Effects of phosphorus control on primary productivity and deep-water oxygenation: insights from Lake Lugano (Switzerland
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PRIMARY RESEARCH PAPER
Effects of phosphorus control on primary productivity and deep-water oxygenation: insights from Lake Lugano (Switzerland and Italy) Fabio Lepori
. Camilla Capelli
Received: 30 April 2020 / Revised: 3 November 2020 / Accepted: 17 November 2020 Ó The Author(s) 2020
Abstract Attempts to restore Lake Lugano, Switzerland and Italy, from eutrophication have produced weak responses in the target variables (primary productivity and hypolimnetic oxygen concentrations), indicating shortcomings in the underlying eutrophication model. An analysis of monitoring data showed that the decrease in phosphorus concentration, although nearly compliant with restoration targets, produced only slight decreases in primary productivity and no change in hypolimnetic oxygen conditions. These target variables were equally or more sensitive to factors external to trophic state, including plankton structure, which influenced primary productivity, and the depth of mixing during turnovers, which influenced hypolimnetic oxygen. To improve the chance of success, the restoration approach should revise the phosphorus concentration target and explicitly account for the influence of external variation, especially mixing depth.
Handling editor: Eric R. Larson.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10750-020-04467-9) contains supplementary material, which is available to authorized users. F. Lepori (&) C. Capelli Institute of Earth Sciences, University of Applied, Sciences and Arts of Southern Switzerland, Campus Trevano, Via Trevano, 6952 Canobbio, Switzerland e-mail: [email protected]
Keywords Eutrophication Path analysis Primary productivity Hypolimnetic oxygen Swiss lakes
Introduction In lakes, cultural eutrophication is the most widespread environmental problem (Smith & Schindler, 2009). Eutrophic lakes have undesirable traits, including algal blooms and the release of harmful gases from the sediments. Because these traits reduce the ability of lakes to provide ecosystem services, eutrophic lakes have long been a priority for ecological restoration (Edmondson, 1970). Aquatic scientists attribute eutrophication to excess phosphorus, which comes from sources like municipal waste and fertilisers (Smith et al., 1999). Consequently, measures to restore eutrophic lakes have focussed on reducing phosphorus inputs (Dillon & Rigler, 1975; Jeppesen et al., 2005; Schindler et al., 2016). A main impetus for the restoration of eutrophic lakes stems from the necessity to mitigate the effects of oxygen depletion on water quality and fish stocks (Vollenweider, 1968). According to lake eutrophication models (Wetzel, 1975; Walker, 1979), oxygen depletion is the end result of a pathway connecting phosphorus, primary productivity and hypolimnetic oxygen consumption (conceptual map in Fig. 1). However, due to lack of data, quantitative assessments
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Hydrobiologia
of these links have been rare and not always supportive
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