Relationship and stratification of multiple marine ecological indicators: A case study in the M2 seamount area of the Western Pacific Ocean

Seawater stratification is an important ecological indicator and typical feature in the marine environment, which is of great significance to the material cycle and distribution of organisms. In March 2016, a survey was conducted in the M2 seamount of the Western Pacific Ocean, seawater stratification of temperature, salinity, dissolved oxygen (DO), nutrients, bacteria, and the impact of the M2 seamount were discussed. It is found that the seawater stratification of each parameter is obvious in this area. At the depth of 0–75 m, there is a mixed layer, and these parameters remain relatively stable; at 75–500 m, there are obvious thermocline, gradient layer of bacteria, and nutricline; at the depth deeper than 500 m, these parameters remain relatively stable. In addition, at 75–150 m, there is a high-salt zone dominated by the North Pacific Tropic Water, while at 270–1370 m, there is an oxygen minimum zone with low DO. There is a close relationship in seawater stratification of different parameters. At 75–500 m, the density gradient dominated by the thermocline limits the water exchange, then directly controls the gradient layer of DO, nutrients, and bacteria. Additionally, the double actions of the high-salt zone and thermocline affect the stratification of other parameters. At 0–75 m, due to the abundant DO, sufficient carbon source, and low nutrients, the bacteria could grow largely, while at 75–500 m, bacteria could consume a large amount of DO to decompose organic matter, leading to a decrease in DO and an increase of nutrients. Although the M2 seamount is a shallow seamount, there is no upwelling around it, which weakly influences the seawater stratification of different parameters. This study will provide a new perspective on seawater stratification and its ecological effects.