Intermingling of two Pseudocalanus species on Georges Bank
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Physical and biological controls on the springtime distributions of Pseudocalanus moultoni and P. newmani on Georges Bank are examined by assimilating observations into a coupled physical-biological model. Monthly snapshots of abundance are derived from U.S. GLOBEC Georges Bank broad-scale surveys during 1997. The forward problem is posed as an advection-diffusion-reaction equation for the copepod concentration. The adjoint method of data assimilation is used to invert for the biological sources and sinks implied by the observed changes in abundance between surveys and the flow during the intervening period. Based on this analysis, the two species appear to have distinct population centers in the late winter/early spring: P. moultoni on the northwest flank of the Bank and P. newmani on the Northeast Peak and the southern tip of Browns Bank. As the growing season progresses, the clockwise circulation around Georges Bank blends reproducing (but not interbreeding) animals from the two source regions, causing their distributions to overlap by early summer. The springtime evolution of Pseudocalanus distributions in this region is driven by a complex mixture of hydrodynamic transport and species-specific population dynamics, including both growth and mortality.