Phosphorus Fractionation in Lake Sediments/fosfori Fraktsioneerimine Jarvesetetes

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INTRODUCTION

Sediment phosphorus has been the subject of a number of studies because of its role in the state and development of lake ecosystems (Dillon & Rigler, 1974; Bostrom et al., 1982; Golterman, 2004). Concentrations of P in lake sediments depend on its concentrations in the lake water, the transport of soluble phosphate between solid components, adsorption.desorption mechanisms, the chemisorption ability of the sediments, and biological uptake (Andersen, 1975; Sondergaard et al., 1992; Koski-Vahala & Hartikainen, 2001; Koski-Vahala et al., 2001). Due to the differences in the origin and genesis of the studied water bodies, as well as a large number of external and internal factors, the biogeochemical cycling of P fractions and, thus, their concentrations in sediments can vary greatly. Therefore, it is necessary to analyse different ecosystem components separately and then identify the general rules governing their dynamics (Punning et al., 2007). A prerequisite for sound environmental field studies on phosphorus is application of reliable analytical methodologies. The lack of uniformity between the different extraction methods does not allow comparison of results or validation of procedures. Indeed, the results do not depend only on the extraction method used. For accurate analyses and especially data interpretation, knowledge of P fractions is required, as environmental behaviour is often critically dependent on the physical and chemical form of P. Nevertheless, extraction methods often yield reproducible results that allow a description of the sediment's P fractions and an estimation of the dominant binding compounds in a given sediment (Pettersson et al., 1988). For the study of the biogeochemical cycling of phosphorus, chemical fractionation methods have been predominantly used. These methods involve a successive addition of different extraction media to sediment samples, each of which is expected to extract a particular fraction of phosphorus. Phosphorus bound to metal oxides, mainly those of Fe and Al, is represented by NaOH-P. The concentration of the NaOH-P fraction can be used for the estimation of both short-term and long-term available P in sediments and it is a measure of available algal P (Zhou et al., 2001). The P fraction that is assumed to consist mainly of apatite P is represented by HCl-P. The most important inorganic P pools seem to be NaOH-P and HCl-P (Golterman, 2004). Most methods of phosphorus determination are based on the reaction of P with an acidified molybdate reagent to yield a phosphomolybdate heteropolyacid, which is then reduced to an intensely-coloured blue compound and determined spectrophotometrically (Murphy & Riley, 1962; McKelvie et al., 1995). There is still debate on the methodological question about the duration of reactions and the concentration of reagents in relation to the concentrations of P extracted. Extractions of NaOH were invalidated by Hieltjes & Lijklema (1980), Golterman (1996), Golterman et al. (1998), and Romero-Gonzalez et al. (2001). All these authors demonstrated that the duration of the extraction and the reagent concentration always influence the quantities of P extracted and they stressed that more investigations are needed.

Due to the effects of the physical-chemical and environmental conditions on the interface of water and sediments, the proportion of P compounds could be modified and P compounds that have already accumulated within sediments could be partly released. As permanent P exchange takes place within this interface, over time the lake sediment can act as an internal source of phosphorus for the overlying water (Lijklema, 1986; Ramm & Scheps, 1997; Zhou et al., 2001). The existence of different P fractions in association with fractions of different particle size is affected by the sedimentation environment.

The objective of this research was to estimate the use of the following factors in the measurement of phosphorus fraction concentrations in lake sediments as a tool for understanding the potential mobility of P from sediments to the overlaying water: (1) reagent concentration, (2) duration of extraction, (3) stability and reproducibility of the concentrations of extracted P, and (4) lithological composition of the sediment. …