Modeling the
Seasonal Decoupling of
the Carbon and Nitrogen Cycles in Santa Monica Bay, CA
Hartmut Frenzel, Anita Leinweber, Nicolas Gruber,
Rebecca F. Shipe, and James C. McWilliams
It is standard practice to use the concept of constant
stoichiometric
ratios, i.e. the so-called Redfield ratio concept, to
derive the net
carbon balance from photosynthesis and respiration from changes in
nutrient concentrations in the euphotic zone. However, long-term
observations in Santa Monica Bay show repeated episodes of anomalous
drawdowns of dissolved inorganic carbon (DIC) that cannot be explained
by concomitant changes in nitrate, suggesting that carbon and nitrogen
are often strongly decoupled, perhaps as a result of faunal shifts in
the upper ecosystem and greater flexibility in nutrient to carbon
ratios (see abstract by Leinweber et al.).
Here, we investigate this
hypothesis using a coupled three-dimensional
physical/biogeochemical/ecological model for Santa Monica Bay and its
vicinity. The biogeochemical/ecological model includes multiple
limiting nutrients (N, P, Si, and Fe) and four phytoplankton
functional groups (small phytoplankton, diatoms, coccolithophorids,
and diazotrophs). While the ratios between the limiting nutrients and
between nutrients and chlorophyll are allowed to vary based on ambient
conditions, the C:N ratio is fixed in the base version of the
model. The observations suggest, however, that we need to allow
variations in the C:N ratio of diatoms, which is the dominant group
during upwelling. In our model simulations, the observed DIC
drawdown during nutrient-deplete conditions can only in part be
explained by nitrogen fixation. We therefore run case studies in which
we (i) modify the model to include a variable C:N ratio for diatoms,
(ii) permit the small phytoplankton functional group that
represents dinoflagellates to migrate vertically depending on their
internal nutrient status, and (iii) modify the parameterization of
nitrogen fixation. Differences between these cases and our
standard model will then be used to determine the net impact of these
changes in physiology and behavior of phytoplankton on the upper
ocean inorganic carbon balance.
For citations, please use:
Frenzel,H., A. Leinweber, N. Gruber, R. F. Shipe, and
J. C. McWilliams (2006), Modeling the
Seasonal Decoupling of
the Carbon and Nitrogen Cycles in Santa Monica Bay, CA, Eos
Trans.
AGU, 87(36), Ocean Sci. Meet. Suppl., Abstract OS43L-04