Distribution of diatoms, coccolithophores and planktic foraminifera in the Arabian Sea

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 concentration up to 14 µmol NO3/l and 1.2 µmol PO4/. 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*10**3 cells/l) at high [NO3] and [PO4], and high numbers (>70*10**3 cells/l) at low [NO3] and [PO4]. The components of the complex microplankton succession, diatoms, coccoliths 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.

1997年5—6月及1995年7—8月期间，阿曼沿岸上升流区至阿拉伯海中部的表层海洋（0~100米）中，硅藻（diatoms）、颗石藻（coccolithophores）与浮游有孔虫（planktic foraminifers）的分布格局，精准反映了该区域的水文与营养盐环境状况。在上升流引发的局部低温、高营养盐（硝酸盐、磷酸盐、硅酸盐）水体中，硅藻丰度显著升高。沿岸上升流区内的硅藻群落以角毛藻（Chaetoceros）营养细胞占绝对优势。向东南方向的营养盐匮乏且层化加剧的表层水体过渡时，硅藻丰度逐渐降低，颗石藻与浮游有孔虫的丰度则逐步上升，其群落组成亦发生相应改变。尤为关键的是，阿曼沿岸富营养上升流区与贫营养阿拉伯海中部的过渡带，呈现中等浓度的营养盐水平，同时伴随较高的颗石藻与有孔虫丰度。此前常被归类为“低光带物种”的深丰球藻（Florisphaera profunda），在水深仅20米的水体中即较为常见，且在硝酸盐浓度高达14 μmol/L、磷酸盐浓度达1.2 μmol/L的高营养盐环境中亦有大量分布。在辐散区东南部的贫营养水域中，颗石藻的细胞密度逐渐下降，不规则伞球藻（Umbellosphaera irregularis）成为整个水柱（至100米水深）中的优势类群。总体而言，颗石藻总丰度受营养盐阈值浓度限制：在高硝酸盐与高磷酸盐环境中，其丰度较低（<10×10³ 个/升）；而在低硝酸盐与低磷酸盐环境中，丰度则较高（>70×10³ 个/升）。微浮游植物演替复合体的各组成部分——硅藻、颗石藻与浮游有孔虫（或其他类群）——理论上应联合用作古海洋学代用指标。因此，浮游生态学相关模型至少需纳入季节变化因素，以抵消古海洋学转换计算中的偏差。