Phosphorus dynamics in a small eutrophic Italian lake.

Phosphorous dynamics within Lake Sirio (NW Italy) were investigated, considering both water and sediments. The total phosphorus (TP) concentration in the water is about 79 ug•l-1 after the winter mixing, that is in homogeneous conditions; then TP content increases up to 360 ug•l-1 in late autumn in the deep hypolimnium (30–45 m). This deep lake portion accounts for only 1/12 of the water volume. Sediment sampled at depths of 20 and 33 m contains less than 2,000 mg•kg-1 of TP, whereas cores from the deepest sediments (46 m) display TP values of 2,000–4,000 mg•kg-1 at the water-sediment interface, increasing with depth to 16,000 mg•kg-1 at about 60–100 cm. In these deep sediments the main chemical form is the Al-Fe-Mn bound P (about 90% in the high TP cores) and Fe and Mn are also highly enriched (3 and 9 times more than in the shallow sediments respectively). The P-Fe association is confirmed by SEM-EDS and XRD analyses. The vertical distribution of the P content in the water column is consistent with its release from sediments, but in this hypothesis an unrealistic P release rate from 8.1 g m-2y-1 to 3.0 g m-2y-1 was estimated. A more complex model is therefore proposed, involving a process of P concentration in the sediments of the central (deepest) part of the lake, and a short term sediment-water exchange. The TP vertical variability and speciation in the cores suggests a change in the sediment retention capacity, connected to the lake shift to more eutrophic conditions.