplankton cells, the atomic ratioof Si/P is about 35, on average. Using this value, it wasfound that the ratio Si/P was lower than 35 in the ACCwaters, while in the high-latitude modification waters, it washigher than is necessary for phytoplankton uptake.[53] Owing to intermixing of the above-mentioned waters,

an area is formed within the WSC where the atomic ratio ofsilicate to phosphate is equal to that in which it is consumedby phytoplankton. As a result, the values of atomic Si/P ratioin the zone itself varied insignificantly over the year,irrespective of the changes in silicate and phosphate con-centrations (Figures 8 and 9). Studies have shown that in the

Figure 9. Some summer characteristics of the Weddell-Scotia Confluence western from the AntarcticPeninsula: (a) dynamic topography of surface waters relative to 1000 m (in dyn. m, each isoline wasplotted in 10 m) [Makarov et al., 1982], and surface distribution of (b) oxygen saturation (%), (c) silicate(mM), and (d) atomic Si/P ratio (data from XV cruise of the RV Academic Knipovich, January–February,1976), after Arzhanova [1982]. This picture presents the sharp lateral gradient of the atomic Si/P ratio,which marks the WSC position, while oxygen saturation and silicate distribution bordering thephytoplankton bloom are patchy.


Scotia Sea Arc, the characteristic values of Si/P for the WSCranged from 20 to 35–40 (Table 4, Figure 8).[54] The general spatial patterns of the chemical proper-

ties along the WSC route depicted in Figures 8 and 9 aresimilar to the geopotential anomaly of the sea surfacerelative to the 3- and 5-MPa surfaces reported by Stein[1988] for spring-summer 1977, summer 1978, the end ofsummer to the beginning of autumn 1978, autumn 1978,summer 1981, autumn 1981, spring 1983, and autumn1985. A similar relationship is observed along the mainline of the WSC in complexity and patchiness, and evencoincidence of the quasi-stationary cyclonic and anticyclon-ic meanders (for example, a cold meander northwest of theKing George Island). Patterson and Sievers [1980] havemade an attempt to trace the position of the WSC using alarge-scale database of temperature, salinity, dissolvedoxygen, and density. They found patchiness and complexityin the horizontal distribution of all parameters at depths of20 and 500 m; they attributed this result to the use of non-synoptic data.[55] Describing the frontal zone between the southern

boundary of the ACC and waters in the polar regime, Orsiet al. [1995] demonstrated that this frontal zone coincidedwith the position of the WSC. As a criterion for defining theformer frontal zone, they used the distribution of dissolvedoxygen on an isopycnal defined by so = 27.6 kg m�3 (s2 =36.8 kg m�3) and determined that this isopycnal surfacerose from about 1600 m near the STFZ to the 200 m layerover the region corresponding to the WSC. They showedthat, across this narrow zonal band, oxygen values increasefrom less than 240 mol kg�1 to greater than 336 mmol kg�1

and that temperature and salinity decreased from above 1�Cto below �1�C and from 34.5 to less than 34.4. As seen inFigure 8, oxygen concentration ranges from 192 to 288mmol kg�1 and coincides with depths of minimum oxygenlocation and gradient of silicate and atomic ratio Si/P at thesurface. According to Orsi et al. [1995], both of thesefrontal zones are part of the continuous circumpolar circu-lation. Consequently, it is possible to use the chemicaltracers noted above to define the location of the frontalzone between the southern boundary of ACC and waterswith a polar regime (e.g., the Weddell and Ross Gyres).

4. Conclusions

[56] The differences of physical and chemical characteri-stics of water masses are most pronounced in frontal zones.During periods prior to blooms of phytoplankton in theAtlantic sector of the Southern Ocean, it is possible to usethe chemical properties, along with the temperature andsalinity, to identify the frontal zones. The northern boundaryof the STFZ is defined by the lateral gradient of dissolvedoxygen; the southern boundary of the STFZ is identified bythe gradient of nitrate. The Subantarctic Front is distin-guished by the lateral gradient of dissolved oxygen. ThePolar Front can be traced by the gradient of silicate. Theonly exception is the Southwestern Atlantic sector wherethe Polar Frontal Zone is under the influence of SubantarcticMode Water. Tracers of the Weddell-Scotia Confluence atthe surface include the gradient of silicate, and the atomicratio Si/P, while on the upper boundary of the CircumpolarDeep Water, the WSC is defined by the lateral gradient of

dissolved oxygen and the depth of its location. In somecases, the use of chemical parameters is more informativeand useful than the use of thermohaline tracers. This isparticularly true for identification of the Weddell-ScotiaConfluence in the surface layer, where the atomic ratioSi/P can be used over the whole year.[57] During periods of intense phytoplankton growth in

the PFZ and WSC, ‘‘biological’’ gradient zones of chemicalproperties, which follow phytoplankton concentrations, areformed. The existence of such ‘‘biological fronts’’ is limitedto the spring-summer period and by intensity of the windmixing. During anomalously warm springs, phytoplanktonblooms in Antarctic waters adjacent to the APF begin andend earlier. During cold springs the opposite situationoccurs; blooms start and finish later. In spite of the highcontents of phosphate and nitrate in Antarctic waters adja-cent to the APF, a decrease of silicate concentration toundetectable values suggested that phytoplankton growthwas severely limited by silicate. The convergent nature ofthe frontal zones, and their high productivity, cause theaccumulation and subsequent biodegradation of organicmatter and appearance of ‘‘new’’ indices of the frontalzones such as particulate barium stocks [Dehairs et al.,1997], dissolved iron [de Baar et al., 1995], particulate ironand aluminium [Loscher et al., 1997], trace metals [Bessonet al., 1995], and iodate [Lucia et al., 1995], among otherchemical properties.[58] We hypothesize that these peculiarities of identifying

the boundaries of frontal zones by chemical parameters arecircumpolar and, apparently (except for the southeast Pa-cific, southwest Atlantic, and Crozet-Kerguelen area of theIndian sectors), can be used to distinguish fronts in both thePacific and Indian sectors of the Southern Ocean.

[59] Acknowledgments. We are grateful for the enthusiastic supportof the officers, engineers, and crews of the RV Academic Knipovich and theRV Vozrozhdeniye. These vessels were kindly made available by theRussian Federal Research Institute of Fisheries and Oceanography(VNIRO). Constructive comments by Hein De Baar and Andrew Dicksonled to major improvements in the manuscript, and finally, our special thanksto Donna Witter for the English correction of the manuscript. This paper iscontribution 260 from the Center for Marine Sciences, University of NorthCarolina at Wilmington. This paper is contribution 260 from the Center forMarine Sciences, University of North Carolina at Wilmington and contri-bution 6181 from SOEST (E.D.).

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��N. Arzhanova, V. Maslennikov, Y. Mikhailovsky, N. Mordasova,

I. Naletova, V. Sapozhnikov, P. Selin, and V. Zubarevich, Russian FederalResearch Institute of Fisheries and Oceanography (VNIRO), Department ofMarine Ecology, V. Krasnoselskaya 17 A, 107140 Moscow, Russia.E. Dafner, Department of Oceanography, School of Ocean and Earth

Science and Technology, University of Hawaii Manoa, Honolulu, HI96822, USA. ([email protected])


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