Postdepositional mobility of phosphorus in lake sediments 1

Fifteen sediment cores from Lake Memphremagog showed pronounced peaks of total phosphorus concentration within the 210 Pb‐defined mixed layer, suggesting that P was migrating upward and accumulating near the mud surface. The mobility of P in the sediments was confirmed by the rapid development (5 weeks) of a marked total P maximum in the upper centimeter of a previously homogenized sediment. Associated with the P migration was a steep gradient of soluble reactive P (SRP) concentration, presumably the result of a redox gradient in the sediments. Similar gradients of total P and SRP were observed in nondisturbed sediments, strongly suggesting that such an upward migration mechanism also operates under natural conditions. The remobilized P seems to be isotopically exchangeable (mobile) under anaerobic conditions. These observations indicate that profiles of sediment total P may not always be useful in determining historical P loading or P sedimentation rates in lakes.