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 Seamount (northeast Pacific) on phytoplankton biomass and primary production. In August 1991, we sampled along six 30 km spokes radiating outward from the summit of Cobb Seamount. Surface chlorophyll a (Chl a) and incident light conditions were relatively uniform over this mesoscale sampling grid, but primary production varied over a ten-fold range. Statistical analyses linked these production variations to a subsurface (>50 m) thermohaline front skirting the flanks of the seamount. The thermal stratification of surface waters (<50 m) also changed across the front, generally increasing toward the seamount. These physical changes in turn were associated with a shallowing and intensification of subsurface Chl a maxima, increases in diatom biovolume, and increases in production to biomass ratios. Although nutrient isolines domed slightly upward over the seamount, this physical feature did not seem to enrich 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. Possible associations of these physicalbiological interactions with the seamount are discussed.