Land Ocean CouplingCoupling riverine fluxes of nutrients to a Global

Biogeochemical Ocean General Circulation Model

WWW.BJERKNES.UIB.NO

Christophe Bernard, Christoph HeinzeGeophysical Institute, Bjerknes Center for Climate Research

University of Bergen

• NPZD model with colimitation of Nutrients such as N, P,Si,Fe

• orthogonal curvilinear C-grid with a formal resolution of 3◦

• 20 km in the Arctic and about 350 km in the Tropics• 40 vertical levels with level thickness increasing with depth• North pole is located over Greenland and the other over

Antarctica.

The HAMOCC5-MPIOM

Application of the coastal segmentation : estimating natural silica fluxes to the coastal zone

Dürr and Meybeck, 2007

Relative silica flux

Global mean silica yield 3.3 t SiO2 km-2 yr-1

Hyperactive

Very active

Active

Sub-active

Hypo-active

Inactive

‘Dead’

10

5

2

0.5

0.2

0.1

0.01

-> < 20 % of area responsible for >50% of natural silica yield

6,2 teramoles of silicon per year

The riverine inputs

• Figure 1. Integrated annual flux of silica as added in the model grid, according to the 129 coastal segments from the COSCAT approach. Riverine silica inputs are given in megamoles Si per year.

The coupling• Flux computed at each time step: 10 times a day • It includes :Si, DIN, DIP, DON,DOP,PP,PN and POC

• Homogeneous along the coastal segment

• Constant over time

With riverine silicate

Without riverine silicate

Computing the difference between the 2 runs

Riverine contribution to the Opal export

production

Fate of riverine Si depends on the level of primary production.

Limiting nutrientAnnual photosynthesis

the computation of the photosynthesis is driven by the less available nutrient corrected by a multiplying factor.

XK

XtzIJPhyPhoto

POPhy

4

*)),((*

PFePN R

Fe

R

NOPOX

::

34 ,,min

Bernard et al. 2008, submitted

Example of the Amazon contribution

Figure 4. The seasonal cycle of nutrient limitation (element equivalent phosphorous) in the Amazon plume, (lower panels) with (left) and without (right) riverine silica inputs. Opal and calcium carbonate export (F opal and F calcarb) at 10m depth in response to photosynthesis (upper panels). Nutrient limitation is expressed as the concentration adjusted to the necessary stoichiometric concentration of nitrogen and iron relative to phosphate. The limitation of photosynthesis is driven by the lowest concentration equivalent phosphate (iron, nitrate or phosphate). Opal and calcium carbonate competition is driven by the silica concentration.

Bernard et al. 2008, submitted

Continental margins in the model’s grid

• Defined as the 8% shallowest grid points

Marge vs global ocean: the riverine nutrients

contribution Carbon

Silica

0.5 Gt C

Relative contribution of riverine nutrients to the export production of Opal and C

All Nut No Nut No Si No Org No Part

No DIN No DIP

C

Si

Marginal seas….

All Nut No Nut No Si No Org No Part

No DIN No DIP No C All DI form

0,09 Gt C

To summarise… and conclude.

• Human activity (urban development, land use, damming) changes the river load of nutrients to the ocean(decreased Si, increased N and P).

• Changes in the riverine inputs of nutrients do have an impact on a global scale.

• Marginal seas are more sensitive to river load changes.

• Eutrophication leads to a larger burial of Opal on the continental shelf.