Phosphorus release rates from freshwater sediments affected by fish farming

The main wastes from fish production are fish faeces and uneaten feed, rich in nitrogen and phosphorus (P), which have the potential to alter the trophic state of the water body. It has been estimated that about 66% of P in fish feed accumulates on the bottom sediments. The P associated with these sediments can be released to the water column. The objectives of this work were to: (a) assess the release of phosphorus from sediments affected by intensive production of salmonids in two lakes in Patagonia, Argentina: Lago Moreno (PMOR) and Alicura Reservoir (PALI); (b) evaluate the efficiency of anionic exchange membranes (AEMs) for the determination of P release rates from sediments and compare this technique with the traditional chemical method; and (c) assess the usefulness of AEMs in estimating the labile fraction of phosphorus from sediments affected by fish farms. For all the conditions assayed, the release rates estimated with AEMs were higher than those estimated by syringe (n = 23, P = 0.001). The AEMs appear to have higher sensitivity than chemical methods, particularly at low concentrations. The AEMs are more reliable for the estimation of labile P, particularly in sediments low in labile P concentration. The difference between the release rates estimated by the AEMs and the syringe are indicative of the amount of labile P that will be available in the water over the short term. The release rates were higher under anaerobic conditions than under aerobic conditions for the two types of sediments affected by fish farms. The calcium and organic matter content of sediments, rather than the iron concentration, appears to control the availability of P.