『Abstract
This study investigates the important role of Fe-oxide/hydroxide-rich
bottom sediments, from two Brazilian tropical reservoirs, on P
sequestration from the overlying waters, which decreases the risk
of eutrophication. Data on P fractionation indicates that P adsorption
capacity of sediments is predominantly associated with Fe, followed
by orthophosphate adsorbed onto Al hydr(oxides) and onto different
forms of apatite (Ca-P). The soluble and loosely bound P-forms
are low. To have a better understanding of the role of Fe oxides
on the adsorption and sequestration of P in the reservoirs, data
obtained by sequential extraction of P have been related to concentrations
of total Fe and of various Fe-fractions extracted through an optimized
fractionation scheme. These analyses indicated high levels of
Fe bound to oxides/hydroxides. These mineral oxides, and the preponderance
of pH-dependent charged clay minerals (kaolinite) in the clay
fraction, induce the sequestration of the P entering the lakes,
mainly in particulate form, by strong retention of PO43-
through surface adsorption and precipitation phenomena. Phosphorus-retention
provided by the replacement of OH- groups on the basal
plane of the minerals, for P anions, decreases the negative impact
of P inputs in these water systems. There is significant correlation
between the concentrations of soluble reactive P in the water
(SRP) and major chemical components of sediments (SiO2,
Fe2O3, Al2O3, K2O, P2O5), which are related to the dominant bedrock
geology. The interaction of SRP with bed-sediments and the chemical
conditions of the environment (decrease of redox potential with
depth and, in the dry period, a low O2 abundance),
can explain fluctionations of SRP and TP (total P) in the water
column. Due to the dynamic equilibrium in the sediment-water interface,
in the dry season, reducing conditions in hypoliminia enhance
the reduction of Fe(III) to the more soluble Fe(II) with subsequent
release of the strongly retained P. This mechanism increases the
soluble P fraction in sediments and the soluble reactive P in
the water column, in this period. The straight linkage between
geochemical properties of sediments and P concentration in the
water column reported in this article suggests that study of the
mineralogical and geochemical composition of bottom sediments
could help to improve model predictions of P concentrations in
surface waters.』
1. Introduction
2. Description of the study areas
3. Materials and methods
3.1. Water analysis
3.2. Sediment analysis
3.2.1. Major elements
3.2.2. Grain-size analysis
3.2.3. Characterization of clay minerals
3.2.4. Organic carbon
3.2.5. pH
3.2.6. Soluble and organic/inorganic P-form
3.2.7. Inorganic-P fractionation
3.2.8. Fe-fractionation
3.3. Data analysis
4. Results and discussion
4.1. Textural and mineralogical characteristics of the sediments
4.2. Chemical behavior of the water and sediments
4.2.1. Water
4.2.2. Sediments
4.3. Dynamics of phosphorus
4.3.1. Phosphorus in the water
4.3.2. Phosphorus in the sediments
4.3.3. Dynamics of phosphorus in the interface sediment-water
5. Conclusions
Acknowledgements
References