Photosynthetic energy conversion efficiency along the West Antarctic Peninsula

Phytoplankton dynamics in the West Antarctic Peninsula (WAP), a highly productive polar ecosystem, is subject to intense bottom-up control by light and iron availability. Long term studies have shown a strong spatial gradient in abundance and primary productivity between the coast and continental slope. Due to rapid climate change along the WAP, dynamics in light, associated with changes in mixed layer depth and water column stability, and iron availability from glacial sources are altering. How these changes will alter primary productivity remains an open question, as the relative role of each control mechanism is unclear. Here, we assessed temporal and spatial responses in phytoplankton photophysiology to the availability of iron and light from the combination of variable fluorescence and fluorescence lifetime measurements, using custom-built fluorometers deployed on the 2017 Palmer Long Term Ecological Research cruise during the austral summer. Changes in photophysiology, reflecting variations in nutrient and light availability, indicate altered partitions between the three dissipation pathways (photochemistry, fluorescence and thermal dissipation) and energy conversion efficiencies of absorbed photons in photosystem II. Our results show diel cycles in photophysiological parameters, driven by light availability, however with a substantially different range of values in the coast and slope as a result of iron limitation. On the other hand, spatial gradients in photophysiology strongly correlated with potential iron availability. Further off the coast into the continental shelf and slope photochemical quantum yields decreased by 50%, with a proportional increase in quantum yields of thermal dissipation. Similarly, the quantum yields of fluorescence, as well as the effective cross sections of PSII, increased by 70% and 50%. Combined, our analysis implies that in the outer shelf waters of the WAP, phytoplankton abundance and primary productivity is controlled primarily by iron availability.