Limnetica 31

Temporal and spatial variations in the nutrient concentrations of lake water and surface sediments and in settling and resuspension rates were assessed in a Mediterranean shallow lake (Medina Lake, southern Spain) using a combination of shortterm and long-time monitoring. Our results confirmed the high temporal (inter- and intra-annual) variability characterising Mediterranean shallow lakes, which is also enhanced by water level fluctuations. The results also underlined the crucial role of phosphorus (P) exchange across the sediment-water interface in controlling P dynamics in lake water. This statement is supported by (i) the existence in the upper 1 cm of the sediment of 18 times the mass of TP of the whole water column and, hence, the high potential impact of P released from the sediment into the overlying column and by (ii) the strong P limitation of planktonic primary production, as reflected by a DIN:TP atomic ratio much higher than 16. Resuspension, co-precipitation with CaCO₃ and adsorption onto iron hydroxides (FeOOH) all had major effects on the P exchange across the sediment-water interface. Wind-induced resuspension (31 ± 13 % of the settled matter) explained the extremely high gross sedimentation rates (40 ± 11 g m^–2 d^–1) recorded for a 24 h period. P adsorption onto FeOOH controlled internal P loading during the winter (FeOOH: P_mobile > 15). During the summer, the low availability of FeOOH (Fe:P_mobile < 15) reflected the inability of FeOOH to control P adsorption. The SRP concentrations in lake water were much higher than necessary for CaCO₃ and P co-precipitation, explaining the high contribution of P bound to CaCO₃ (P_HCl) to the total P in the sediment of the study site and demonstrating the importance of CaCO₃ precipitation for removing P from lake water.