Light Is the Primary Driver of Early Season Phytoplankton Production Along the Western Antarctic Peninsula

Light and dissolved iron (dFe) availability control net primary production (NPP) in much of the Southern Ocean, but the primary controller during spring in the western Antarctic Peninsula has never been assessed. Underwater light and dFe availability are sensitive to climate‐induced changes in upper ocean circulation, stratification, and sea ice cover, which can affect NPP and phytoplankton community composition, both of which can alter carbon drawdown and food web structure. We estimated in situ NPP, net community production, and heterotrophic respiration and contextualized our field measurements with satellite‐based historical NPP estimates. Average light exposure mainly controlled NPP, while low dFe was associated with greatest NPP, indicating that spring phytoplankton growth is light‐limited and not dFe‐limited. Using experiments that simulated varying mixed layer depths by exposing phytoplankton to a short period of in situ surface light (up to 150× the mean light in the mixed layer, comparable to the difference in light experienced by phytoplankton mixed from 50 m to the surface), we assessed the effect of phytoplankton photoacclimation on NPP and relative success of individual taxa. At moderate light exposure (<30×), phytoplankton experienced little photodamage or changes in NPP, and Phaeocystis antarctica grew more than diatoms. Conversely, phytoplankton exposed to high light (>60×) experienced significant photodamage, declines in NPP, and declines in P. antarctica, with no consistent changes in diatoms. These results support the idea that P. antarctica is better adapted to variable light than diatoms and suggest that deeper mixed layers with variable light will favor P. antarctica.