Phosphorus fractionation and mobility in Loch Leven sediments

Abstract

ABSTRACT1. This paper reports on the application of a chemical fractionation scheme for assessing the amounts, forms and potential mobility (recycling potential) of phosphorus in the sediments of the freshwater Loch Leven, which is currently highly topical in relation to eutrophication and its manifest dense, potentially toxic, blue-green algae. The findings are placed in the context of the biological importance of the loch, the problems it is experiencing and some management options for its restoration.2. The fractionation scheme adopted enabled quantification of phosphorus in sediment pore water and in sedimentary solid phases classified as 'loosely-bound', 'reductant-soluble', 'oxide-adsorbed', 'organic', 'apatite-bound' and 'residual' in the sections of a 16-cm core collected in January 1990.3. Vertical profiles of reductant-soluble and pore water phosphorus and iron show redox-controlled remobilization and cycling within the sediment to be the principal mechanisms leading to the surface sediment enrichment of phosphorus observed during the winter months.4. The association of phosphorus with iron oxyhydroxides in the reductant-soluble fraction, which accounts for 31% of the total phosphorus (2470 mg kg-I) in the 0-1 cm section compared with 0.5% loosely-bound, 25.6% oxide-adsorbed, 27.9% organic, 9.9% apatite-bound and 5.1 Yo residual, restricts the release of phosphorus to the overlying water column in winter.5 . Extrapolation of the data to the whole loch suggests that in early 1990 more than 40 tonnes of phosphorus were associated with the surfaces of iron oxyhydroxides in the upper 10 cm of Loch Leven sediment, about five times that required to raise loch water concentrations to maxima of 0.15 mg L-I observed during the algal blooms of summer under conditions favourable to the release of phosphorus from the sediments.6. Despite recent measures to reduce external inputs of phosphorus and thereby limit the degree of eutrophication of Loch Leven, there appears to have been little overall change in the inventory of phosphorus associated with iron in the upper 10 cm of sediment since 1970.7. There should be more detailed seasonal and spatial studies of the chemical fractionation and mobility of phosphorus in Loch Leven sediments to investigate the extent to which release from the reductant-soluble iron oxyhydroxide fraction (internal loading) contributes to the highly elevated phosphorus concentrations in loch water during summer.

Sediment phosphorus cycling in a large shallow lake: spatio-temporal variation in phosphorus pools and release

“…However, prolonged internal loading coupled with a low flushing rate (2-3 lake volumes per year) resulted in poor water quality being maintained, with an improvement in water column SRP concentration only being observed in recent years (Carvalho & Kirika, 2003). This improvement in water quality has not been mirrored in the sediment P composition (Farmer et al, 1994;Spears et al, 2006). Field measurements, water chemistry, and sediment P fractionation Monthly sampling was conducted between April 2004 and April 2005 from six sites along a depth transect (2, 2.5, 3.5, 5.5, 10, 22 m overlying water depth), to allow comparison of shallow and deep regions of the loch throughout the year.…”

“…Inorganic nutrient analyses were carried out using the methods of Wetzel and Likens (2000). Sediment P fractionation was carried out using a sequential chemical extraction technique based on Psenner et al (1988) and Farmer et al (1994). Two sub-samples of homogenised slurry, from each site, were subject to the following extraction procedure on each sample date: (1) repeated extraction in 1 M NH 4 Cl for 2 h and quantification of SRP [labile P]; (2) repeated extraction with 0.11 M NaHCO 3 /0.11 M Na 2 S 2 O 4 for 1 h and quantification of SRP [reductantsoluble P] and total soluble phosphorus (TSP) [reductant-soluble soluble un-reactive phosphorus (SURP)]; (3) repeated (for 1 h) extraction in 1 M NaOH for 18 h followed by quantification of SRP (metal-oxide adsorbed P), and TSP; (4) repeated (for 1 h) extraction with 0.5 M HCl for 18 h and quantification of SRP [apatite-bound P]; (5) digestion with 30% (v/v) H 2 SO 4 and 8% K 2 S 2 O 4 at 121°C for 30 min followed by SRP quantification [residual P].…”

Long-term variation and regulation of internal phosphorus loading in Loch Leven

“…that associated with Fe and, therefore, releasable under reducing conditions) in the upper 4 cm of sediment was about 20 t P. Using a water column mass balance approach they also estimated that P cycling between the sediments and the water column amounted to a maximum sediment uptake of 136 kg TP day -1 , a maximum sediment release of 88 kg TP day -1 . Advances in chemical speciation techniques facilitated a more comprehensive assessment of sediment P fractions in 1990, immediately 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 , 1968-2008 after significant P loading reductions (Farmer et al, 1994). The authors found that a large proportion (*30%) of the 68.8 t of TP contained in the upper 4 cm of sediments within the loch was release sensitive.…”

Manganese behavior in the sediments of diverse Scottish freshwater lochs

“…The indirect effect of an algal bloom on the behavior of Mn has also been suggested as a possible cause of Mn nodule formation in freshwater, eutrophic Lake Oneida, New York (Dean and Greeson 1979;Dean et al 1981). Because Fe and P are known to be released from Loch Leven sediments during calm, warm weather conditions (Farmer et al 1994), it might be anticipated that Mn would also be released under such conditions. The greater Mn inventory in the summer core, however, indicates that the reprecipitation and sedimentation of Mn must outweigh any release from the sediment.…”

“…Chl a levels can reach 160 pg liter-' in summer. Summer algal blooms are a subject of concern due to potential harmful effects of toxins released by blue-green algae in this loch, which is famous for trout fishing (Bailey-Watts et al 1994). Water column stratification is relatively uncommon, intermittent, and affects only the deep kettle holes (22-23 m).…”