Eawag Überlandstrasse 133 8600 Dübendorf Schweiz Telefon +41 (0)58 765 55 11 Telefax +41 (0)58 765 50 28 www.eawag.ch Swiss lake temperature monitoring program Eawag: Swiss Federal Institute of Aquatic Science and Technology Commissioned by the Federal Office for the Environment (FOEN) Kastanienbaum, February 2019 Eawag: Das Wasserforschungsinstitut des ETH-Bereichs Swiss lake temperature monitoring program IMPRINT Commissioned by Federal Office for the Environment (FOEN), Hydrology Division, CH-3003 Bern. The FOEN is an agency of the Federal Department of the Environment, Transport, Energy and Communications (DETEC). Contractor Eawag Authors Damien Bouffard, Josquin Dami, Martin Schmid FOEN support Thilo Herold Suggested citation Bouffard D, Dami J, Schmid M (2019). Swiss lake temperature monitoring program. Report commissioned by the Federal Office for the Environment (FOEN), Eawag, Kastanienbaum. Note This study/report was prepared under contract to the Federal Office for the Environment (FOEN). The contractor bears sole responsibility for the content. Front cover figure Map of all lakes of Switzerland and Liechtenstein with an area larger than 0.5 ha. Data provided by Swisstopo (2016) Vector200, Bundesamt für Landestopographie (Art.30 Geo IV): 5704 000 000, reproduced by permission of swisstopo / JA100119. 2/54 Swiss lake temperature monitoring program EXECUTIVE SUMMARY Climate change and other anthropogenic factors affect lakes in many ways that can lead to important effects on freshwater ecosystems. For this reason, the Swiss Federal Office for the Environment (FOEN) is currently evaluating options to add a nation-wide lake temperature monitoring program to their monitoring activities, which already includes a network of river temperature monitoring. In this work, we discuss the benefits and limitations of temperature data obtained by a monitoring program depending on the spatial and temporal resolution of the measurements. We conclude that for accurately observing trends in the thermal structure of lakes caused by climate change, a high time resolution is required that cannot be provided by traditional temperature profiling. We therefore recommend installing moorings where temperature can be continuously monitored. Individual lakes react differently to climate change according to internal and external parameters, such as their size, trophic state, or altitude. These different reactions should be considered when selecting lakes to be included in the monitoring network. We therefore discuss the factors that influence a lake’s reaction to climate change, and present four different scenarios for a monitoring network. The first scenario includes the modelling of lakes that are currently monitored, without adding new monitoring sites. The second scenario represents the installation of moorings in all lakes above a certain size. The third scenario is the monitoring of lakes at different altitudes. Scenario 4 is a combination of Scenario 2 and Scenario 3 with the addition of other lakes with various characteristics to broaden the perspective. The advantages and disadvantages of the four scenarios are assessed. Finally, we discuss the opportunity of measuring other parameters (e.g., meteorological forcing, chemical parameters…) that would increase the coherence and impacts of the monitoring program. We also shortly discuss options for data management and data quality control. 3/54 Swiss lake temperature monitoring program TABLE OF CONTENTS 1 Introduction .................................................................................................................................... 6 2 Motivations and goals for a lake monitoring program ................................................................... 6 2.1 Motivations for a lake temperature monitoring program in Switzerland .............................. 6 2.2 Aims of the monitoring program ............................................................................................ 7 2.3 Observed impacts of climate change ...................................................................................... 7 2.4 Parameters characterizing the thermal structure ................................................................ 10 3 Overview of existing monitoring systems ..................................................................................... 15 3.1 In Switzerland ........................................................................................................................ 15 3.2 In other countries.................................................................................................................. 15 3.3 Concluding remarks .............................................................................................................. 17 4 Spatial & temporal variability – Analysis, description and results ................................................ 18 4.1 Methodology ......................................................................................................................... 18 4.2 Spatial resolution .................................................................................................................. 19 4.3 Temporal resolution .............................................................................................................. 22 4.4 General conclusions and recommendations ........................................................................ 27 5 Monitoring network ...................................................................................................................... 28 5.1 Selection of measurement location within a lake ................................................................ 28 5.2 Lakes in Switzerland .............................................................................................................. 28 5.3 Criteria for selecting monitoring sites................................................................................... 29 5.4 Choice of lakes for a monitoring network ............................................................................ 30 6 Scenarios for a lake monitoring network ...................................................................................... 31 6.1 Introduction .......................................................................................................................... 31 6.2 First scenario - Status quo ..................................................................................................... 31 6.3 Second scenario - Largest lakes ............................................................................................ 31 6.4 Third scenario - Altitude gradient ......................................................................................... 32 6.5 Fourth scenario - High variability .......................................................................................... 33 6.6 Final remarks ......................................................................................................................... 33 7 Combinations with other measurements and methods ............................................................... 35 7.1 Meteorological forcing .......................................................................................................... 35 7.2 Remote sensing and modelling ............................................................................................. 35 7.3 Biochemical parameters and water chemistry ..................................................................... 36 8 Data management ........................................................................................................................ 37 8.1 Open access data platform ................................................................................................... 37 8.2 Data quality control .............................................................................................................. 37 4/54 Swiss lake temperature monitoring program 9 Conclusion ..................................................................................................................................... 38 10 Appendix ................................................................................................................................... 39 10.1 List of lakes ............................................................................................................................ 39 10.2 Detailed Methodology of the analysis .................................................................................. 42 10.3 Temporal variability – other results ...................................................................................... 44 10.4 Survey presentation .............................................................................................................. 44 10.5 Mixing regime dependency to altitude and average depth ................................................. 49 References ............................................................................................................................................ 50 5/54 Swiss lake temperature monitoring program 1 INTRODUCTION The Swiss Federal Office for the Environment (FOEN) is currently evaluating options to add a nation- wide lake temperature monitoring program to their monitoring activities, which already includes a network of river temperature monitoring. The objective of this report is to provide an assessment about the following questions from the perspective of a research institute: • Why do