Mississippi river chemistry impacts on the interannual variability of
aragonite saturation state in the Northern Gulf of Mexico
Abstract
In the northern Gulf of Mexico shelf, the Mississippi-Atchafalaya River
System (MARS) impacts the carbonate system by delivering freshwater with
a distinct seasonal pattern in both total alkalinity (Alk) and dissolved
inorganic carbon (DIC), and promoting biologically-driven changes in DIC
through nutrient inputs. However, how and to what degree these processes
modulate the interannual variability in calcium carbonate solubility
have been poorly documented. Here, we use an ocean-biogeochemical model
to investigate the impact of MARS’s discharge and chemistry on
interannual anomalies of aragonite saturation state (ΩAr). Based on
model results, we show that the enhanced mixing of riverine waters with
a low buffer capacity (low Alk-to-DIC ratio) during high-discharge
winters promotes a significant ΩAr decline over the inner-shelf. We also
show that increased nutrient runoff and vertical stratification during
high-discharge summers promotes strong negative anomalies in bottom ΩAr,
and less intense but significant positive anomalies in surface ΩAr.
Therefore, increased MARS discharge promotes an increased frequency of
suboptimal ΩAr levels for nearshore coastal calcifying species.
Additional sensitivity experiments further show that reductions in the
Alk-to-DIC ratio and nitrate concentration from the MARS significantly
modify the simulated ΩAr spatial patterns, weakening the positive
surface ΩAr anomalies during high-discharge summers or even producing
negative surface ΩAr anomalies. Our findings suggest that riverine water
carbonate chemistry is a main driver of interannual variability in ΩAr
over river dominated ocean margins.