Mesotrophy is not enough: re-assessing phosphorus objectives for the restoration of a deep Alpine lake (Lake Lugano, Switzerland and Italy)

The restoration of eutrophic lakes requires the identification of phosphorus objectives, i.e., the phosphorus reductions needed to achieve desired water quality goals. Due to inherent uncertainty, phosphorus objectives need periodic revision as the restoration progresses. We used monitoring data from a deep southern Alpine lake (Lake Lugano, Switzerland and Italy) to assess restoration progress and revise the current phosphorus objective of 30 mg m–3. Because one basin of the lake is meromictic (North basin) and the other is holomictic (South basin), restoration focussed on the mixolimnion for the North basin and the entire water column for the South basin. Time series analyses indicated that, thanks to restoration, phosphorus concentrations in the lake declined to values compliant with the objective (~20-30 mg m–3). In contrast, little progress was observed towards achieving the main water quality goals (chlorophyll a ≤4 mg m–3, primary production ≤150 g C m–2 year–1 and oxygen concentrations ≥4 mg L–1). Using predictive models, we estimated that achieving these goals requires a phosphorus objective of <10 mg m–3, which would bring the lake back to the original oligotrophic state. The concentration of <10 mg m–3 is lower than the objectives predicted for other (mainly northern) deep Alpine lakes. The apparent sensitivity of Lake Lugano, which we attribute to unfavorable hydrodynamic conditions common in lakes south of the Alps (weak mixing and long stratification), calls for particularly attentive phosphorus management.