Department of Physics, Chemistry and Biology (IFM)

Eutrophication is one of the main cause of pollution in aquatic systems. Phosphorus (P) is a major element that plays a key role in eutrophication. This is due to the fact that P is a limiting nutrient for plant production in freshwater systems. Sources of P in the waterbodies are mainly characterized under external and internal loadings. The external loadings include run-offs from agricultural farmlands, treatment plants etc, whereas internal loading is a result of release of P from the sediments. Internal loading occurs under conditions like high pH, high temperatures (summer seasons), depletion of dissolved oxygen (DO) etc. (Bostrom, 1988). Studies have shown that external loading have been reduced or managed well during the past years and the major cause of eutrophication in some lakes is the release of P from sediments. (Abrams and Jarell, 1995; Xie et al., 2003). Sediments have been seen to play an essential role in the determination of the concentration, the transport and eventually the fate of Phosphorus in aquatic ecosystems. This is due to its acting as the transport mechanism as well as its accumulation and subsequent release (Søndergaard et al., 1996; Kleeberg et al., 1997). 

Sediment is of interest due to its ability to adsorb to nutrients, pesticides and other pollutants which end up in water bodies (Dorioz and Ferhi, 1994; Svendsen et al., 1995).

Due to different forms of P adsorb on sediments, there is need to assess the forms of P and the potential of its release under certain environmental conditions. Phosphorus fractionation could be used in the assessment This is used because total phosphorus (TP) only gives the total concentration and that in its sense does not give room for predicting potential ecological danger (Psenner et al., 1984). Phosphorus fractionation, which is a sequential extraction of P, has been noted as a useful technique in characterizing various P compounds (Zhou et al., 2001; Psenner et al., 1998; De Groot, 1990; Pardo, 1998). The fractionation scheme is based on differences in reactivity of solid phases to different extractant solutions (Hieltjeset al., 1980). 

2.1 Phosphorus Fractionation:

This is a scheme mainly to ascertain the various fractions or P forms found in the sediments. Psenner et al. (1984) proposed the very first fractionation scheme, which was then modified by subsequent scientist notably is Jensen and Thamdrup, (1993). Different fractions from the scheme were noted to be: labile P or loosely bound P, Redox sensitive Iron (Fe)-bound P, hydrated ions of Al-bound P, calcium (Ca)-bound P, Residual-P which consists mainly of organic P and Apatite-P.   

The principle behind the scheme is based on solubility and reaction with chemicals.

The scheme is designed to extract loosely bound P and sequentially to the very tightly bound P (Ruttenberg, 1992).  

Orthophosphate anion (HPO₄^2-) is the main P dissolved ion, which could be adsorbed to sediments as well as been dissolved in water. Orthophosphate (PO₄-P) is found to be bound to Fe and Al in hydrated oxides, also the mineral lattices with a specific ligand is seen to be a suitable surface with which this bond can occur. Water or hydroxide in this chemical reaction is replaced by PO₄-P anion (Hingston et al. 1972, 1974). Fine clay particles are known to have high content of Al and Fe oxides, also with a large surface area for P-binding reactions. This coupled with the slowly settling nature in water column makes it very efficient in source of sediment P. 

Loosely-P, Fe-P and Al-P are known to be the Biovailable P (BAP) since they could be released in the event of depletion of oxygen.

Steinberg and Muenster (1985), showed that the P in organic matter could be part of the organic molecule itself or could be bound to the organic matter via metallic cations like Al³⁺ and Fe³⁺.

 

 

2.2 P release from sediments

Dissolved oxygen (DO) is known to be one of the factors that really influences the release of mobile P from sediments. P is often released when O₂ is being depleted and P form bound to Fe is mainly the one released (Einsele 1938; Mortimer 1941, 1942). As time went on studies showed that even in non-depleted O₂ conditions P could be released. This was mostly the case in very shallow eutrophic lakes. (Nriagu and Dell 1974; Boström et al. 1982; Hupfer et al. 2004). The increase in saturation of P binding sites in sediments or the degradation of organic materials by microbes were plausible reasons for the release of P in oxic conditions. 

Another instance where P release could be possible in oxic conditions, is when in algal bloom with very high pH surface water mixes with the fluffy surface of sediments of low pH. This elevates the concentration of hydroxyl ion (OH^-) which tends to induce the release of P (Drake and Heaney, 1987). Studies have shown how changes in pH could result from photosynthetic activity in which carbon dioxide (CO₂) is removed by algae or plants. The removal of CO₂ tends to elevate the pH even to a pH of 10 or more (Boers1991, Frodge 1991). The release of P at high pH is attributed to the desorption of P from ferric hydroxide, where P is replaced with hydroxide (Jensen et al., 1992; Boers, 1991; Drake and Henry, 1987).    

P release mostly is the largest source of internal P loading in a stratified lake with anoxic hypolimnion (Nürnberg 1984, 1987). Fluctuations in localized oxygen also contributes to P release (Penn et al., 2000). Studies have revealed the role sulphur plays in P release under anoxic conditions (Carco et al., 1993). At lower redox potentials, sulphates and iron are reduced, Iron sulphide (FeS) precipitates and Iron are removed from anoxic water by sulphides. In the absence of reoxygenation, P that was bound to Iron oxide would then be released (Caraco et al., 1993).    

The county of Östergötland has been recently awarded a grant to restore lakes by removing the upper sediment layer. The study which is in connection with county’s is to estimate the amount of BAP in the P forms elucidated by the fractionation procedure and to ascertain the possibility of P released in the event of oxygen depletion.  

The hypothesis then is,

1. How much of the TP is BAP?
2. Is there a possibility of P release under different levels of DO (Anoxic and Oxic conditions)?