Impact of Dynamic Phytoplankton Stoichiometry on Global Scale Patterns
of Nutrient Limitation, Nitrogen Fixation, and Carbon Export
Abstract
Phytoplankton stoichiometry modulates the interaction between carbon,
nitrogen and phosphorus cycles, yet most biogeochemical models represent
phytoplankton C:N:P as constants. This simplification has been linked to
Earth System Model (ESM) biases and potential misrepresentation of
biogeochemical responses to climate change. Here we integrate key
elements of the Adaptive Trait Optimization Model (ATOM) for
phytoplankton stoichiometry with the Carbon, Ocean Biogeochemistry and
Lower Trophics (COBALT) ocean biogeochemical model. Within a series of
global ocean-ice-ecosystem retrospective simulations, ATOM-COBALT
reproduced observations of particulate organic matter N:P, and compared
to static N:P, exhibited reduced phytoplankton P-limitation, enhanced
N-fixation, and increased low-latitude export, leading to improved
consistency with observations. Two mechanisms together drove these
patterns: the growth hypothesis and frugal P-utilization during
scarcity. The addition of translation compensation- differential
temperature dependencies of photosynthetic relative to biosynthetic
processes- led to relatively modest strengthening of N:P variations and
biogeochemical responses relative to growth-plus-frugality. Comparison
of the multi-mechanism model herein against frugality-only models
suggest that both can capture observed N:P patterns and produce
qualitatively similar biogeochemical effects. There are, however,
quantitative response differences and different responses across N:P
mechanisms are expected under climate change- with the growth rate
mechanism adding a distinct biogeochemical footprint in
highly-productive low-latitude regions. These results suggest that
variable phytoplankton N:P makes some biogeochemical processes resilient
to environmental changes, and support using dynamic N:P formulations
with the ocean biogeochemical component of next generation of ESMs.