RIBEIRO, D.C., MARTINS, G., NOGUEIRA, R., CRUZ, J.V., BRITO, A.G. (2008) – Phosphorus fractionation in volcanic lake sediments (Azores – Portugal). Chemosphere 70: 1256-1263.

​The phosphorus distribution in volcanic sediments of three lakes that are under different anthropogenic pressures in São Miguel island (Azores – Portugal) was evaluated using a sequential extraction scheme. The P-fractionation scheme employs sequential extractions of sediment with NH₄Cl, bicarbonate-dithionite (BD), NaOH (at room temperature), HCl and NaOH (at 85 °C) to obtain five P-fractions. The P-fractionation shows that in lakes with higher trophic status (Lake Furnas and Lake Sete Cidades), the NaOH extracted P is the dominant fraction, that contribute with more than 50% to total sedimentary phosphorus. The rank order of P-fractionation for these two lakes was NaOH>NaOH (85°C)>HCl>BD>NH₄Cl for Furnas lake and NaOH>HCl>NaOH (85°C)>NH₄Cl>BD for Sete Cidades lake. On the other hand, the trend of P contribution in the oligotrophic lake Fogo shows that the most inert P pools have the higher concentrations, with more than 50% of the P contribution from the last extraction step with NaOH at 85 °C. For this lake, the rank order of P-fractionation was NH₄Cl > BD > NaOH > HCl > NaOH (85 °C). The Phosphorus Maximum Solubilization Potential (P-MSP) was also calculated and the results show that for the more bio-available P-fractions (first and second extraction step), the P-MSP values for Furnas and Sete Cidades lakes are sensibly higher than the results obtained in Fogo lake, an indication of the non-point diffuse load discharged in the first ones.