Spatial patterns of chlorophyll, primary production, macrozooplankton biomass, and physical structure in the central North Pacific Ocean

The relation between the large-scale horizontal patterns of biological properties (primary production and the standing stocks of phytoplankton and macrozooplankton) and physical structure in the North Pacific central gyre is described based upon sampling on a north-south section in August 1980 (expedition FIONA). Primary production in the central North Pacific is nutrient limited and the large-scale patterns in the measured biological properties appear to be determined by physical processes which affect the vertical flux of new nutrients from deeper water to the euphotic zone. The observed biological patterns can thus be used to infer horizontal variations in physical processes which affect the rate of nutrient supply to the euphotic zone. A mesoscale eddy in the southern part of the section decreased the local level of primary production by pushing density and nutrient surfaces to deeper depths. In addition, there was a large-scale, south-to-north decrease within the central gyre in chlorophyll, primary production and macrozooplankton biomass. This biological gradient was not related in any simple way to the horizontal distribution of light penetration or the depths of the nutricline or chlorophyll maximum layer. The hypothesis that horizontal patterns of nutrient input and production were determined by vertical Fickian diffusion and, thus, should be positively correlated with the vertical nutrient gradient and negatively correlated with the vertical density gradient, was tested. This simple model however was not supported, suggesting that either some other process drives the large-scale patterns of vertical nutrient input to the euphotic zone, or that k 2 (the vertical eddy diffusivity) is uncorrelated with the mean vertical density gradient. There was some support for a previous hypothesis that enhanced mixing is associated with the density variance maximum, and that the relation of the depth of the variance maximum to the depth of the top of the nutricline may affect the large-scale biological patterns.