Sedimentary phosphate fractions related to calcite precipitation in an eutrophic hardwater lake (Lake Alserio, northern Italy)

P-fractional composition has been studied in a 64-cm long sediment core collected in an eutrophic hardwater lake (Lake Alserio, northern Italy) in an attempt to identify the main mechanisms (chemical or biologic) controlling CaCO3 precipitation in the water column. The results of the sedimentary phosphate fractionation showed that the most important P fraction was an organic fraction extracted with hot NaOH: org-Palkali (26% of Psed). A digestion of the supernatant of the P-fraction bound to CaCO3 allowed the detection of a large pool of org-P (19% of Psed). Although the nature of this fraction is unclear, we suggest that it may be involved in the biologically mediated precipitation of CaCO3. The high CaCO3 concentration in the entire vertical sediment profile, as well as the presence of inorg- and org-P bound to Ca in both laminated and non-laminated zones, suggest that the CaCO3 precipitation is not a recent process but is a process that has been occurring in the lake for some time. Accordingly, the change from a homogeneous sediment to a laminated sequence might be caused by the eutrophication process of Lake Alserio occurring since the 1960s, which has allowed the preservation of the varved sediment as a consequence of the drastic reduction in bioturbation.