Interannual variations in nutrients, net community production, and biogeochemical cycles in the Ross Sea

The Ross Sea continental shelf is dominated by the seasonal appearance of a large phytoplankton bloom. This bloom is regularly dominated by diatoms and the haptophyte Phaeocystis antarctica, and significant nutrient (nitrogen and silicon) reductions within the water column occur during the growing season (early November to late February). Diatoms mediate silicic acid removal, whereas both taxa remove nitrate. Dissolved and particulate nitrogen and silica concentrations were collected from a series of cruises to the southern Ross Sea over 3 years. Simple, one-dimensional nutrient budgets were generated for nitrogen and silica, and estimates of vertical flux were derived from these budgets. Substantial variations among years are observed to occur in seasonal community production, assemblage composition, Si:N uptake ratios, and export, and standard deviations are equal to ∼30% of the mean. During 2003–2004 a large Phaeocystis antarctica bloom occurred in December, and was followed by a bloom of diatoms. This secondary bloom was equal in magnitude to that of the initial P. antarctica bloom. In contrast, no secondary bloom was observed in 2001–2002. Continuous fluorescence measurements suggested that the spatial–temporal mosaic of phytoplankton dynamics in the Ross Sea is far more complex than previously thought. We hypothesize that variations occur between years not only in terms of both magnitude and composition of the bloom, but also in the controlling mechanisms.