Exploring Long-Term Changes in Silicon Biogeochemistry Along the River Continuum of the Rhine and Yangtze (Changjiang)

This paper presents
the spatially explicit (0.5° spatial resolution) Dynamic InStream
Chemistry (DISC)-SILICON module, which is part of the Integrated Model
to Assess the Global Environment-Dynamic Global Nutrient Model global
nutrient cycling framework. This new model, for the first time, enables
to integrate the combined impact of long-term changes in land use,
climate, and hydrology on Si sources (weathering, sewage, and soil
loss) and sinks (uptake by diatoms, sedimentation, and burial) along
the river continuum. Comparison of discharge and dissolved silica
results with observations shows good agreement both in the Rhine and
Yangtze. The simulated total Si export for the Rhine is stable during
the period 1900–2000. The total Si export for the Yangtze decreased
(155–51 Gmol yr–1) because of damming and
transformation of 40% of the natural vegetation to cropland. As a
result of dam construction in the Yangtze, diatom primary production
(from 24 to 48 Gmol yr–1) and burial (15 to 32 Gmol
yr–1) increased and the DSi export decreased (139–46
Gmol yr–1) from the 1950s to 1990s. The Three Gorges
Reservoir has a large contribution to diatom primary production (11%)
and burial (12%) in the Yangtze basin. DISC-SILICON reproduces a flooding-induced
increase in Si inputs and burial and the legacy of this temporary
storage in subsequent dry years.