Nutrient Dynamics Associated With Mesoscale Eddies In The Western South China Sea

Mesoscale eddies in the ocean are believed to induce discrepancies in the biogeochemical responses at different stages of eddy development,which may result in different behaviors of nutrient dynamics.The area of this study is located between 11°N and 16°N in latitude and 110°E and 115°E in longitude in the western South China Sea(WSCS).In our study,soluble reactive phosphate(SRP),inorganic nitrogen(NO₃^-+NO₂^-,referred as N+N hereafter) and silicate(SiO₃)were monitored, along with other parameters-sea surface height altimetry,hydrography and biology to examine the influence of eddy development on nutrient dynamics in the upper ocean in the WSCS.Note that for samples with low SRP concentrations(＜500 nM), measurements were taken with a home-made ship-board C18 enrichment-flow injection analysis system.Two cyclonic mesoscale cold eddies were monitored in August and September 2007 in the WSCS,and were named as eddyâ… and eddyâ…¡.The dimension of eddyâ… and eddyâ…¡was～170 km and～200 km,respectively.Hydrographic surveys revealed that at the horizon of 25 m,the core of eddyâ… was ca.7.6℃cooler,1.4 saltier,and 3.3 kg m^-3 denser than periphery,while the core of eddyâ…¡was ca.6℃cooler,1.7 saltier,and 2.9 kg m^-3 denser.The concentrations of SRP or N+N decreased from the core to periphery in both cold eddies:at the horizon of 25 m,the values of SRP and N+N decreased from 275 nM and 3.3μtM of the core to 5 nM and＜0.3μM of the periphery in eddyâ… ,and in eddyâ…¡the values decreased from 265 nM and 7.8μM of the coteto 6 nM and＜0.3μM of the periphery.The 0-100 m nutrient inventories of eddy core were 4-7 folds higher than eddy periphery.The 0-100 m SRP inventories in the core(n=4 stations) of eddyâ… and eddyâ…¡were 54.8±6.5 mmol P m^-2 and 66.1±9.4 mmol P m^-2,and 0-100 m N+N inventories were 792±98 mmol N m^-2 and 1107±166 mmol N m^-2,respectively.While the 0-100 m SRP inventories in the periphery (n=1 station) of eddyâ… and eddyâ…¡were 14.1 mmol P m^-2 and 11.5 mmol P m^-2,and 0-100 m N+N inventories were 193.7 mmol N m^-2 and 152 mmol N m^-2,respectively. In winter(n=1 station),0-100 m SRP inventory was 20 mmol P m^-2 and N+N inventory was 224 mmol N m^-2.We made a preliminary partition with eddy development:"spin up-mature" stage(September 1-8),"mature-degrading" stage (August 15-24),"relaxation" stage(August 25-31) during the southwest monsoon in 2007 and "no eddy" stage(December,as a reference) during the northeast monsoon in 2006.We observed unexpected large-amplitude fluctuations of N:P ratio of 1-36 mol mol^-1(eddyâ… ) and 3-44 mol mol^-1(eddyâ…¡) atσ_θ＜24.4 in the upper 100 m.As eddyâ… decayed,N:P ratio declined back to 3-～20 mol mol^-1;similar to the "no eddy" condition(5-20 mol mol^-1).Plots of N:P ratio versusσ_θimplied that any nutrient injection from deeper South China Sea should deliver N+N and SRP at molar ratio of approximately 13.7(s.d.=0.3).The maintenance of high N:P ratio required a net uptake of SRP atσ_θ＜24,4,and visa versa.Meanwhile,a bloom of Trichodesmiums was observed,nitrogen fixation may maintain elevated proportions of N+N relative to SRP,as compared to the Redfield ratio.Here we present a rapid change of N:P ratios at the euphotic zone with eddy development.N₂-fixer sitting in the oligotrophic surface is stimulated almost instantaneously as the occurrence of eddies.N₂ fixation may be triggered by a series of factors:(1) SRP and Fe injected by eddy pumping from deeper water,(2) iron from Mekong river plume.The estimated nutrient assimilated percentage varied from 32%(eddyâ…¡) to 74%(eddyâ… ) with eddy development.