Biological Processes in the Soil and Floodwater - The Biogeochemistry of Submerged Soils - page 152

of CO 2 and O 2 are inversely related, the pH and O 2 concentration in the flood-
water tend to be positively correlated. Diurnal variations of floodwater CO 2 and
pH are discussed in Section 3.3.
Inorganic Nutrients
The supply of nutrients in the floodwater depends on the composition of the
water arriving in the field and on the nutrient status and transport properties
of the underlying soil. As discussed above, P is most often the most limiting
nutrient in natural wetlands and also in ricefields (Roger, 1996). Concentrations
in irrigation and flood waters are generally small ( ≤ 10 µ M ) and diffusion from
the soil into the floodwater is slow, particularly if P is adsorbed to a large extent
on iron oxides in the oxic floodwater-soil interface. The effect of turbation by
animals burrowing into the soil may therefore be important. The calculations in
Chapter 2 show that the flux of P into the floodwater increases several fold with
realistic tubificid populations and soil parameters.
5.2.4 FLOODWATER FLORA
The photosynthetic aquatic biomass comprises cyanobacteria (formerly called
blue-green algae), planktonic, filamentous and macrophytic algae, and vascu-
lar macrophytes. The net productivity of the floodwater depends on the level
of primary production by the photosynthetic biomass versus its consumption
by grazing animals, particularly cladocerans, copepods, ostracods, insect larvae
and molluscs. Their role will change as the canopy develops and at a leaf area
index of about 6-7 there will be no more photosynthetically active radiation
available to them.
The cyanobacteria and algae are confined to the water and upper few mm of
soil, whereas the macrophytes may be either floating—for example, in ricefields
the water fern Azolla -or rooted in the soil to depth. Planktonic and filamentous
algae move up and down in the water column over the day as their buoyancy
changes with photosynthetic O 2 production. Table 5.1 compares the compositions
of cyanobacteria and aquatic macrophytes obtained from ricefields. The data show
that cyanobacteria have lower dry matter contents and higher N contents than
the macrophytes. Aquatic macrophytes notably have much smaller dry matter
contents than terrestrial plants and greater ash contents as % of dry matter. The
P contents of the cyanobacteria and aquatic macrophytes in Table 5.1 are in both
cases smaller than critical values for deficiency (about 1%), which indicates that
in these ricefields P was the limiting nutrient.
Information on the productivity of the floodwater is scarce. Data compiled for
ricefields by Roger (1996) give an average of 0.35 t dry wt ha − 1 of cyanobacteria,