Extended Abstract

A transition zone in phytoplankton concentration running across the North Pacific basin at 30 to 40 north latitude corresponds to a basin wide front in surface chlorophyll observed in a composite of Coastal Zone Color Scanner images for May, June and July 1979-1986. This transition zone with low chlorophyll to the south and higher chlorophyll to the north can be simulated by a simple model of the concentration of phytoplankton, zooplankton and dissolved nutrient (nitrate) in the surface mixed layer of the ocean applied to the North Pacific basin for the the climatological conditions during oceanographic springtime (May, June and July). The model is initialized with a 1 gridded estimate of wintertime (February, March and April) mixed layer nitrate concentrations calculated from an extensive nutrient database and a similarly gridded mixed layer depth data set. Comparison of model predictions with CZCS data provides a means to evaluate the dynamics of the transition zone. We conclude that in the North Pacific, away from major boundary currents and coastal upwelling zones, wintertime vertical mixing determines the total nutrient available to the plankton ecosystem in the spring. The transition zone seen in basin-scale CZCS images is a reflection of the geographic variation in the wintertime mixed layer depth and the nitracline, leading to a latitudinal gradient in phytoplankton chlorophyll.

The cover plate of global phytoplankton distribution that accompanies Feldman et al. [1989], clearly shows a transition zone between low pigment levels in the central gyres and high concentrations at middle and higher latitudes of both the Atlantic and Pacific oceans. A boundary in ocean transparency along middle latitudes in both the N. Pacific and N. Atlantic basins is also evident in Plate 1 of Lewis, et al. [1988]. We hypothesize that the wintertime mixing regime combines with the nutrient structure of the ocean and plankton ecosystem dynamics to determine the transition zone along 30 to 40N latitude in both the N. Pacific and N. Atlantic.

In this paper we present a composite CZCS image for the North Pacific in spring 1979--1986. We then use a simple plankton dynamics model [ Wroblewski et al., 1988] to explore the patterns of plankton distributions in the upper Pacific ocean. We review the climatological mixed layer and nutrient databases used to solve this model for the North Pacific during the oceanographic spring. We present model predictions of phytoplankton, zooplankton and dissolved nutrient distributions, making comparisons to observations. We then discuss the relationship of distributional boundaries to wintertime mixing regimes, results from sensitivity analysis, and limitations of the model.

The ocean basin-scale pattern seen in a CZCS composite image for surface chlorophyll in the North Pacific during the oceanographic springtime (May, June, July) can be crudely reproduced by a simple plankton model which includes the supply of nutrients to the surface layer by wintertime convective mixing. The major transition zone with low chlorophyll to the south and higher chlorophyll to the north, running along 30-- 40N latitude is a reflection of the geographic variation in the wintertime mixed layer depth and the depth of the nitracline. The dynamics of the transition zone in both the North Pacific and North Atlantic appear to be similar.