Linking Physical Dynamics and Biological Productivity in a Coastal Mesoscale Eddy

Abstract:

The Santa Barbara Channel (SBC) eddy is a cyclonic mesoscale eddy located off the coast of Southern California, USA. In the summer of 1998 and 1999, the SBC eddy was surveyed for juvenile fishes. In 1998, very high numbers of juvenile fishes were observed within the eddy, but not in 1999. The ocean conditions that contributed to the differences in fish abundances inside the eddy were investigated with three-dimensional numerical modeling. The physical dynamics of the SBC eddy, which included eddy size, three-dimensional rotational structure, and isopycnal uplift, were evaluated using a three-dimensional Regional Ocean Modeling System (ROMS). The retention ability of the eddy was quantified using a three-dimensional particle tracking model driven by the ROMS. The physical dynamics and particle retention of the SBC eddy were found to differ significantly in 1998 and 1999. In 1998, when the SBC eddy was rotating at a steady rate spatially and temporally and cycling consistently in and out of solid-body rotation, the particle retention was high and the isopycnal uplift sustained. However in 1999, when the SBC eddy was rotating unsteadily in space and time and did not have periods of solid-body rotation, the particle retention was low and the isopycnal uplift unstable. We theorize that the steady symmetric rotation of the eddy in 1998 had two important impacts on the biological productivity inside the eddy. First, it provided a prolonged period of cold nutrient rich water uplifting into the euphotic zone, which stimulated productivity and consequently attracted zooplankton. Second, it allowed the zooplankton, prey for the juvenile fish, to be retained inside the eddy, which attracted the juvenile fish. We conclude that biological productivity inside mesoscale eddies may be linked to the stability of its three-dimensional rotational structure and consequently its ability to retain particles.