Self-shading and meltwater spreading control the transition from light
to iron limitation in an Antarctic coastal polynya
Abstract
{Dotson Ice Shelf (DIS) in West Antarctica is undergoing rapid basal
melting driven by intrusions of warm, saline Circumpolar Deep Water
(CDW) onto the continental shelf. Meltwater from DIS is thought to
influence biology in the adjacent Amundsen Sea Polynya (ASP), which
exhibits the highest Net Primary Productivity (NPP) per unit area of any
coastal polynya in the Southern Ocean. However, the relative importance
of iron and light in colimiting the spring phytoplankton bloom in the
ASP remains poorly understood. In this modelling study we first
investigate the mechanisms by which ice shelves impact NPP, then map
spatio-temporal patterns in iron-light colimitation, and finally examine
the environmental drivers of iron and light supply. We find that ice
shelf melting leads to greater upper ocean iron concentrations, both
directly due to release of iron from sediments entrained at the glacier
bed, and indirectly via a buoyancy driven overturning circulation which
pulls iron from CDW to the surface. Both of these mechanisms increase
NPP compared to experiments where ice shelf melt is suppressed. We then
show that the phytoplankton self-shading feedback delays the bloom and
reduces peak NPP by 80\% compared to experiments where
light penetration is independent of chlorophyll. Iron limitation due to
phytoplankton uptake is more important a) later in the season, b) higher
in the water column and c) further from the ice shelf; as compared to
light limitation. Finally, sensitivity experiments show that variability
in CDW intrusion influences NPP by controlling the horizontal spreading
of iron-rich meltwater.}