Oligotrophic sea area mainly distributes in tropical and subtropical ocean, which have a profound impact on the marine material cycle and the structure of biological community. Based on the comprehensive survey in M4 seamount area of the Tropical Western Pacific Ocean in August 2017, the nutrients characteristics were explained, the formation mechanism of oligotrophic characteristics in this region was explored, and the influence of M4 seamount on oligotrophic sea area was analyzed. The results showed that the M4 seamount area is a typical oligotrophic sea area. In the water column of 0-100 m, the nutrients levels are extremely low, and NO3-N, PO4-P and SiO3-Si are lower than 0.55, 0.15 and 1.75 μmol/L, respectively; in the water column of 100-500 m, the nutrients concentrations rise rapidly, forming nutriclines. Seawater stratification is one of the main reasons for the formation of oligotrophic characteristics in this area. The thermocline in the water column of 100-500 m and the high-salt area in the water column of 100-230 m form a dual effect, which hinders the upward transport of waters with high nutrients concentrations at the bottom. In addition, Synechococcus and bacteria grow and multiply in a large amount in the water column of 0-100 m, absorb and utilize nutrients, and aggravate the oligotrophic characteristics of this area. There are significant upwellings near the summit of the M4 seamount, which promotes the waters with high nutrients concentrations at the bottom to break through the barriers of the thermocline and high-salt area and to transport upwards into the euphotic layer, which can be better utilized by phytoplankton. This study showed that seamounts may have an important effect on improving the oligotrophic characteristics of local sea areas, and then promote the growth of phytoplankton.

中文翻译：

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海水分层与浮游生物的贫营养机制：以西太平洋 M4 海山地区为例。

贫营养海域主要分布于热带和亚热带海洋，对海洋物质循环和生物群落结构产生深远影响。以2017年8月热带西太平洋M4海山区综合调查为基础，解释了营养盐特征，探索了该区贫营养特征的形成机制，分析了M4海山对贫营养海区的影响。结果表明，M4海山区是典型的贫营养海区。在0-100 m水柱中，营养物含量极低，NO3-N、PO4-P和SiO3-Si分别低于0.55、0.15和1.75 μmol/L；在100-500米的水体中，养分浓度迅速上升，形成营养线。海水分层是该区形成贫营养特征的主要原因之一。100-500 m水柱内的温跃层和100-230 m水柱内的高盐区形成双重效应，阻碍了底部营养物质浓度高的水域向上输送。此外，聚球藻和细菌在0-100m水体中大量生长繁殖，吸收利用养分，加剧了该地区的贫营养特征。M4海山峰顶附近有明显的上升流，促使底部营养物质浓度高的水域突破温跃层和高盐区的障碍，向上输送进入透光层，可以更好地利用通过浮游植物。