Biogeochemical climatologies in the Ross Sea, Antarctica: seasonal patterns of nutrients and biomass

The seasonal patterns of nutrient (nitrate and silicic acid) and chlorophyll distributions in the Ross Sea are formulated by two independent methods. The first procedure compiles all available data from cruises from 1970 to the present and generates a three-dimensional grid for the months from November through February using an iterative difference-correction scheme. The second method uses a three-dimensional circulation model and the phytoplankton standing stock climatology to investigate the effects of currents and phytoplankton uptake on nutrient distributions. The two approaches produced similar results, although the circulation model produced distributions that were more variable in space due to its finer resolution. The nutrient distributions were characterized by elevated concentrations in early spring and gradual reductions to ca. 15 and 40 μM (nitrate and silicic acid, respectively) in summer. Nutrient depletion did not occur despite the favorable growth conditions (elevated macronutrient concentrations, strong vertical stratification) in summer, suggesting that an alternative limitation (such as by dissolved iron concentrations) occurs. Chlorophyll concentrations reached ca. 6 μg l−1 in December and declined thereafter. Seasonal primary production calculated from the nitrate deficits and the circulation model suggested that production was ca. 73 g C m−2, slightly lower but similar to other estimates using independent methods. Using the nutrient climatology, losses (vertical flux plus respiration) through Feb. 15 from the upper 100 m were ca. 50% of the seasonal production, and the rest of the organic production was removed after that date. Results also suggest that carbon export from the surface layer may vary significantly in space and time.