8 YEARS OF INTERNAL PHOSPHORUS LOADING AND CHANGES IN THE SEDIMENT PHOSPHORUS PROFILE OF LAKE SOBYGAARD, DENMARK

During each of the first 8 years following an 80-90% reduction in external phosphorus loading of shallow, hypertrophic Lake Sobygaard, Denmark in 1982, phosphorus retention was found to be negative. Phosphorus release mainly occurred from April to October, net retention being close to zero during winter. Net internal phosphorus loading was 8 g p m-2 y-1 in 1983 and slowly decreased to 2 g P m-2 y-1 in 1990, mainly because of decreasing sediment phosphorus release during late summer and autumn. The high net release of phosphorus from Lake Sobygaard sediment is attributable to a very high phosphorus concentration and to a high transport rate in the sediment caused by bioturbation and gas ebullition. Sediment phosphorus concentration mainly decreased at a depth of 5 to 20 cm, involving sediment layers down to 23 cm. Maximum sediment phosphorus concentration, which was 11.3 mg P g-1 dw at a depth of 14-16 cm in 1985, decreased to 8.6 mg P g-1 dw at a depth of 16-18 cm in 1991. Phosphorus fractionation revealed that phosphorus release was accompanied by a decrease in NH4Cl-P + NaOH-P and organic phosphorus fractions. HCl-P increased at all sediment depths. The Fe:P ratio in the superficial layer stabilized at approximately 10. Net phosphorus release can be expected to continue for another decade at the present release rate, before an Fe:P ratio of 10 will be reached in the sediment layers from which phosphorus is now being released.