PRIMARY PRODUCTION IN THE DEEP BLUE SEA

Primary production rates and concentrations of photosynthetic pigments were measured on 5 days during March and April 1987, at 26°N, 155°W in the North Pacific subtropical gyre. The primary production measurements were carried out using on-deck incubators with blue and blue-violet filters which simulated both the intensity and spectral characteristics of submarine light. Dawn-to-dusk production rates averaged 777±219 mg C m-2. The quantum requirements implied by photosynthetic rates measured near the base of the euphotic zone approached the theoretical minimum of 8 Ein mol-1C if the chlorophyll a-specific absorption coefficient (kc) was assumed to equal 16 m2 g-1 Chl a. Both the absorption characteristics of extracted pigments and of particulate material collected on glass fiber filters indicated that kc was constant and equal to 16.0±1.6 m2 g-1 Chl a over the depth of the euphotic zone if submarine light were assumed to have the same spectral characteristics as surface light. When the spectral characteristics of submarine light were properly modeled, kc was found to increase by about a factor of three between the surface and approximately the 14% light level and to equal 44±7 m2 g-1 Chl a throughout the lower 75% of the euphotic zone. The dramatic difference in the ability of phytoplankton to absorb equal quantum fluxes of white and blue light implies that areal primary production rates can be underestimated by about a factor of two in the open ocean if incubations are conducted using surface light attenuated with neutral density filters. © 1990.