Exploration Of "Seamount Effect" On The Biogenic Elements In The Seamount Waters Of The Western Pacific Ocean

Seamount is a unique landform in the deep ocean,which has a unique ecosystem.As the material basis of the marine ecosystem,the study of the biogenic elements in the seamounts is of great significance to reveal the uniqueness of the biogenic elements in the seamounts,to clarify the mechanism of the"seamount effect",and to clarify the key processes of the material cycling and energy flow in the seamount ecosystem.Based on surveys of five seamount areas in the Western Pacific Ocean(37.62-153.42°E,8.76-17.78°N)including Y3,M2,M4,Magellan and M5(peak depths were 315,34,110,1195 and 106 m,respectively),this study systematically described the distribution characteristics of the biogenic elements in the seamounts,and explored the coupling relationship between the biogenic elements and the ecological environment in the seamounts.A series of results and understandings are as follows:1.The distribution of biogenic elements in euphotic zone of the Y3,M4,and M5 seamounts had a significant relationship with the seamount terrain,forming the"seamount effect",while there were no significant"seamount effect"in the M2 and Magellan seamounts.The"seamount effect"was controlled by the interaction between North Equatorial Current(NEC)and the seamount terrain,and was more prominent in the section A of the Y3 and M5 seamount.However,the"seamount effect"existed in both sections A and B of the M4 seamount due to a circulation around the seamount.The upwelling around the seamount is one of the important drivers of the"seamount effect",and the uplift of the isotherm or the salinity line is the main reference for judging the upwelling.There were uplifts of the isotherms or isalilines at different depths in the five seamount areas,and the amplitudes and positions of the isotherm and the isalilines were similar,indicating the existence of upwelling.Y3,M4 and M5(northern part)seamounts were all shallow or medium-depth seamounts,and the upwellings near the summit were more significant.The response of NO₃-N,PO₄-P,Si O₃-Si and DIC to the upwelling was the most significant,and the isolines of the concentrations of these parameters tended to show similar uplifts as the isotherms or isalilines,indicating that they were affected by the strong upwelling.The average concentrations of most biogenic elements on stations near the summits of the Y3,M4,and M5 seamouns areas were often higher than those on staions far from the seamount,further confirming that upwelling around the seamount was beneficial to increase NO₃-N,PO₄-P,Si O₃-Si and DIC concentration.Particulate nitrogen and phosphorus and NO₂-N,DOC and POC were relatively weak in the effect of upwelling due to their high concentration in the upper waters.However,the average concentrations of these parameters were higher near the seamount summit,and there was still a certain"seamount effect".In the Y3 and M5 seamount areas,the average concentrations of particulate nitrogen and phosphorus in the 200 m water column in section A showed a"seamount effect",there were not these phenomenons in the other sections.The upper waters in the depth of 0-200 m in this area was mainly affected by NEC from east to west,so the section A in the east-west direction was more likely to cause upwelling,which in turn caused significant uplifts of parameters such as particulate nitrogen and phosphorus.The“seamount effect”with high average concentration of particulate nitrogen and phosphorus occurred both in section A and section B in the M4 seamount area,that is,not only an upwelling,but also a significant circulation around the M4 seamount,causing the stronger"seamount effect"in the M4 seamount area.2.In the Western Pacific Warm Pool region where the five seamount areas were located,the water layer could be divided into Surface Water(SW,except Magellan seamount),North Pacific Tropic Water(NPTW),North Pacific Intermediate Water(NPIW)and Deep Water(DW).The biogeochemical processes and typically environmental characteristics such as high salinity zone and thermocline,oceanic hypoxic zone,and Deep Chlorophyll Maximum Layer(DCML)in water layers controlled the morphology,distribution and cycling of the biogenic elements.In SW,there were strong light,high water temperature and massive heterotrophic bacteria.Strong light and high water temperature were the main reasons for the distribution of phytoplankton in the subsurface layers,which in turn affected the distribution of biogenic elements such as NO₂-N and DOC.High temperature also reduced the solubility of CO₂,making the concentration of DIC lower.The overlap between high salinity zone and thermocline below the SW restricted the upward transport of biogenic elements at the bottom,which was an important reason for the extremely low levels of NO₃-N,PO₄-P and Si O₃-Si in the SW.The proliferation of heterotrophic bacteria might cause DIN:PO₄-P to be much lower than 16:1,and Si O₃-Si:DIN to be much higher than 1:1,exacerbating the nitrogen deficiency in seawater.In NPTW,photosynthesis by phytoplankton and decomposition by microorganisms were strong.DCML mainly distributed in the NPTW of the five seamount areas,which was the area with the most intense phytoplankton activity,causing a large increase in NO₂-N and DOC.Massive organic matter was generated in this area,significantly improving the PON/TPN and POP/TPP.Meanwhile,the metabolism of phytoplankton might cause DIN:PO₄-P and Si O₃-Si:DIN turn into phosphorus and silicon limitation.The decomposition of organic matter consumed O₂,resulting in a decrease in DO of1.32,0.55,0.10,1.54,and 0.55 mg/L,and a significant increase in AOU compared to SW.DIC was affected by both photosynthesis and decomposition.Photosynthesis caused a decrease in DIC.However,the overall DIC concentration increased continuously with the increase of water depth,indicating that the rate of supplementation of DIC by organic matter decomposition was much higher than the consumption rate of DIC.In NPIW,the decomposition by microorganisms was dominant.The area basically coincidds with the range of the oceanic hypoxic zone,where the DO was consumed in large quantities.Decomposition of organic matter by microorganisms caused rapid conversion of biogenic elements from organic to inorganic,and the PON/TPN and POP/TPP gradually decreased.Meanwhile,dramatic reduction in DO was significant for the nitrogen cycle,which was beneficial for the more completed enitrification,and the NO₂-N,as the intermediate product in this process,rapidly reduced.Meanwhile,the conversion of NO₃-N to N₂ was also accelerated,causing the lower DIN:PO₄-P and higher Si O₃-Si:DIN.In DW,the decomposition by microorganisms tended to the end,and the biogenic elements were greatly affected by the oceanic thermohaline circulation,which could carry DO-rich Antarctic bottom water to increase the DO and decrease the AOU in the five seamount areas.Meanwhile,the oceanic thermohaline circulation also had a profound impact on the distribution of biogenic elements due to water exchange.In particular,the particulate nitrogen and phosphorus,and POC concentrations increased slightly.On the one hand,they may be affected by sediment resuspension,on the other hand,they may also be affected by oceanic thermohaline circulation.The weak decomposition by microorganisms kept the PON/TPN,POP/TPP,DIN:PO₄-P,and Si O₃-Si:DIN stable.The NO₂-N also remained stable due to the attenuated denitrification.In DW,the lower water temperature and higher pressure made Ca CO₃accelerate the dissolution,which made the DIC in this area continue to increase.3.There was a significant Taylor column phenomenon in the M4 seamount.The isotherms and isalilines,Acoustic Doppler Current Profilers(ADCP)data and theoretical calculations together indicated the existence of the Taylor column in the M4 seamount.The Taylor column led to the isolines uplifts of nutrients,Chlorophyll a(Chl a)and heterotrophic bacteria around the M4 seamount,and the higher average concentrations of these parameters in the 200 m water column near the semount summit.There are complex hydrological dynamic when the current flows through the seamount,and the Taylor column is one of the most primary hydrology features in seamount.The three separate parts,the upwelling based on the uplifts of isotherms and isohalines,the anti-cyclonic circulation based on current data,and the theory calculation based on the environmental conditions around the seamount showed that the Taylor column indeed exists around the M4 seamount during our cruise.The uplifts of isotherms and isohalines around the seamount in temperature and salinity profiles indicated the existence of upwelling.Meanwhile,the ADCP profiles showed the approximately opposite current directions on east-west sides and north-south sides of the seamount at the water layers ranged from 200 to 300 m,and a brief current plot planar further indicated an anti-cyclonic circulation in this region intuitively.In addition,the theory calculation results showed that values of Blocking Parameter and Rossby Number was 4.922 and 0.195,indicating the environmental conditions were sufficient enough to support a formation of Taylor column in M4 seamount.The biophysical coupling was common around the seamount due to the influence of hydrological dynamic on biological processes.The isolines of the nutrients,Chl a and heterotrophic bacteria showed approximately consistent uplifts with isotherms and isohalines on the west of the summit in section A and above the two summits in section B,indicating the effect of the biophysical coupling,especially the upwelling was strong enough to influence the distribution of these parameters.Meanwhile,the planar graphs of nutrients,Chl a and heterotrophic bacteria in water column above 200 m showed the relatively high values around the seamount,indicating the effect of Taylor column on the enhancement of productivity,which could be explained by the“classic theory”.Moreover,the ratios of nutrients concentrations,Chl a concentrations and heterotrophic bacteria abundances between A6 station and the other five stations were always greater than 1.0,further indicating the effect of biophysical coupling.Furthermore,the range of the effect of biophysical coupling in M4 seamount was with O station(140.13°E,10.48°N)as center point and 6.1 km as radius,roughly estimated based on a standard of Chl a concentration ratio?1.0.