OBSERVATIONS AND MODELING OF THE ANTARCTIC PHYTOPLANKTON CROP IN RELATION TO MIXING DEPTH

The multi-disciplinary program RACER (Research on Antarctic Coastal Ecosystem Rates) conducted eight surveys of a 69-station grid in a 100 x 250 km area in the southwestern Bransfield Strait from December 1986 to March 1987. Mean phytoplankton crop size in the upper 50 m during December, January, February and March was 291, 176, 58 and 50 mg Chl a m-2, respectively, and was inversely proportional to the increasing mean depth of the upper mixed layer (UML) (15, 17, 26 and 30 m, respectively). Massive mid-summer phytoplankton blooms (> 10 mg Chl a + phaeo m-3) were persistent nearshore where we observed shallow UMLs (< 20 m) caused by meltwater stabilization (DELTA-sigma-t 0.25-1.0 from 0 to 75 m). Drake Passage waters were low in phytoplankton biomass (< 1.0 mg Chl a + phaeo m-3), and had deep UMLs (> 20 m) with small density gradients (DELTA-sigma-t 0.05-0.20 from 0 to 75 m). Proximity to stabilizing meltwater and protection from intense Antarctic storm activity appear to be essential for the development of persistent massive blooms. A model of Antarctic phytoplankton growth based on mixing depth and pigment-specific light attenuation and in situ photosynthesis-irradiance relationships indicates that the depth of the UML (Z(UML)) can be used to predict the upper limit of the phytoplankton crop size. Observed phytoplankton biomass for diverse Southern Ocean ecosystems is discussed in relation to the mean light level of the UML, growth and loss rates of Antarctic phytoplankton, and the depth and duration of stratification required before a bloom ensues. Assuming nutrients do not limit the crop size, a best-fit to observations indicates specific loss rates must be approximately 0.3-0.35 day-1 and massive blooms occur only if Z(UML) < 25 m. The grazing component of this predicted loss rate is higher than previously estimated. We conclude that grazing rates are greater than previously reported, or vertical flux rates of nutrients limit massive blooms.