On the role of mesoscale eddies for the biological productivity and biogeochemistry in the eastern tropical Pacific Ocean off Peru

[摘要] Mesoscale eddies seem to play an important role for both the hydrography andbiogeochemistry of the eastern tropical Pacific Ocean (ETSP) off Peru.However, detailed surveys of these eddies are not available, which has sofar hampered an in depth understanding of their implications for nutrientdistribution and biological productivity. In this study, three eddies along asection at 16°45´ S have been surveyed intensively during R/VMeteor cruise M90 in November 2012. A coastal mode water eddy, an open oceanmode water eddy and an open ocean cyclonic eddy have been identified andsampled in order to determine both their hydrographic properties and theirinfluence on the biogeochemical setting of the ETSP. In the thermocline thetemperature of the coastal anticyclonic eddy was up to 2 °Cwarmer, 0.2 more saline and the swirl velocity was up to 35 cm s⁻¹. Theobserved temperature and salinity anomalies, as well as swirl velocities ofboth types of eddies were about twice as large as had been described for themean eddies in the ETSP. The observed heat and salt anomalies (AHA, ASA) ofthe anticyclonic eddy near the shelf-break of 17.7 × 10¹⁸ J and36.6 × 10¹⁰ kg are more than twice as large as the mean AHA and ASA forthe ETSP. We found that the eddies contributed to the productivity bymaintaining pronounced subsurface maxima of chlorophyll of up to 6 μgL⁻¹. Based on a comparison of the coastal (young) mode water eddy andthe open ocean (old) mode water eddy we suggest that the ageing of eddieswhen they detach from the shelf-break and move westward to the open oceaninfluences the eddies' properties: chlorophyll maxima are reduced to abouthalf (2.5–3 μg L⁻¹) and nutrients are subducted. However,different settings at the time of formation may also contribute to theobserved differences between the young and old mode water eddies. Thecoastal mode water eddy was found to be a site of nitrogen (N) loss in theOMZ with a maximum ΔNO₃⁻ anomaly (i.e. N loss) of about−25 μmol L⁻¹ in 250 m water depth, whereas, the open ocean modewater and cyclonic eddies were of minor and negligible importance for the Nloss, respectively. Our results show that the important role of eddies forthe distribution of nutrients, as well as biogeochemical processes in theETSP (and other OMZ/upwelling regions) can only be fully deciphered andunderstood through dedicated high spatial and temporal resolutionoceanographic/biogeochemical surveys.

[学科分类] 地球化学与岩石