Phosphorus, carbon and nitrogen enrichment during sedimentation in a seasonally anoxic lake (Lake Lugano, Switzerland)

Sedimentation fluxes of major nutrients are investigated during 1996 and 1997 at three different depths and two locations in eutrophic southern basin of Lake Lugano (Switzerland). Horizontal differences between the two sites are on the order of 10-40% (but can exceed 50%), whereas differences related to interannual oscillations range between 5 and 24%. Particulate organic carbon (POC) and nitrogen (PN) fluxes show a constant increase of 5-20% from the upper to the bottom trap. This tendency remains more or less constant during the year. On the contrary, particulate phosphorus (PP) shows a seasonal variation, with higher accumulation rates from the I to the III trap in autumn and winter which can exceed +1200%. This phenomenon is due to the interaction between the dissolved phosphorus (DP) and the iron(oxi)hydroxides (Fe(OH)3) near the oxycline. Fe(OH)3 precipitates at the iron redox boundary, scavenging DP. This enrichment flux increases together with the development of the anoxic benthic layer. The efficiency of the iron redox layer in trapping upward diffusing P is related to the concentration of dissolved iron in the anoxic hypolimnion. In Lake Lugano the two considered sites present major difference of iron concentration, and this difference is reflected in the P sedimentation fluxes. The exposition of an additional sedimentation trap above the maximal oxycline height has allowed to gain insight into this phenomenon.