Decadal increase in phytoplankton community photoprotective pigments associated with shoaling mixed layers along a warming West Antarctic Peninsula

Abstract In the Southern Ocean, phytoplankton are critical drivers of biogeochemical cycling and food web dynamics, and show sensitivity to shifting climates. Along the West Antarctic Peninsula, climate‐driven variations in sea ice and hydrography have been linked to long‐term changes in summer phytoplankton productivity. Such changes are hypothesized to reflect decadal shifts in algal light environments, but diagnostic evidence of this light‐dependent response is limited. Using a 27‐yr timeseries of summer phytoplankton pigments and productivity collected along the West Antarctic Peninsula by the Palmer Long Term Ecological Research (LTER) program, we quantified trends in surface phytoplankton photophysiology, productivity, and composition in response to environmental change. Our results revealed a decadal doubling in proportions of phytoplankton photoprotective pigments correlated with long‐term shoaling and strengthening of mixed layer depths. This biophysical signature is consistent with photophysiological response to increased light supply, signifying a long‐term shift in light environments for surface phytoplankton. The long‐term change in community pigment signature could not be explained by trends in phytoplankton composition alone, indicating a key role of photoacclimation. Phytoplankton community biomass, productivity, and production efficiency (chlorophyll‐normalized productivity) were similarly correlated to upper ocean structure and increased decadally, demonstrating a functional benefit from enhanced light availability. These physiological findings support the hypothesis that decadal trends in summer production along the peninsula are light‐dependent. Such large‐scale shifts in community pigment signature, linked to oceanographic forcing, suggest that photophysiological indices provide useful insights into how shifting climates will influence phytoplankton communities.