Environmental context. Eutrophication can lead to the production of harmful algal blooms and is one of the world’s most serious water quality issues. Phosphorus is potentially the limiting macro-nutrient in fresh, estuarine and some marine waters. Consequently, it plays a crucial role in determining the ecological status of many aquatic ecosystems. Considerable effort has been invested in monitoring dissolved reactive phosphorus and total phosphorus in the water column, but less is known about the speciation of phosphorus, particularly in the sediment. This compartment is an important and dynamic reservoir of phosphorus and a potential long-term source of phosphorus release to the water column by the sediment–water interface. This paper investigates the solid-phase speciation and reorganisation of phosphorus within the sediments of a shallow lake system in south-east Australia (the Gippsland Lakes) which suffers from recurring harmful algae blooms. Various strategies are proposed to determine the minimum realistic timescale required to deplete the sediment of labile and reactive phosphorus species.

Abstract. Solid-phase phosphorus pools in the sediments of two shallow lakes (Wellington and Victoria) in the Gippsland Lakes coastal lagoon system of south-east Australia are discussed. Cores (20-cm depth) were taken in summer and winter in both lakes and a sequential extraction scheme (SEDEX) was used to profile the exchangeable P (P_ex), iron oxide/hydroxide bound P (P_Fe), authigenic P (P_auth), detrital P (P_det) and organic P (P_org). Pore-water (P_pw) dissolved reactive phosphorus concentration profiles were also measured. The dominant forms of P were P_Fe (up to 53%) and P_org (35–55%), with the P_Fe fraction playing a key role in the short-term retention of P in the sediment. Benthic phosphorus fluxes at the sediment–water interface (μmol m^–2 d^–1) were determined from the sequential extraction data. The results were compared with flux measurements from the complementary approaches of benthic chamber experiments and Fickian diffusion calculations, to allow an insight into the nature and seasonal variations of the fluxes. The burial flux of phosphorus was also estimated from excess ²¹⁰Pb profiles in the sediment of the lakes. All of these data were used to produce a phosphorus budget for the Gippsland Lakes which suggested that the sediment represents a substantial source of phosphorus within the lakes and thus clearly highlights the importance of the sedimentary compartment in shallow eutrophic ecosystems. Minimum realistic timescales for complete labile phosphorus depletion from the sediment (assuming no resupply from the sediment–water interface) were calculated and ranged from 8 to 22 years.