Spatial and temporal variations of phytoplankton biomass related to environmental factors in the Western Ross Sea.

Three years of investigation in the western Ross Sea reveal that despite the genera1 abundance of phytoplankton biomass, the ratios between biomass and nutrient availability are low due to high nutrient concentrations.The NIP ratio is practically identica1 to that of Redfield; however, during blooms the ratio can be severely altered due to very low phosphate concentrations. Phytoplankton biomass during summer shows a bimodal cyclic pattern with two blooms in early and late summer.The spatial and temporal distribution of these blooms follows the latitudinal distribution of daily irradiance, which determines the advance of sea-ice melting from south to north forming an upper mixed layer rich in nutrients.This mixed layer is well illuminated and warmer than the deeper layer, thereby favouring the development of the early bloom. Afterwards, in the mixed water column, biomass decreases and reaches the summer minimum. A new stabilization of the water column is followed by a second bloom, less extensive than the first. In al1 3 years, similar cycles have been recorded, giving corroborative evidence for the existence of a second bloom.