Distribution of Diatoms, Coccolithophores and Planktic Foraminifers Along a Trophic Gradient During SW Monsoon in the Arabian Sea
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Marine Micropaleontology 51 (2004) 345–371 www.elsevier.com/locate/marmicro Distribution of diatoms, coccolithophores and planktic foraminifers along a trophic gradient during SW monsoon in the Arabian Sea Ralf Schiebela,*,1, Alexandra Zeltnerb, Ute F. Treppkea, Joanna J. Waniekc, Jo¨rg Bollmannd, Tim Rixene, Christoph Hemlebena a Institute of Geosciences und Palaontologie, Tu¨bingen University, Sigwartstrasse 10, D-72076 Tu¨bingen, Germany b Wilonstrasse 19, 72072 Tu¨bingen, Germany c Southampton Oceanography Centre, SOC, European Way / Empress Dock, Southampton SO14 3ZH, UK d Department of Earth Sciences, Swiss Federal Institute of Technology, ETHZ, Sonneggstrasse 5, 8092 Zurich, Switzerland e Zentrum fu¨r Marine Tropeno¨kologie, Bremen University, ZMT, Fahrenheitstr. 6, 28359 Bremen, Germany Received 1 September 2003; received in revised form 19 January 2004; accepted 14 February 2004 Abstract The distribution of diatoms, coccolithophores and planktic foraminifers mirrored the hydrographic and trophic conditions of the surface ocean (0–100 m) across the upwelling area off the Oman coast to the central Arabian Sea during May/June 1997 and July/August 1995. The number of diatoms was increased in waters with local temperature minimum and enhanced nutrient concentration (nitrate, phosphate, silicate) caused by upwelling. Vegetative cells of Chaetoceros dominated the diatom assemblage in the coastal upwelling area. Towards the more nutrient depleted and stratified surface waters to the southeast, the number of diatoms decreased, coccolithophore and planktic foraminiferal numbers increased, and floral and faunal composition changed. In particular, the transition between the eutrophic upwelling region off Oman and the oligotrophic central Arabian Sea was marked by moderate nutrient concentration, and high coccolithophore and foraminifer numbers. Florisphaera profunda, previously often referred as a ‘lower-photic-zone-species’, was frequent in water depths as shallow as 20 m, and at high nutrient À 1 À 1 concentration up to 14 Amol NO3 l and 1.2 Amol PO4 l . To the oligotrophic southeast of the divergence, cell densities of coccolithophores declined and Umbellosphaera irregularis prevailed throughout the water column down to 100 m depth. In general, total coccolithophore numbers were limited by nutrient threshold concentration, with low numbers ( < 10 Â 103 cells À 1 3 À 1 l ) at high [NO3] and [PO4], and high numbers (>70 Â 10 cells l ) at low [NO3] and [PO4]. The components of the complex microplankton succession, diatoms, coccolithophores and planktic foraminifers (and possibly others), should ideally be used as a combined paleoceanographic proxy. Consequently, models on plankton ecology should be resolved at least for the seasonality, to account for the bias of paleoceanographic transfer calculations. D 2004 Elsevier B.V. All rights reserved. Keywords: coccoliths; diatom flora; foraminifera; nutrients; oceanography; paleoceanography * Corresponding author. Tel.: +41-1-6323676; fax: +41-1-6321080. E-mail address: [email protected] (R. Schiebel). 1 Current address: Department of Earth Sciences, Swiss Federal Institute of Technology, ETHZ, Sonneggstrasse 5, 8092 Zu¨rich, Switzerland. 0377-8398/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.marmicro.2004.02.001 346 R. Schiebel et al. / Marine Micropaleontology 51 (2004) 345–371 1. Introduction in coastal waters off India were studied for their standing crop, unfortunately without being discussed The Indian Ocean Dipole is one of the most for their ecology (Prasad and Nair, 1960; Durairat- prominent climate systems on Earth today, and is nam, 1964). Simonsen (1974) gave a broad and connected to other climatic patterns such as the North detailed overview of species in plankton samples from Atlantic Oscillation (NAO), El Nin˜o Southern Oscil- net hauls taken during the International Indian Ocean lation (ENSO) and the Indonesian to Australian region Expedition (IIOE) in 1964/1965, but only little infor- via tele-connections (e.g., Ashok et al., 2003). The mation on diatom ecology was presented. Diatoms of Indian Monsoon affects the precipitation on land as the Arabian Sea have not yet been investigated in a well as the hydrography and the biogeochemistry of temporal and spatial resolution that would allow an the Indian Ocean (e.g., Ittekkot and Nair, 1993; ecological interpretation. Fleitmann et al., 2003). Distribution and the standing The taxonomic composition of coccolithophore stock of marine planktic organisms, including dia- assemblages from the Arabian Sea and northern toms, coccolithophores and planktic foraminifers, is Indian Ocean was investigated by Norris (1965, related to the monsoonal oscillation, which varied 1971, 1983, 1984, 1985), Kleijne et al. (1989) and over the historic past and over geologic time scales Kleijne (1991, 1992, 1993). Bernard and Lecal (1960) (e.g., Gupta et al., 2003; Ivanova et al., 2003). reported the distribution of extant coccolithophores in The fossil remains of microorganisms, for example a general phytoplankton study. Martini and Mu¨ller skeletons and alkenones, are used as paleoceano- (1972) and Guptha et al. (1995) analyzed the cocco- graphic tools although microplankton dynamics and lithophore assemblage along transect at 65jE during trophic conditions are not yet fully understood. In this the late SWM. Sediment trap studies from the north- study, we therefore focus on the effect of the south- eastern Arabian Sea (Andruleit et al., 2000), and from west monsoon (SWM) on the ecology of living the Somalia upwelling region (Broerse et al., 2000) diatoms, coccolithophores and planktic foraminifers, revealed a strong relationship between the monsoon and on the differential population dynamics among and the seasonal flux of coccolithophores. A first these three microplankton groups. comparison between monsoon induced changes of Different phytoplankton groups co-occur accord- the living coccolithophore assemblage in the north- ing to ecological demand on trophic (nutrients, food), western Arabian Sea, compared to Holocene and physical (e.g., light, mixing, temperature) or biolog- Quaternary assemblages, was carried out by Woellner ical factors (e.g., competition, predation) (cf. Smayda, et al. (1988). Holocene sediment assemblages in the 1986). In particular, we focus on the effect of Arabian Sea are dominated by Gephyrocapsa oce- hydrography on nutrient concentration in the upper anica (Martini and Mu¨ller, 1972; Guptha, 1985; 100 m of the water column and production of Houghton and Guptha, 1991; Andruleit and Rogalla, autotrophic organisms (diatoms and coccolitho- 2002). Potential environmental control of coccolitho- phores). This is pursued by quasi-synoptic investiga- phores in surface waters off Pakistan was investigated tions on the spatial and temporal succession of by Andruleit et al. (2003). diatoms, coccolithophores and planktic foraminifers Planktic foraminifers were investigated for their along a productivity gradient from the coastal general distribution, ecology and fossil record in the upwelling off Oman (eutrophic) to the stratified Indian Ocean by Be´ and Tolderlund (1971), Be´ and (oligotrophic) central Arabian Sea. Hutson (1977) and Guptha et al. (1994). Seasonal changes of the planktic foraminiferal fauna and stable 1.1. Previous studies on diatoms, coccolithophores isotope composition over the course of the monsoons and planktic foraminifers in the Arabian Sea are described by Kroon and Ganssen (1988) and Curry et al. (1992). Kroon (1988) and Ivanova et al. Diatoms of the Indian Ocean were studied by (1998) reported Globigerinita glutinata, Neoglobo- Cleve (1901), Karsten (1907), Taylor (1967), Sournia quadrina dutertrei and Globorotalia menardii as up- (1968), Hendey (1970), Thorrington-Smith (1970) welling indicators in the Arabian Sea. While and Mathur and Singh (1993). Certain diatom taxa Globigerina bulloides occurs mainly during the late R. Schiebel et al. / Marine Micropaleontology 51 (2004) 345–371 347 SWM (September, after Kroon and Ganssen, 1988), the method of Kleijne (1991). Coccolithophores were N. dutertrei is more frequent during the early SWM, filtered onto a 50-mm diameter, regenerated cellulose in June and July, along with decreased sea surface membrane filter with a pore size of 0.45 Am (SAR- temperatures, which is again indicative of upwelling. TORIUS) using a vacuum pump. The vacuum was adjusted to 200 mbar and the actual filtration area was 1249 mm2. All filters were air dried for 12 h, stored 2. Materials and methods in plastic petri dishes and were kept dry in closed boxes with silica gel. For the analysis on a scanning Water and microplankton was collected from the electron microscope (SEM), circular pieces (area of upper 100 m of the water column in the Arabian Sea. 133 mm2) were punched out of the center of the filter During R/V METEOR cruise 32/5 (M32/5), July 14 to membrane and mounted onto aluminium stubs using August 14, 1995, nine stations along a transect perpen- double-sided adhesive tape. Subsequently, samples dicular to the coast of Oman towards 14jN/65jE were coated with 20 nm of gold/palladium, and (Central Arabian Sea Station, CAST) and at 10jN/ colloidal silver suspension was put on the border of 65jE (Southern Arabian Sea Station, SAST) were the membrane to provide optimal conductivity of the sampled (Table 1, Fig. 1). During R/V SONNE cruise sample. The stubs were examined on a Cambridge 119 (SO119), May 12 to June 10, 1997, nine stations Stereoscan