Translating an open-ocean biogeochemistry code with cryptic sulfur
cycling to Chesapeake Bay requires considering the impacts of burial,
dissolved organic matter, and optics
A number of models have been developed to simulate hypoxia in the
Chesapeake Bay, but these models do not agree on what processes must be
included. In this study we implemented a previously published
biogeochemical (BGC) code developed for open-ocean waters that includes
“cryptic” microbial sulfur cycling, a process that can increase
denitrification and anammox rates in anoxic waters. We ran this BGC code
within the ChesROMS physical model of the Chesapeake Bay, then compared
the results to those of a ChesROMS simulation with an estuarine BGC code
previously implemented and calibrated in the Bay. The estuarine BGC code
neglects sulfur cycling but includes burial of particulate organic
matter (POM) and cycling of dissolved organic matter (DOM) and uses
different values for many parameters governing phytoplankton growth and
particle dynamics. At a key test site (the Bay Bridge Station), the
model with sulfur cycling gives better results for oxygen and nitrate.
However, it also gives a worse overprediction of ammonium-suggesting
that its greater accuracy in predicting these two variables may result
from cancellation of errors. By making comparisons among these two
models and derivatives of them, we show that the differences in modeled
oxygen and ammonium are largely due to whether or not the BGC codes
include cycling of DOM and sedimentary burial of POM, while the
differences in modeled nitrate are due to the other differences in the
modeled biogeochemical processes (sulfur cycling/anammox/optics).
Changes in parameters used in both BGC codes (in particular particle
sinking velocities) tended to compensate the other differences.
Predictions of hydrogen sulfide (H 2 S) within the Bay are very
sensitive to the details of the simulation, suggesting that it could be
a useful diagnostic.