It is important for lake management and policy to estimate the timescale of recovery from long-term P release from sediment after a reduction in the external load. To provide a scientific basis for this, a condensed model was elaborated, applied and evaluated in four lakes. The model is based on first order kinetics, with an overall rate constant composed of the rate of diagenesis of labile P (kd,2) and rate of burial of P (kb) below an active sediment layer. Using the variation of P fractions in dated sediment cores, kd,2 varied from 0.0155 to 0.383 yr −1, kb from 0.0184 to 0.073 yr −1 and the overall rate constant from 0.0230 to 0.446 yr −1. The active layer depths, 8 to 29 cm, and kd,2 values are within the ranges found by others. The time for a 75% reduction (t 75) of labile P in the active layer is 60 years in Lough Melvin, 3 in Ramor, 33 in Sheelin and 41 in Neagh, although P release is only important in Ramor and Neagh. Combining the kd,2 values with other estimates (mean 0.0981 yr −1, median 0.0426; n=14) produces a t 75 value of less than 14 and 33 years. A review of other models indicates a timescale of one to two decades and from lake monitoring also of one to two decades. It is desirable to estimate the timescale directly in all lakes if sediment P release is important, but, generally, it should take between one and three decades.
Bibliographical noteFunding Information:
We would like to thank the two funders, the Environmental Protection Agency of Ireland for the work on Lough Melvin, Ramor and Sheelin through the DETECT Project (2015-W-LS-9) and the Department for Agriculture, Environment and Rural Affairs in Northern Ireland for Lough Neagh. We would also like to thank the following for the use of boats and invaluable assistance to retrieve sediment cores from the lakes: Hugh Gillespie and Kevin McCloskey for Lough Melvin, Fergus Lynch for Lough Ramor and Stephen Ryan, Frankie Conlon and Martin Devlin for Lough Neagh. Thanks go to Handong Yang of the Environmental Radiometric Facility at University College London for completing the dating of the sediment cores. We would also like to thank Dr Shane O'Boyle of the Environmental Protection Agency for valuable comments on the manuscript and two anonymous reviewers for comments and suggestions that improved the clarity of the text.