A quantitative analysis of sources for summertime phytoplankton variability over 18 years in the South Shetland Islands (Antarctica) region

Eighteen years of summertime hydrographic and chlorophyll-a (Chl-a) data (∼2700 stations) from the South Shetland Islands (Antarctica) region show that a “bell-shaped” (unimodal) distribution of phytoplankton biomass results annually when plotted against the inshore to offshore gradient in surface salinity. The maximum for this unimodal Chl-a distribution corresponds with a shallow upper mixed layer (UML) in iron-rich waters that occurs at salinities ∼34. Methods of gradient analysis are used to distinguish sources of variability for bloom development among years. The control of phytoplankton biomass is resolved across the salinity gradient that separates the co-limiting conditions of deep UML depths and low-iron concentrations as opposing end-members. Chlorophyll-fluorescence yield data (a proxy for Fe-stress) showed that at salinities ∼34, phytoplankton biomass was unlikely to be limited by Fe. Instead, blooming at salinities ∼34 (1.3±1 mg Chl-a m−3) co-varied with shallow UML depths (41±19 m) that occurred as a function of higher UML temperature (1.5±0.5 °C) among years, and is evidence that atmospheric climate variability impacts summertime phytoplankton biomass and production in this Southern Ocean seascape.