Estimation of the processes controlling variability in phytoplankton pigment distributions on the southeastern U.S. continental shelf

Abstract

A series of five CZCS images from the southeastern U.S. continental shelf for April 10–16, 1980, were combined with concurrent moored current meter measurements to estimate the processes controlling the observed variability in chlorophyll concentration and distribution in this region. The approach used for this analysis is a series of calculations with the equation governing the space and time distribution of a nonconservative quantity such as chlorophyll. The CZCS chlorophyll distributions and current meter measurements allow estimation of the terms in the governing equation which gives an indication of the relative contribution of advective, diffusive, and local processes in producing the observed chlorophyll distributions on the southeastern U.S. continental shelf. The estimates of the terms in the governing equation from observations show that advective, diffusive, and local processes all contribute to the phytoplankton distributions and that the relative contribution of each changes with time and location. For the 5 days considered in this analysis, the order of importance of processes controlling variability in phytoplankton distributions determined from observations was biologicalvertical processes (36%), diffusion (33%), advection (16%), and local time change (15%). As a comparison, a similar series of calculations were performed with simulated chlorophyll distributions obtained from a numerical physical‐biological model constructed for the southeastern U.S. continental shelf ecosystem that used circulation fields derived from an optimal interpolation of the current meter observations. The ranking of the importance of processes obtained from the simulated distributions is; advection (32%), biological‐vertical processes (27%), diffusion (20%), and local time change (21%). Thus the two approaches provide different estimates of the processes controlling phytoplankton variability on the southeastern U.S. continental shelf. The most likely cause of the different results obtained with the two methods is error introduced into the calculations by the transient nature and sharp gradients associated with Gulf Stream frontal eddies. In spite of these results, the approach described in this analysis may have promise for computing chlorophyll budgets from ocean color measurements obtained in less dynamic environments.