Phytoplankton taxonomic variability in nutrient utilization and primary production in the Ross Sea

Patterns of nutrient utilization and primary productivity (PP) in late austral spring and early summer in the southwestern Ross Sea were characterized with respect to phytoplankton taxonomic composition, polynya dynamics, and upper ocean hydrography during the 1996-1997 oceanographic program Research on Ocean-Atmosphere Variability and Ecosystem Response in the Ross Sea. Phytoplankton biomass in the upper 150 m of the water column ranged from 40 to 540 mg chlorophyll a (Chl a) m(-2), exceeding 200 mg Chl a m-2 everywhere except the extreme northern and eastern boundaries of the Ross Sea polynya. Diatom biomass was greatest in the shallow mixed layers of Terra Nova Bay, while the more deeply mixed waters of the Ross Sea polynya were dominated by Phaeocytstis antarctica. Daily production computed from the disappearance of NO3 (1.14 g C m(-2) d(-1)) and total dissolved inorganic carbon (TDIC, 1.29 g C m(-2) d(-1)) is consistent with estimates made from an algorithm forced with satellite measurements of Chl a (1.25 g C m(-2) d(-1)) and from measurements of C-14 uptake (1.33 g C m(-2) d(-1)). Phytoplankton PP in the Ross Sea averaged 100 g C m(-2) yr(-1) during 1996-1997. Despite the early formation of the Terra Nova Bay polynya the diatom bloom there did not reach its peak PP until middle to late January 1997 (most likely because of more intense wind mixing in November), similar to 6 weeks after the P. antarctica bloom in the Ross Sea polynya had reached the same stage of development. From 70 to 100% of the C and N deficits in the upper 150 m could be accounted for by particulate organic matter, indicating that there had been little dissolved organic matter production or export of particulate material prior to our cruise. This suggests that early in the season, PP and zooplankton grazing are decoupled in the southwestern Ross Sea. The NO3:PO4 disappearance ratio in waters dominated by P. antarctica (19.0 +/- 0.61) was significantly greater than in waters where diatoms were most common (9.52 +/- 0.33), and both were significantly different from the Redfield N:P ratio of 16. Vertical profiles of TDIC suggest that P. antarctica took up 110% more CO2 per mole of PO4 removed than did diatoms, an important consideration for climate models that estimate C uptake from the removal of PO4.