Effects of phosphorus control on primary productivity and deep-water oxygenation: insights from Lake Lugano (Switzerland and Italy)

Lepori, Fabio; Capelli, Camilla

doi:10.1007/s10750-020-04467-9

Cited by 10 publications

(9 citation statements)

References 51 publications

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“…Similar effects have been described for the deep subalpine lakes Garda (Salmaso et al, 2018) and Lugano (Lepori et al, 2018) as well for other deep lakes North of the Alps (Yankova et al, 2017;Schwefel et al, 2019). These results overall are extremely important for water quality objectives: restoration targets should be revised taking into account the influence of climate-related drivers such as mixing depth and stability (Lepori and Capelli, 2021).…”

Section: External and Internal Drivers Of The Long-term Evolution Of Nutrients Concentrationssupporting

confidence: 75%

Temporal changes in nutrients in a deep oligomictic lake: the role of external loads <em>versus</em> internal processes

Rogora

Austoni²,

Caroni

et al. 2021

J Limnol

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The impact of climate change on stratification and mixing patterns has important effects on nutrient availability and plankton dynamics in deep lakes. We demonstrate this in a long-term study of Lake Maggiore, a deep oligomictic lake located in the subalpine lake district in Northern Italy. Studies on physical, chemical and biological features of the lake have been performed continuously since the 1980s. The lake recovered from eutrophication in response to a reduction of catchment nutrient loads and reached a stable oligotrophic status by the end of the 1990s, with average total phosphorus concentrations in the water column around 10 µg L-1. However, both reactive and total phosphorus have slightly increased since 2010, leading to a shift in the lake trophic state towards mesotrophy. The increase in phosphorus has been limited to the hypolimnetic layers, concentrations being fairly stable or decreasing in the epilimnion. Reactive silica also progressively increased in the hypolimnion, while nitrate and total nitrogen concentrations have steadily decreased in both deep and surface layers, especially in the summer period. These changes were assessed in relation to catchment loads, atmospheric deposition and climate-related variations in stratification and mixing patterns and in nutrient retention. Long-term changes in primary production, represented by chlorophyll levels, and biovolume of the main algal groups were also considered. During the eutrophication period and until the 1990s, in-lake phosphorus concentrations were tightly related to external loads; successively, phosphorus and its vertical distribution up the water column became more controlled by internal processes, in particular by stratification and mixing regime. An increase of thermal stability and a reduced frequency and intensity of deep mixing events has fostered oxygen depletion and phosphorus and silica accumulation in the hypolimnion. Another consequence of reduced deep mixing events, has been a reduction in nutrient replenishment of the upper layers at spring mixing. External loads are still the main driver of change for nitrogen compounds: the decrease in the atmospheric load of nitrogen that occurred in the Lake Maggiore area over the last decade, as an effect of reduced nitrogen emissions, has caused decreasing concentration of inorganic nitrogen in the lake. However, the phytoplankton community changes observed might also play a role in nitrogen dynamics, particularly in the nitrate minima observed during summer in recent years.

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Section: External and Internal Drivers Of The Long-term Evolution Of Nutrients Concentrationssupporting

confidence: 75%

Temporal changes in nutrients in a deep oligomictic lake: the role of external loads <em>versus</em> internal processes

Rogora

Austoni²,

Caroni

et al. 2021

J Limnol

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“…We addressed these questions by analysing time series of monitoring data (question 1) and using a predictive modelling approach (questions 2 and 3). The time-series analysis summarizes and updates previously-published work (Lepori, 2019a;Lepori and Capelli, 2021). The predictive modelling is new.…”

Section: Articlementioning

confidence: 73%

“…An explanation for the model tendency to overestimate the observed values in this study is that the initial oxygen concentration (HO LW ) in Lake Lugano probably differed from those of the calibration set [Müller et al (2019) mention 10 mg L -1 as a typical HO LW value, whereas we observed average values of 5-6 mg L -1 ]. This difference would bias the predictions because, in deep Alpine lakes, AHOD tends to be positively associated with HO LW (higher HO LW results in faster hypolimnetic oxygen depletion; Rogora et al, 2018, Lepori andCapelli, 2021). Therefore, in Lake Lugano, lower HO LW values could have caused lower AHOD values relative to lakes in the calibration set with similar phosphorus concentrations.…”

Section: Discussionmentioning

confidence: 99%

“…We included data from the period 1984-2021, for which systematic monitoring data are available for both stations. The time series of phosphorus, chlorophyll a, primary production, and areal hypolimnetic oxygen depletion were presented in specific papers, which provide detailed method information (Lepori, 2019a;Lepori and Capelli, 2021). In short, the average yearly total phosphorus concentration in the lake (P, in mg m -3 ) was calculated from samples collected monthly at different depths throughout the water column.…”

Section: Data Sourcesmentioning

confidence: 99%

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

et al. 2022

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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.

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“…Phosphorus reduction can also reduce the frequency and severity of harmful algal blooms and sustain the habitat for fish and other species that are sensitive to hypoxia. Various mitigation measures have successfully reduced P loads, , but the effects of P reduction on primary production tend to show little change in deep-water O 2 consumption during the early phases of recovery from eutrophication. − Changes in primary production and subsequent hypolimnetic O 2 consumption may become evident only at TP concentrations significantly lower than those previously expected . Oxidation of sediment layers and their reduced substances may limit improvements in hypolimnetic O 2 concentrations. , Furthermore, primary production in lakes varies seasonally and interannually, and the detection of trends in the rates of production typically depends on the long-term monitoring data.…”

Section: Introductionmentioning

confidence: 99%

Increasing Carbon-to-Phosphorus Ratio (C:P) from Seston as a Prime Indicator for the Initiation of Lake Reoligotrophication

Müller

Steinsberger

Stöckli³

et al. 2021

Environ. Sci. Technol.

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Decline in total phosphorus (TP) during lake reoligotrophication does not apparently immediately influence carbon assimilation or deep-water oxygen levels. Traditional monitoring and interpretation do not typically consider the amount of organic carbon exported from the productive zone into the hypolimnion as a measure of net ecosystem production. This research investigated the carbon-to-phosphorus ratios of suspended particles in the epilimnion, (C:P) epi , as indicators of changing productivity. We report sestonic C:P ratios, phytoplankton biomass, and hypolimnetic oxygen depletion rates in Lake Hallwil, a lake whose recovery from eutrophic conditions has been documented in 35 years of historic water-monitoring data. This study also interpreted long-term (C:P) epi ratios from reoligotrophication occurring in four other lakes. Lake Hallwil exhibited three distinct phases. (i) The (C:P) epi ratio remained low when TP concentrations did not limit production. (ii) (C:P) epi increased steadily when phytoplankton began optimizing the declining P and biomass remained stable. (iii) Below a critical TP threshold of ∼15 to ∼20 mg P m −3 , (C:P) epi remained high and the biomass eventually declined. This analysis showed that the (C:P) epi ratio indicates the reduction of productivity prior to classic indicators such as deep-water oxygen depletion.

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Effects of phosphorus control on primary productivity and deep-water oxygenation: insights from Lake Lugano (Switzerland and Italy)

Cited by 10 publications

References 51 publications

Temporal changes in nutrients in a deep oligomictic lake: the role of external loads <em>versus</em> internal processes

Temporal changes in nutrients in a deep oligomictic lake: the role of external loads <em>versus</em> internal processes

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

Increasing Carbon-to-Phosphorus Ratio (C:P) from Seston as a Prime Indicator for the Initiation of Lake Reoligotrophication

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