Phosphorus and Iron Dynamics in Lake Sediments. Implications for nutrient loading of aquatic ecosystems

Many eutrophic lakes do not show significant improvement in water quality upon substantial reduction of the external nutrient supply. Internal loading (recycling) of phosphorus (P) from lake sediments is one of the causes for the delayed recovery. In the present study, solid-phase and pore water geochemistry were characterized in sediment cores collected in Lake 227 of the Experimental Lakes Areas (ELA), Ontario, Canada. This lake has been artificially fertilized through yearly P additions since 1969. Particular attention is given to the speciation of P, iron (Fe) and sulfur (S) in the sediments. The results show that in the highly eutrophic Lake 227 organically-bound P plus humic-metal-phosphate complexes represent the major pool of reactive sediment P. Pyrite formation in the sediments of the anoxic hypolimnion reduce the retention of P by iron minerals, while Fe(III) oxyhydroxides scavenge upward diffusing pore water phosphate in sediments deposited in the oxygenated epilimnion. Enzymatic hydrolysis experiments using phytase, alkaline phosphomonoesterase and phosphodiesterase reveal the presence of labile organic P in the surface sediments. Thus, the sediments have accumulated a large reservoir of reactive P that may continue to contribute to long-term internal P loading even when external P additions are halted.