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Abstract：
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.
摘要：
贫营养海域主要分布在热带和亚热带海洋,对海洋物质循环和生物群落结构产生了深远的影响。在2017年8月对热带西太平洋M4海山区进行综合调查的基础上,探索了该地区贫营养特征的形成机理，分析了M4海山对贫营养海域的影响。结果表明，M4海山区是典型的贫营养海域。在0-100米的水柱中，营养水平极低，和NO3-N,PO4-P和SiO3-Si低于0.55、0.15和1.75μmol/L，分别在100-500米的水柱中，营养物质浓度迅速上升，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。100-500m水柱中的温跃层和100-230m水柱中的高盐区域形成双重效应，这阻碍了底部营养物质浓度高的水的向上运输。此外,复合球菌和细菌在0-100m的水柱中大量生长和繁殖，吸收和利用营养，并加剧了该地区的贫营养特征。M4海山山顶附近有大量的建筑，这促进了底部高养分浓度的水域突破了温跃层和高盐区域的屏障，并向上输送进入富营养层，可以更好地利用浮游植物。这项研究表明，海山可能对改善当地海域的贫营养特征具有重要作用，然后促进浮游植物的生长。
