Worldwide anthropogenic activity has hindered the water quality of lakes and reservoirs in the last decades. Highly fertilize land fields leached nutrients (nitrogen and phosphorus) to the water bodies leading to eutrophication and consequently to a bad ecological status. In lakes systems, eutrophication is limited by phosphorus (P) concentration in the water column. Azorean lakes Furnas and Sete-Cidades are examples of eutrophic lakes as consequence of external inputs of nutrients resulting also in P accumulation in the sediments for many years. With the objective of achieving a good water quality and ecological status until 2015, it is essential to implement restoration measures. The reduction of external inputs of nutrients (especially P) is the primary concern in several lakes and reservoirs. But, results from several researchers showed that even reducing external P load, the sediments would continue to release P disabling the recovery process. Thus, the mobility of P in the sediment turned to be one of the most important factors to delineate recovery processes of the lakes’ water quality. In this context, the present work has the objectives of exposing several theories for P internal load and of understanding the factors that contribute to P mobility in a geochemical perspective, leading to suggestions for P internal input reduction in a management point of view. Sediments samples from three Azorean lakes (Fogo, Furnas and Sete-Cidades) were collected and analyze through a P sequential extraction procedure (PSE) to understand how P was geochemically bound. For lake Fogo (an oligotrophic lake), results showed that the P concentration values increase from the most labile fraction to the most stable fraction (refractory-P). Based on this trend, lake Fogo will not have significant internal P input (concomitant with its present trophic status), unless these P fractions change in the future. In Lake Furnas (eutrophic status) the major P quantity is adsorbed to Al and Fe oxide/hydroxide fractions, with no changes in several years, which can mean that the adsorption capacity might be already saturated for this fraction, leading to P diffusion through the sediment-water interface (SWI). The lake Sete-Cidades (meso-eutrophic status) seems to have the same trend with higher ~P concentration found in the metallic oxide/hydroxide. The phosphorus maximum solubilization potential (P-MSP) was calculated for the three lakes and the higher value were found in Lake Furnas, which indicates a high potential of P release, turning this lake into the most endangered ecosystem of the three studied lakes. With the objective of understanding the effect of O2 concentration with simultaneous change in temperature, sediments from Furnas Lake were placed in a reactor, with temperature and oxygen concentration control. With the aid of microsensors, pH, O2 and H2S where monitored in sediment depth. Results showed that variations in O2 concentration can force changes in pH which will change the equilibrium of P adsorption to metallic minerals. We think that the pH shift is due to microbial activity and adaptation to the aerobic/anoxic conditions, and that this shift can influence P release that is adsorbed to Al minerals. As in situ measuring of changes that happen in the several P-fractions is extremely difficult, we decided to use mathematical models to predict changes in P mobility. The AQUASIM platform was used to model OM mineralization, acid/base equilibrium, precipitation equilibrium as well as P adsorption equilibrium in the sediments. According to the model, the release of P from the dissolution of FeOOH-P complex plus the P released from OM mineralization in anoxic conditions is lower than P release from OM mineralization in aerobic conditions. In addition, during anoxic period Fe(II) is produced in major quantities which leads to phosphate removal as it precipitates in the form of Fe3(PO4)2. Moreover, sulfate reduction activity by bacteria produces hydrogen sulfide (H2S) that react with Fe(II) precipitating as FeS. This process can permanently remove Fe from the cycle in lower anoxic layers leaving less Fe available to remove P from the pore water as Fe3(PO4)2 or as FeOOH-P when condition at SWI will turn aerobic. With this entire picture in mind, we can understand that if sediments will act as a sink or not depends largely on the P adsorption capacity of its natural mineral constitution and the time that those mineral remain unsaturated. The presence of H2S in sediment can reduce the availability of Fe(II) and Fe(III) minerals and consequently reduce de retention capacity of sediments.