Zonal and vertical distribution of radiolarians in the western and central Equatorial Pacific in January 1999

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Zonal and vertical distribution of radiolarians in the western and central Equatorial Pacific in January 1999
  Deep-Sea Research II 49 (2002) 2823–2862 Zonal and vertical distribution of radiolarians in the westernand central Equatorial Pacific in January 1999 Hitoshi Yamashita, Kozo Takahashi*, Naoki Fujitani Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, Hakozaki 6-1-10, Higashi Ku,Fukuoka 812-8581, Japan Received 26 July 2000; received in revised form 22 December 2000; accepted 25 July 2001 Abstract The zonal and vertical distribution of radiolarians was studied to document their relationship with their environmentin the Equatorial Pacific in January 1999. Vertical plankton tows using a closing-type net with 63 m m mesh were madebetween the sea surface and 1000m depth on board R/V  Mirai   at five stations located between 140 1 E and 170 1 W, i.e. inthe western Pacific warm pool (WPWP) Region in the west and the Upwelling Region located in the east. The conditionduring the time of sampling was more or less La Ni * na, characterized by a zonal extension of the upwelling region at160 1 E. Our survey yielded 261 radiolarian taxa including 114 Nassellaria, 120 Spumellaria and 27 Phaeodaria. Totalradiolarian standing stocks increased from west to east, responding to the general increase in nutrients, chlorophyll- a ,and diatom numbers. Three major radiolarian depth communities were identified: surface dwellers (0–120m),subsurface dwellers (120–200m), and intermediate-water dwellers (200–1000m). When the vertical profiles wereexamined longitudinally, their abundance preference was characterized as follows: four taxa preferred the high-abundance water masses of the Upwelling Region:  Lithomelissa setosa ,  Pseudocubus obeliscus ,  Stylodictya multispina ,and  Stylodictya validispina . On the other hand,  Collosphaera tuberosa  was most abundant in the WPWP.  r  2002Elsevier Science Ltd. All rights reserved. 1. Introduction Radiolarians are known to live at various depthsin the epipelagic and mesopelagic zones (e.g.,Renz, 1976; McMillen and Casey, 1978; Caseyet al., 1979; Kling, 1979; Boltovskoy and Jankile-vich, 1985; Dworetzky and Morley, 1987; Klingand Boltovskoy, 1995; Welling et al., 1996;Abelmann and Gowing, 1997; Welling and Pisias,1998) and are known to contribute to both thesilica and organic carbon fluxes in the ocean. Theequatorial region of the ocean displays a highradiolarian diversity (e.g., Takahashi, 1991) andprimary production (Koblentz-Mishke et al., 1970;Berger et al., 1987; Barber and Chavez, 1991;Longhurst, 1998; Le Borgne et al., 2002) and theirstudy can provide insights into the relationshipbetween vertical radiolarian transport and thecarbon cycle (Gowing, 1986, 1993). In addition,radiolarians comprise one of the dominant plank-ton and microfossil groups found in marinesediments. Therefore, a better knowledge of theecology of living radiolarians can improve ourunderstanding of the upper water circulation and *Corresponding author. Tel.: +81-92-642-2656; fax: +81-92-642-2686. E-mail address:  kozo@geo.kyushu-u.ac.jp (K. Takahashi).0967-0645/02/$-see front matter r 2002 Elsevier Science Ltd. All rights reserved.PII: S0967-0645(02)00060-7  past climate. Since radiolarian abundance is likelyinfluenced by various environmental factors (e.g.,hydrographic parameters, water mass movements,food availability), one of our objectives was toevaluate the relationship between the radiolarianspecific composition and the physico-chemicalproperties of the tropical region.Samples were collected as part of the GlobalCarbon Cycle and Related Mapping based onSatellite Imagery Program (GCMAPS) in thewestern and central Equatorial Pacific. This regioncomprises the western Pacific warm pool (WPWP)and the equatorial upwelling region, east of it. TheWPWP registers sea-surface temperatures (SST)exceeding 28 1 C (Yan et al., 1992) and both itstemperature and geographic extension fluctuatefrom year to year in relation to El Ni * no-SouthernOscillation (ENSO) events (e.g., Picaut et al., 1996;Inoue et al., 1996; Le Borgne et al., 2002). DuringEl Ni * no years, the WPWP expands and/or movestoward the eastern Pacific, and retracts to thewestern Pacific during La Ni * na years. Zonalextension of the equatorial divergence follows thereverse pattern. The velocity of the EquatorialUndercurrent (EUC), which has an eastwardcomponent, decreases during an El Ni * no event(Kessler and McPhaden, 1995).Previous studies have shown that radiolarianassemblages are correlated to the eastward exten-sion of WPWP from the western Pacific (Wellinget al., 1996). Similarly, the conspicuous changes inorganic carbon flux due to the El Ni * no of 1983were reflected by changes in radiolarian speciesabundance in the region slightly north of theEquatorial Pacific (11 1 N, 140 1 W) (Pisias et al.,1986). As a baseline study, the present work willshed light on the fundamental understanding of ENSO events. 2. Samples and methods Plankton samples for this study were collectedat several depth-intervals from 0 to 1000m at fivestations (from west to east: Stations 2, 3, 6, 9, and12) in the western Equatorial Pacific duringMR98-K02 Cruise (2–15 January 1999) on boardR/V  Mirai  , operated by the Japan Marine Scienceand Technology Center (JAMSTEC) (Table 1,Fig. 1). Station 2 was located under the influenceof the North Equatorial Counter Current(NECC), and Stations 3, 6, 9, and 12 were locatedunder the influence of the South Equatorial Table 1Logistic and sample information for the vertical plankton towsconducted at five stations during R/V  Mirai   Cruise MR98-K02Station No. SampleNo.Depthinterval(m)TotalspecimenscountedSt. 2 1 0–10 4012 10–30 403Latitude 05 1 02.49 0 N 3 30–50 403Longitude 140 1 07.90 0 E 4 50–75 399Date 2 Jan. 1999 5 75–100 399Sampling time 0:36–3:17 6 100–125 401Bottom depth 4175m 7 125–150 4078 150–200 4069 200–500 40010 500–1000 402St. 3 11 0–40 40512 40–80 406Latitude 0 1 00.37 0 N 13 80–120 405Longitude 144 1 59.94 0 E 14 120–160 403Date 4 Jan. 1999 15 160–200 405Sampling time 0:23–3:21 16 200–500 404Bottom depth 3638m 17 500–1000 418St. 6 18 0–40 40619 40–80 399Latitude 0 1 01.38 0 N 20 80–120 401Longitude 159 1 59.88 0 E 21 120–160 405Date 7–8 Jan. 1999 22 160–200 396Sampling time 23:58–2:45 23 200–500 421Bottom depth 2800m 24 500–1000 431St. 9 25 0–40 40526 40–80 405Latitude 0 1 01.96 0 N 27 80–120 405Longitude 174 1 58.74 0 E 28 120–160 405Date 12 Jan. 1999 29 160–200 405Sampling time 0:32–2:52 30 200–500 405Bottom depth 4831m 31 500–750 405St. 12 32 0–40 406Latitude 0 1 00.55 0 N 33 40–80 404Longitude 170 1 07.96 0 W 34 80–120 405Date 14–15 Jan. 1999 35 120–160 405Sampling time 0:07–2:09 36 160–200 404Bottom depth 5397m 37 200–500 406 H. Yamashita et al. / Deep-Sea Research II 49 (2002) 2823–2862 2824  Current (SEC). Samples from below 750m atStation 9 and below 500m at Station 12 were notcollected due to strong currents. The samples wereobtained using a flow-metered closing planktonnet with 63- m m mesh size, between 1:00 and 3:00a.m. at each station, and preserved in 4% bufferedformaldehyde. The amount of water filtered in thenet was computed by the UNESCO formula(Cassie, 1968).Using a Folsom plankton splitter, an optimalconcentration was attained by repeated splitting(e.g., mainly 1/8–1/32 with a range of 1/8–1/128aliquot size) in order to perform the most efficientand accurate radiolarian counts. The split samplewas heated to 80 1 C after adding 5ml of 1 N-hydrochloric acid and 30ml of 10%-hydrogenperoxide in order to digest calcium carbonate andorganic matter. The digested sample was filteredthrough 47mm Gelman s grid membrane filterswith 0.45 m m sized pores. At the end of thefiltration, the filter was rinsed with distilled waterand dried overnight in an oven at 40 1 C. Thefiltered sample was mounted on a standard glassslide with Canada Balsam.In the eastern Equatorial Atlantic the shellssmaller than 40–60 m m (mostly juvenile) represent50% of total polycystine shells (Boltovskoy et al.,1993). Thus, the o 63- m m radiolarian shells cannotbe disregarded in the total abundance. Because oursamples were obtained with a 63- m m mesh net, ourresults in the western Equatorial Pacific are likelyto represent partial abundances.For diversity indices, the Shannon–Wiener log-base 2 formula was used (Shannon and Weaver,1949). A cluster analysis, using the Pearson’scorrelation coefficient average linkage methodwith Systat v. 5.2, was applied to the radiolarianstanding stocks to test the relationships betweenthe taxa. One hundred and one radiolarian taxa,which were determined to have sufficient countnumbers, were applied to the cluster analysis. Therelatively rare radiolarian taxa were eliminated inthe cluster analysis computations.Species identification of radiolarians was per-formed using the following works: Bj  rklund(1976), Renz (1976), Nigrini and Moore (1979),Takahashi and Honjo (1981), and Takahashi(1991). Taxonomic identification down to specieslevel was made on complete specimens (in the caseof broken specimens, >50% of the skeleton) formost of the abundant polycystine (Nassellaria andSpumellaria) and phaeodarian radiolarians. Whenradiolarians could not be identified to species level,they were recorded in the following categories: 140 ° E 160 °  180 °  160 ° W 15 ° S 15 ° S10 °  10 ° 5 ° 5 ° 0 °  0 ° 5 °  5 ° 10 °  10 ° 15 ° N 15 ° N0 400km Longitude    L  a   t   i   t  u   d  e Station 2Station 3 Station 6 Station 9 Station 12 WPWP RegionUpwelling RegionSalinity Front NECNECCSECSEC Fig. 1. Locations of the vertical plankton tow stations and oceanographic features during Cruise MR98-K02 in the western andcentral Equatorial Pacific. Major surface currents (NEC: North Equatorial Current, NECC: North Equatorial Countercurrent, SEC:South Equatorial Current) and characterized water masses (WPWP Region, salinity front, Upwelling Region). H. Yamashita et al. / Deep-Sea Research II 49 (2002) 2823–2862  2825  ‘‘other nassellarians’’, ‘‘other spumellarians’’, and‘‘other phaeodarians’’. If these specimens were notidentified to the suborder level, they were recordedas ‘‘other radiolarians’’. 3. Results 3.1. Oceanographic setting Since this study was part of a multidisciplinaryprogram conducted on board R/V  Mirai  , pertinenthydrographic and other physico-chemical databecame available (Suzuki et al., 1999). Thecondition during the time of sampling was LaNi * na, marked by a zonal extension of theequatorial upwelling west of the dateline (180 1 ).La Ni * na followed the termination of the1997–1998 significant El Ni * no event. From westto east the following systems were observed: theWPWP with low SSS (34.0=34.7), and higherthan normal SST (>29.4 1 C). At approximately156 1 E, the SSS increases abruptly from 34.4 to35.0 PSU, indicating the western limit of theequatorial upwelling. The WPWP extends down toapproximately 100m where a strong thermoclinedevelops, isolating the subsurface water below thisdepth and making it oligotrophic. In this regionrainfall strongly influences the water mass condi-tion, including a ‘‘barrier layer’’ (Lukas andLindstrom, 1991). Conversely, occurrence of sur-face nutrients in the Upwelling Region make iteutrophic. Such characteristic distribution basedon the nutrient concentrations in this region can beclearly illustrated by 2-D (W–E and vertical)distributions of nutrients such as nitrate andsilicate (Figs. 2 and 3). Although the western frontof the equatorial upwelling is not well recognizedfrom SST, the edge of this region is clearly markedby a surface-salinity front (Rodier et al., 2000). (b) 36 Temperature ( ° C) Depth (m) 0 10 20 300500100034 35 36 St. 2 34 35 St. 3 0 10 20 30 Salinity (PSU) St. 6 34 35 360 10 20 30 St. 9 34 35 360 10 20 30 St. 12 TemperatureSalinity34 35 360 10 20 30 (a) Longitude 140 ° E 150 °  160 °  170 °  180 °  170 ° W24 Temperature (              ° C) 2526272829303134 Salinity (PSU) 34.53535.5 Sea Surface Temperature Sea Surface Salinity St. 2 St. 3 St. 6 St. 9 St. 12 Fig. 2. (a) Hydrographic data (sea surface temperature and salinity) obtained in the Equatorial area, taken every ten minutes duringCruise MR98-K02. (b) Vertical profiles of temperature and salinity from 0 to 1000m at each station during Cruise MR98-K02 in theEquatorial Pacific. H. Yamashita et al. / Deep-Sea Research II 49 (2002) 2823–2862 2826  The displacement of this front is closely related tothe Southern Oscillation Index (SOI: Inoue et al.,1996) and, for example, was located at 167–169 1 Eduring non-El Ni * no and at 172 1 W during El Ni * no(Rodier et al., 2000). 3.2. Total radiolarian standing stock  A total of 261 radiolarian taxa were encounteredin this study: 114 Nassellaria, 120 Spumellaria,and 27 Phaeodaria (Appendix A). Identifiedspecimens were counted and expressed in radi-olarian standing stocks (i.e. the number of radi-olarians m  3 ) (Fig. 4). The vertical distribution of total radiolarians varied among stations (Fig. 4b).Standing stocks at Stations 2, 3 and 9 showed theirmaximum values in the surface layer (0–40m), andgradually decreased with depth. On the otherhand, those of Stations 6 and 12 were low in thesurface layer. The maximum abundance recordedappeared in the 80–120m interval at Station 6, andin the 40–80m interval at Station 12. In the 200– 500m depth interval, the standing stocks atStations 6 and 12 were higher than those at otherstations.Integrated standing stocks per m 3 from thesurface to 500m increased from west to east(Fig. 4a): the standing stock at Station 12(1,156,361 shellsm  2 ) was about four times higherthan that of Station 2 (315,144 shellsm  2 ). 3.3. The radiolarian diversity index Radiolarian diversity indices at each station anddepth interval are illustrated in Fig. 5. In generalterms, diversity increased with depth at allstations. Radiolarian diversity indices and totalradiolarian standing stocks were weakly andnegatively correlated (Fig. 5b;  r ¼ 0 : 445 ; N   ¼ 37). 3.4. Contributions of Nassellaria, Spumellaria, and Phaeodaria The vertical distributions of the contributions of the three radiolarian suborders (Nassellaria, Spu-mellaria, and Phaeodaria) from 0 to 1000m areillustrated in Fig. 4c. Proportions of Nassellariawere much lower in the surface layer of Station 6(30%) than in the surface layers of the otherstations (57–64%). The maximum relative abun-dance of Nassellaria (82%) occurred in the 80– 120m interval of Station 6. Phaeodarian contribu-tions were noticeable only below 160m, withhighest proportions (11%) in the 200–500minterval at Station 6. In the WPWP Region(Stations 2 and 3) and the area just west of theUpwelling Region (Station 6), proportions of Spumellaria increased with depth. However, inthe Upwelling Region (Stations 9 and 12) thepercentages appeared roughly uniform, albeitpartially unresolved due to the lack of data fromthe deep intervals. 3.5. Vertical distribution patterns Many characteristic patterns of vertical radi-olarian distribution could be discerned from the 020406080100120140160180200140 ° E 150 °  160 °  170 °  180 °  170 ° WSt. 2 St. 3 St. 6 St. 9 St. 12    D  e  p   t   h   (  m   ) LongitudeStation No.5201015 Nitrate (µM) (a) 020406080100120140160180200140 ° E 150 °  160 °  170 °  180 °  170 ° WSt. 2 St. 3 St. 6 St. 9 St. 12    D  e  p   t   h   (  m   ) LongitudeStation No.5510151015 (b) Silicate (µM) Fig. 3. Vertical distribution of nutrients along the Equator(Stations 3, 6, 9, and 12) and at 5 1 N, 145 1 E (Station 2)measured during Cruise MR98-K02. H. Yamashita et al. / Deep-Sea Research II 49 (2002) 2823–2862  2827
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