胶州湾海洋生态系统国家野外研究站知识库

The tropical western Pacific, one of the most oligotrophic seas on Earth, has a large number of seamounts. However, the phytoplankton study of seamount in this sea area is at the initial stage, and the community structure characteristics of seamount and the effect of seamount on phytoplankton are still unclear. The study of phytoplankton in seamount in this region is not only helpful to understand the characteristics of seamount ecosystem and primary production process in tropical oligotrophic waters, but also can support for understanding the mechanism of "seamount effect".

In this paper, phytoplankton biomass (in terms of chlorophyll a), group composition and primary productivity of four seamounts in the western tropical Pacific were investigated for five cruises（Y3: December 2014；M2: March 2016；C4: August 2017 and May 2019；Kocebu：March 2018）. Based on this, the relationships between phytoplankton community structure characteristics and environmental factors were analyzed. In addition, the comparison of phytoplankton community structure between seamount and surrounding stations was performed, and the physical, chemical and biological coupling processes of each seamount were analyzed. The study drew the following conclusions：

1. At study area, the concentration of chlorophyll a (Chl a) at surface layer is very low, and it increases above isotherms and then decreases with the increase of water depth. The deep Chl a maximum (DCM) is mainly located at 75-150 m, and the level of primary productivity is low.

The growth of phytoplankton in the upper water was mainly restricted by nutrients, and the concentrations of Chl a in the surface water were about 0.05 mg/m³, and the water column integrated concentration of Chl a ranged from 11.2 to 19.4 mg/m². In the thermocline, nutrients gradually increased with depth, but the light intensity was low, resulting in the DCM were located 20 – 30 m below the initial depth of the thermocline, and the variation range of the (DCM) was about 0.1 – 0.35 mg/m³. Primary productivity at Y3, M2 and C4 seamounts were less than 100 mgC/(m²×d), and at Kocebu Seamount, it was about 150 mgC/(m²×d).

2. The dominant groups were picoplankton such as prochlorophytes and cyanobacteria (mainly Synechococcus) were the dominant groups, and followed by nanoplankton such as haptophytes, chrysophytes, cryptophytes.

At M2, C4 and Kocebu seamounts, picophytoplankton such as prochlorophytes and Synechococcus were the dominant groups, contributing approximately 0.85 of total Chl a. Nanophytoplankton such as haptophytes, chrysophytes, cryptophytes were followed, contributing about 0.10 of Chl a, and micro-phytoplankton such as diatoms and dinoflagellates contributed lowest Chl a. At Y3 seamount, picophytoplankton was still the dominant group, but its relative contribution to Chl a was lower than the other seamounts, and the relative contributions of nanophytoplankton and micro-phytoplankton were higher.

Cyanobacteria and prochlorophytes were dominated above the thermocline, and below thermocline, the dominant position of cyanobacteria was gradually replaced by chrysophytes and cryptophytes. Prochlorophytes was dominant group in the whole euphotic layer, and the haptophytes occupied a certain proportion in the euphotic layer.

3. In the sampling area, the changes of phytoplankton group composition were obvious in temporal dimensions.

The Chl a was lowest in December, and followed by August, and the Chl a concentrations were higher in March and May. The primary productivities were close in March, May and August, and it was the lowest in December. Micro and nanophytoplankton had highest relative contribution rate of Chl a in December, and the size-fractionated structures of phytoplankton were similar in March, May, and August. In the same month, the surface Chl a at Kocebu seamount was slightly higher than that at M2 seamount, while the the size-fractionated structures and DCM showed no significant difference between the two seamounts.

4. Enhanced phytoplankton was observed at Y3 Seamount, C4 Seamount and Kocebu Seamount.

Although the upwellings in Y3, C4, and Kocebu seamount were weakened by stratification, the nutrients supplemented still promoting phytoplankton biomasses at the bottom of eutrophic layer, due to the deep distribution of phytoplankton. Enhancements of phytoplankton were observed at section A of Y3 Seamount and both sections of C4 Seamount in 2017, with the DCM at some seamount stations were twice that of the surrounding water. There might were eddy at the peak of the Y3 Seamount, and "Taylor cap" may exist at C4 Seamount. At stations which existed high Chl a patches, the uplifted isotherm and nutrients was observed, which showed good physico-chemical-biological coupling process. The fluctuations of isotherms near the peak of Kocebu Seamount seemed to be transferred to the thermocline, and there is an obvious uplift of nutrients above the peak. Unlike Y3 and C4 seamounts, the high Chl a patches were observed at the downstream stations instead of the peak station, and that was consistent with the 200 m water column concentrations of nutrients. It may be because the effect of retention or capture of deep seamount will not extend to the euphotic layer, and the nutrient-supplemented water was rushed to the downstream in a short time, and formed high nutrient and Chl a pathes there.

There is no "seamount effect" in M2 Seamount. Comparing to M2 Seamount, Y3 and C4, which are elongated along the N-S axis, seem to interact more strongly with westward-flowing currents than does M2, and these interactions serve to drive the biological responses.

5. Seamounts affect the phytoplankton groups composition and distribution.

at Y3, C4 and Kocebu seamount, some nano- or micro-phytoplankton Chl a concentrations increased, For example, above the summit of Kocebu Seamount, the concentration of micro-Chl a was the highest, which was about three times that in the surrounding water, and Diatoms had a higher biomass at the A5-8 station between the two peaks. The results of similarity analysis showed that phytoplankton group composition at C4 seamount were significant differences from that at control stations in 2019.

Seamounts can not only promote the growth of phytoplankton by upwellings and Taylor column, but also accelerate the transfer of phytoplankton or organic particles to the deep ocean by some vertical processes such as internal waves with large amplitude, secondary circulation, and Ekman bottom boundary flow. Given there are numerous seamounts in the tropical Western Pacific, the potential contribution of seamounts to carbon fluxes or geochemical cycles in the region deserves further attention and study.