Shelled Pteropod individual-based model output for the publication: The impact of aragonite saturation variability on shelled pteropods: An attribution study in the California current system

Observations from the California Current System (CalCS) indicate that the
long-term trend in ocean acidification (OA) and the naturally occurring
corrosive conditions for the CaCO3 mineral aragonite (saturation
state Ω < 1) have a damaging effect on shelled pteropods, a
keystone group of calcifying organisms in the CalCS. Concern is heightened
by recent findings suggesting that shell formation and developmental
progress are already impacted when Ω falls below 1.5. Here, we quantify
the impact of low Ω conditions on pteropods using an individual-based
model (IBM) with life-stage-specific mortality, growth, and behaviour in a
high-resolution regional hindcast simulation of the CalCS between 1984 and
2019. Special attention is paid to attributing this impact to different
processes that lead to such low Ω conditions, namely natural variability,
long-term trend, and extreme events. We find that much of the observed
damage in the CalCS, and specifically >70% of the shell
CaCO3 loss, is due to the pteropods’ exposure to naturally
occurring low Ω conditions as a result of their diel vertical migration
(DVM). Over the hindcast period, their exposure to damaging waters (Ω
< 1.5) increases from 9% to 49%, doubling their shell CaCO3 loss,
and increasing their mortality by ∼40%. Most of this increased exposure is
due to the shoaling of low Ω waters driven by the long-term trend in OA.
Extreme OA events amplify this increase by ∼40%. Our approach can quantify
the health of pteropod populations under shifting environmental
conditions, and attribute changes in fitness or population structure to
changes in the stressor landscape across hierarchical time scales.