INSHORE-OFFSHORE DIVERGENCE IN THE LONG-TERM EFFECTS OF MIXED LAYER SHOALING ON SUMMER PHYTOPLANKTON COMMUNITIES ALONG THE WEST ANTARCTIC PENINSULA

The West Antarctic Peninsula (WAP) provides an ideal setting to study long-term trends in Southern Ocean phytoplankton ecology under rapidly changing conditions. Over recent decades, shoaling of summer mixed layers on the WAP have been linked to increased surface phytoplankton productivity and CO₂ drawdown via increased light supply to light-limited communities. However, the link between light-dependent physiology and shallowed mixing, as well as responses in offshore iron-limited regions, remains unclear. Using two decades of pigment data from the Palmer Long Term Ecological Research (LTER) program, we analyzed trends in photophysiology, biomass, productivity, and mixing across the WAP to assess the long-term responses of phytoplankton communities to physical change, both in total and between physiochemical regions. We found a doubling of photoprotective carotenoid (PPC) proportions on the WAP, correlated with shallower mixed layers, increased productivity, and increased production efficiency. However, biophysical trends showed notable inshore-offshore gradients, where coastal regions displayed weak changes in mixing and PPC but the highest biomass gains, while slope regions had stronger shoaling and high PPC but minimal change in biomass. These results suggest that while light boosts surface production, iron availability has significantly constrained long-term biomass gains in offshore regions. Our findings support a light-regulated model of productivity for the WAP as a whole, but highlight that growth remains severely dampened offshore.

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Primary Presenter: Quintin Diou-Cass, Rutgers University (dioucass@gmail.com)

Authors:

Nicole Waite, Rutgers University  (waite@marine.rutgers.edu)

Oscar Schofield, Rutgers University  (oscar@marine.rutgers.edu)