DATA ASSIMILATION IN BIOGEOCHEMICAL MODELLING
527
vertical DIC gradient, only about a quarter can be explained by the
solubility pump [Sarmiento et al., 1995] with the remaining three quarters being attributed to the “biological” carbon pump [Volk and Hoffert,
1985]. The driving agent of the biological pump is photosynthesis that
generates organic carbon and thereby reduces DIC concentrations, and
accordingly the partial pressure of CO 2 , in the surface ocean. Respiration of organic carbon by metabolic processes in bacteria, higher trophic
levels, and in the photosynthetically active phytoplankton itself reverses
this process. As a result of mixing and advection along the vertical light
gradient and because of the formation of biogenic particles that sink
through the water instead of moving with it, respiration occurs generally deeper in the water column than photosynthesis. This decoupling of
photosynthesis and respiration generates vertical gradients of DIC. To
make things more complicated, some organisms form calcium carbonate
“hard parts” which, on formation, sinking, and dissolution also affect
the carbonate chemistry of sea water and result in an alkalinity pump.
Because the formation of calcium carbonate in surface water increases
surface pCO 2 , this constitutes a counter pump in terms of CO 2 which
partly compensates the pCO 2 effect of the organic carbon pump. A
bust mechanistic understanding of the formation and biotically aided
dissolution of calcium carbonate shells is not yet available, and many
models so far assume that a fixed fraction of all biogenic particulate carbon sinking out of the light-lit euphotic zone (roughly the upper 100 m)
is associated with calcium carbonate formation.
A close interaction of biology and physics arises not only from the
interplay of physical and biological transport mechanisms on the vertical DIC gradient, but also from the fact that phytoplankton growth
requires the presence of both light and nutrients, which usually have
opposite vertical gradients. Accordingly, light and nutrient levels experienced by a phytoplankton cell are very sensitive to physical transport
processes that may upwell or entrain deeper and nutrient-rich waters,
or may mix or advect phytoplankton cells down into the dark ocean
terior. This physical control on biological production has to be taken
into account when attempting to simulate the marine carbon cycle. A
standard strategy is to couple marine ecosystem models into circulation
models. Validation of such coupled models is not straightforward. For
example, the strong sensitivity of the marine biota to physical transport
processes makes it difficult to separately evaluate the individual model
components. For many applications one can at least safely neglect the
biological impact on the physics via changes in the absorption profile
of solar radiation [Oschlies, 2004]. While this allows to evaluate the
physical model component individually, the reverse is not true for the
ro
in
527
vertical DIC gradient, only about a quarter can be explained by the
solubility pump [Sarmiento et al., 1995] with the remaining three quarters being attributed to the “biological” carbon pump [Volk and Hoffert,
1985]. The driving agent of the biological pump is photosynthesis that
generates organic carbon and thereby reduces DIC concentrations, and
accordingly the partial pressure of CO 2 , in the surface ocean. Respiration of organic carbon by metabolic processes in bacteria, higher trophic
levels, and in the photosynthetically active phytoplankton itself reverses
this process. As a result of mixing and advection along the vertical light
gradient and because of the formation of biogenic particles that sink
through the water instead of moving with it, respiration occurs generally deeper in the water column than photosynthesis. This decoupling of
photosynthesis and respiration generates vertical gradients of DIC. To
make things more complicated, some organisms form calcium carbonate
“hard parts” which, on formation, sinking, and dissolution also affect
the carbonate chemistry of sea water and result in an alkalinity pump.
Because the formation of calcium carbonate in surface water increases
surface pCO 2 , this constitutes a counter pump in terms of CO 2 which
partly compensates the pCO 2 effect of the organic carbon pump. A
bust mechanistic understanding of the formation and biotically aided
dissolution of calcium carbonate shells is not yet available, and many
models so far assume that a fixed fraction of all biogenic particulate carbon sinking out of the light-lit euphotic zone (roughly the upper 100 m)
is associated with calcium carbonate formation.
A close interaction of biology and physics arises not only from the
interplay of physical and biological transport mechanisms on the vertical DIC gradient, but also from the fact that phytoplankton growth
requires the presence of both light and nutrients, which usually have
opposite vertical gradients. Accordingly, light and nutrient levels experienced by a phytoplankton cell are very sensitive to physical transport
processes that may upwell or entrain deeper and nutrient-rich waters,
or may mix or advect phytoplankton cells down into the dark ocean
terior. This physical control on biological production has to be taken
into account when attempting to simulate the marine carbon cycle. A
standard strategy is to couple marine ecosystem models into circulation
models. Validation of such coupled models is not straightforward. For
example, the strong sensitivity of the marine biota to physical transport
processes makes it difficult to separately evaluate the individual model
components. For many applications one can at least safely neglect the
biological impact on the physics via changes in the absorption profile
of solar radiation [Oschlies, 2004]. While this allows to evaluate the
physical model component individually, the reverse is not true for the
ro
in