Journal of Oceanography, Vol. 55, pp. 681 to 691. 1999 Seasonal Response of Planktonic Foraminifera to Surface Ocean Condition: Sediment Trap Results from the Central North Pacific Ocean 1 2 1 NOBUHISA O. EGUCHI , HODAKA KAWAHATA and ASAHIKO TAIRA 1Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164-8639, Japan 2Marine Geology Department, Geological Survey of Japan, Higashi 1-1, Tsukuba, Ibaraki 305-8567, Japan, and Graduate School of Science, Tohoku University, Sendai 980-8578, Japan (Received 7 September 1998; in revised form 29 June 1999; accepted 29 June 1999) The fluxes of planktonic foraminifera (calcareous shell producing zooplankton) were Keywords: examined in order to clarify temporal and regional variations in production in the ⋅ Sediment trap, upper ocean in relation to hydrographic conditions. Three time-series sediment traps ⋅ planktonic were deployed in the central North Pacific along 175°E for about one year, beginning foraminifera, ⋅ in June 1993. Trap sites were located in the subarctic, the transition, and the sub- organic matter, ⋅ seasonal varia- tropical water masses, from north to south. The southernmost site was under the in- tions, fluence of the transition zone in January to May. Both temporal and regional fluxes ⋅ food availability, of planktonic foraminifera showed large variations during the experiment. In the ⋅ surface ocean subarctic water mass, high total foraminiferal fluxes (TFFs) and high organic matter thermal condition, fluxes (OMFs) were observed during summer to fall, suggesting that food availability ⋅ central North is the most important factor for the production of planktonic foraminifera. Further- Pacific. more, low TFFs during winter were ascribed to low food availability and low tem- peratures. The OMFs and TFFs correlated well and increased rapidly after the dis- ruption of the seasonal thermocline during winter, peaking in late February to early March in the transition zone. In the subtropical water mass, both OMFs and TFFs remained low due to lower productivity under oligotrophic conditions. In general, TFFs show a positive correlation with OMFs during the trap experiment, suggesting that food availability is one of the factors controlling the production of planktonic foraminifera in the central North Pacific. Relatively low TFFs during summer to fall in the subtropical water mass may be caused by the thermal structure of the upper ocean. Low SST possibly reduces the production of foraminifera during winter in the subarctic region. 1. Introduction al., 1981). In recent years the development of time-series Planktonic foraminifera have long been used for sediment traps, which allows the collection of consecu- paleoenvironmental reconstruction, based only on a gen- tive, short-duration samples (Honjo et al., 1980; Honjo eral understanding of individual species’ ecology. How- and Doherty, 1988), has led to a better understanding of ever, their correspondence to the present-day ocean is still the sedimentation, or flux, of modern planktonic poorly understood, especially in the central North Pacific. foraminifera. Recent planktonic foraminifera studies us- A better understanding of modern assemblages of ing sediment traps have been conducted in the eastern planktonic foraminifera began with the first documenta- subarctic Pacific (Reynolds and Thunell, 1985; Sautter tion of the seasonal succession of the species composi- and Thunell, 1989), in the San Pedro Basin (Sautter and tion by Bé (1960), based on a year-long plankton tow Thunell, 1991a, b; Thunell and Sautter, 1992), in the study of living foraminifera obtained from surface wa- Panama Basin (Curry et al., 1983; Thunell et al., 1983; ters near Bermuda. Since that time, many towing programs Thunell and Reynolds, 1984), in the Sargasso Sea (Deuser have been conducted in many oceanic regions (e.g., et al., 1981; Deuser, 1987; Deuser and Ross, 1989), in Boltovskoy, 1969; Tolderlund and Bé, 1971; Williams et the Bay of Bengal (Guptha et al., 1997), in the Arabian 681 Copyright © The Oceanographic Society of Japan. Sea (Curry et al., 1992; Steens et al., 1992) and in the 2. Method Japan Trench (Oda, 1989). These studies demonstrate a Three time-series sediment traps (PARFLUX Mark connection between seasonal successions of planktonic 7G-21: opening 0.5 m2) were deployed in the central North foraminifera and surface ocean conditions. Pacific along 175°E for about one year, beginning in June In the present study we document the seasonal vari- 1993. The traps were deployed individually at 1,412 m ations in the total fluxes of planktonic foraminifera as depth at Site 8 (46°07.2′ N, 175°01.9′ E), 1,482 m depth well as the organic matter fluxes through the water col- at Site 7 (37°24.2′ N, 174°56.7′ E) and 3,873 m depth at umn in the central North Pacific along the 175°E merid- Site 6 (30°00.1′ N, 174°59.7′ E), respectively (Fig. 1, ian and clarify the factors that regulate the production of Table 1). Each trap cup was filled with filtered deep sea planktonic foraminifera. water containing a 3% formaldehyde solution with so- Table 1. Organic matter fluxes and total foraminiferal fluxes of three sediment trap sites in the central North Pacific Ocean. No correction is adopted for the time lag between species production and its deposition. Sample number Trap cup Duration Estimate fluxes Open Close Organic matter Total foraminifera (days) (mg m–2day –1) (shells m–2day –1) Site 8 ET106 16 Jun. 93 30 Jun. 93 15 7.02 1451 ET107 1 Jul. 93 31 Jul. 93 31 37.31 4034 ET108 1 Aug. 93 31 Aug. 93 31 25.99 3617 ET109 1 Sep. 93 30 Sep. 93 30 17.50 2987 ET110 1 Oct. 93 31 Oct. 93 31 18.19 2828 ET111 1 Nov. 93 30 Nov. 93 30 17.58 18347 ET112 1 Dec. 93 15 Dec. 93 15 11.40 4471 ET113 16 Dec. 93 31 Dec. 93 16 5.59 2014 ET114 1 Jan. 94 15 Jan. 94 15 5.14 883 ET115 16 Jan. 94 31 Jan. 94 16 3.15 728 ET116 1 Feb. 94 15 Feb. 94 15 4.26 467 ET117 16 Feb. 94 28 Feb. 94 13 3.55 322 ET118 1 Mar. 94 15 Mar. 94 15 4.99 1156 ET119 16 Mar. 94 31 Mar. 94 16 3.62 1168 ET120 1 Apr. 94 15 Apr. 94 15 7.73 3317 Site 7 ET64 1 Jun. 93 15 Jun. 93 15 36.41 1073 ET65 16 Jun. 93 30 Jun. 93 15 41.30 800 ET66 1 Jul. 93 15 Jul. 93 15 15.91 949 ET67 16 Jul. 93 31 Jul. 93 16 8.43 582 ET68 1 Aug. 93 15 Aug. 93 15 7.57 350 ET69 16 Aug. 93 31 Aug. 93 16 10.87 ND ET70 1 Sep. 93 15 Sep. 93 15 8.25 361 ET71 16 Sep. 93 30 Sep. 93 15 3.80 196 ET72 1 Oct. 93 15 Oct. 93 15 5.00 836 ET73 16 Oct. 93 31 Oct. 93 16 7.85 806 ET74 1 Nov. 93 15 Nov. 93 15 12.05 1182 ET75 16 Nov. 93 30 Nov. 93 15 7.71 885 ET76 1 Dec. 93 15 Dec. 93 15 9.29 894 ET77 16 Dec. 93 31 Dec. 93 16 5.97 638 ET78 1 Jan. 94 15 Jan. 94 15 2.21 265 ET79 16 Jan. 94 31 Jan. 94 16 3.42 226 ET80 1 Feb. 94 15 Feb. 94 15 4.46 516 ET81 16 Feb. 94 28 Feb. 94 13 5.48 1019 ET82 1 Mar. 94 15 Mar. 94 15 12.77 418 ET83 16 Mar. 94 31 Mar. 94 16 22.93 2666 ET84 1 Apr. 94 9 Apr. 94 9 11.49 1845 682 N. O. Eguchi et al. Fig. 1. Location of three sediment traps along 175°E longitude with surface hydrography of the central North Pacific Ocean. The Kuroshio axis moves latitudinally, see text for detail. Table 1. (continued). Sample number Trap cup Duration Estimate fluxes Open Close Organic matter Total foraminifera (days) (mg m–2day –1) (shells m–2day –1) Site 6 ET43 16 Jun. 93 30 Jun. 93 15 2.61 ND ET44 1 Jul. 93 31 Jul. 93 31 2.37 265 ET45 1 Aug. 93 31 Aug. 93 31 3.00 199 ET46 1 Sep. 93 30 Sep. 93 30 1.89 265 ET47 1 Oct. 93 31 Oct. 93 31 1.57 451 ET48 1 Nov. 93 30 Nov. 93 30 1.54 300 ET49 1 Dec. 93 15 Dec. 93 15 2.47 414 ET50 16 Dec. 93 31 Dec. 93 16 3.05 396 ET51 1 Jan. 94 15 Jan. 94 15 3.77 ND ET52 16 Jan. 94 31 Jan. 94 16 8.33 898 ET53 1 Feb. 94 15 Feb. 94 15 8.97 843 ET54 16 Feb. 94 28 Feb. 94 13 8.40 604 ET55 1 Mar. 94 15 Mar. 94 15 9.23 571 ET56 16 Mar. 94 31 Mar. 94 16 13.49 1242 ET57 1 Apr. 94 15 Apr. 94 15 14.21 1860 ET58 16 Apr. 94 30 Apr. 94 15 7.49 703 ET59 1 May 94 15 May 94 15 2.80 319 Seasonal Response of Foraminifera in the Central North Pacific 683 dium borate (pH > 8). foraminifera, in terms of number of specimens per square Recovered sample bottles were immediately refrig- meter per day (shells m–2day–1) were then made taking erated on board at approximately 2 to 4°C. In the labora- into account the sample split, the duration of each collec- tory, samples were passed through a 1 mm sieve, after tion period and the size of the opening of the sediment which the <1 mm fraction was split into aliquots with a trap (0.5 m2).