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

Sediment and water column phosphorus fractions were recorded monthly for one year (April 2004–April 2005) in a shallow lake recovering from nutrient pollution (Loch Leven, Scotland). Equilibrium phosphate concentration (EPC0) and gross sediment phosphorus (P) release rates were estimated from laboratory experiments. Pore water and organic P pools were lowest during warm water periods whereas bottom water P was lowest during cold water periods. Reductant-soluble, organic, metal oxide-adsorbed, residual and sediment total phosphorus pools all varied significantly with overlying water depth. Short-term, high magnitude, redox initiated P release events occurred in late summer and winter as a result of anoxic sediment conditions. Lower magnitude long-term release conditions were maintained for most of the year, most likely as a result of organic P cycling and maintenance of high concentration gradients between the pore and bottom water P pools. Estimates of summer P uptake/release rates, across an intact sediment-water interface, suggested that maximum gross internal release was ~12 mg SRP m⁻² lake surface area d⁻¹ with EPC0 values ranging between 180 and 270 μg P L⁻¹. This study highlights the biological mediation of internal loading in shallow eutrophic lakes, and in particular, the role of sediment algae in decreasing, and sediment bacteria in enhancing, sediment P release.