RELATIONSHIP BETWEEN PHYTOPLANKTON PRODUCTION AND THE PHYSICAL STRUCTURE OF THE WATER COLUMN NEAR COBB SEAMOUNT, NORTHEAST PACIFIC

The objective of this study was to determine the impact of the Cobb Se amount (northeast Pacific) on phytoplankton biomass and primary produc tion. In August 1991, we sampled along six 30 km spokes radiating outw ard from the summit of Cobb Seamount. Surface chlorophyll a (Chi a) an d incident light conditions were relatively uniform over this mesoscal e sampling grid, but primary production varied over a ten-fold range. Statistical analyses linked these production variations to a subsurfac e (>50 m) thermohaline front skirting the flanks of the seamount. The thermal stratification of surface waters (<50 m) also changed across t he front, generally increasing toward the seamount. These physical cha nges in turn were associated with a shallowing and intensification of subsurface Chi a maxima, increases in diatom biovolume, and increases in production to biomass ratios. Although nutrient isolines domed slig htly upward over the seamount, this physical feature did not seem to e nrich surface waters or stimulate production. It appeared that growth and production rates were enhanced by increased stability of the upper water column that led to an improved subsurface light regime. Possibl e associations of these physical-biological interactions with the seam ount are discussed.