An offshore study in the California current system preface

Abstract

Eddy systems are a unique ecosystem, usually having high biological masses and primary production in the sea. In this study, both particulate and dissolved phases of cadmium in the water column of 15 stations over a cyclonic eddy in the southern East China Sea were determined to obtain their spatial distributions. This allows us to confirm that cyclonic eddy systems play the role of a Cd pump in the sea.Results showed that particulate Cd (PCd) and dissolved Cd (DCd) concentrations in water varied greatly, ranging over two orders and one order of magnitude, respectively. Large spatial variability was found not only for PCd but also for DCd in the upper water, apparently due to the effects of the cyclonic eddy system on the Cd distributions over the study area. DCd accounts for about 99% of the total Cd. For the surface water, DCd concentration at the eddy center was about five times the average of the water surrounding the eddy center. The depth distributions of DCd exhibited a typical surface depletion and a subsequent increase with depth; however, the PCd distribution showed the opposite, i.e. a surface maximum and a subsequent decline with depth. In general, the DCd maxima were found at depths of 600–1000 m, agreeing well with the literature. It reflected the internal biogeochemical cycling of Cd in the water column, which was driven by the utilization of Cd by plankton in the euphotic zone and by the regeneration of Cd at depth. In addition, a remarkably high DCd concentration existed in near-bottom water around the cyclonic eddy center.The horizontal distributions of both PCd and DCd in the upper water shared a common feature with elevated concentrations centering around the eddy center and a decline in concentration with distance from the eddy center. This shows that the cyclonic eddy could bring up the Cd-rich deep water to the surface water around the eddy center and could then expand toward eddy's vicinity via advection–diffusion. It is thus likely that it produces ample biological masses over the eddy system. Hence, this work can demonstrate that the ascending nutrient-rich water driven by the cyclonic eddies can serve as an important source not only for many nutrients but also for Cd in the sea.