Analysis of differences in nutrients chemistry in seamount seawaters in the Kocebu and M5 seamounts in Western Pacific Ocean

Comprehensive surveys were conducted in the Kocebu deep seamount and the M5 shallow seamount in the Western Pacific Ocean in March 2018 and May 2019, respectively. The distributions of nutrients in euphotic zone of the two seamount-areas were revealed, and the causative controlling factors were analyzed. Results show that the vertical distribution of nutrients in the two seamount-areas accorded with the general law of the oligotrophic ocean. The concentrations of NO3-N, PO4-P, and SiO3-Si generally increased gradually with the increase of water depth, and they were extremely low in water layers within 100 m. The area of high NO2-N concentration well agreed with the Deep Chlorophyll Maximum Layer. On the other hand, the distribution of water masses and phytoplankton and hydrological conditions in the two seamount-areas were different, resulting in lower concentrations of NO3-N, PO4-P, and SiO3-Si in the water layers below 100 m in the Kocebu seamount area than those in the M5 seamount area. In addition, NO2-N was affected by the distribution of phytoplankton, and distributed mainly in the water layers of 150 and 100 m. There was upwelling in the euphotic zone of M5 seamount area, causing accumulations of nutrients and phytoplankton around the seamount, forming a “seamount effect”; however, no such an effect was found in Kocebu seamount area. Affected by the composition of biological community and the “seamount effect”, the nitrogen limitation in the M5 seamount area was not significant, and the dissolved inorganic nitrogen (DLN):PO4-P and SiO3-Si:DIN were closer to the Redfield ratios.