Global Warming Affects Nutrient Upwelling in Deep Lakes Robert
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Manuscript Click here to access/download;Manuscript;Schwefel_Global_Warming_upwelli 1 2 3 4 1 Global warming affects nutrient upwelling in deep lakes 5 6 2 Robert Schwefel1*ǂ, Beat Müller2, Hélène Boisgontier1, and Alfred Wüest1,2 7 8 1 9 3 Physics of Aquatic Systems Laboratory, Margaretha Kamprad Chair, École Polytechnique 10 4 Fédérale de Lausanne (EPFL), Institute of Environmental Engineering, Lausanne, Switzerland. 11 12 5 2Eawag, Swiss Federal Institute of Aquatic Science and Technology, Surface Waters – Research 13 14 6 and Management, Kastanienbaum, Switzerland. 15 16 7 *corresponding author 17 18 8 ǂ Current address: UC Santa Barbara, 3015 Marine Science Building, Santa Barbara, CA 93106- 19 20 9 6150 United States; Phone: +1 805 8952157; E-mail: [email protected] 21 22 10 23 24 11 ORCID: 25 26 27 12 Robert Schwefel: 0000-0003-1610-4181; Beat Müller: 0000-0003-3696-9035; Hélène 28 13 Boisgontier: n/a; Alfred Wüest: 0000-0001-7984-0368 29 30 14 31 32 33 15 ABSTRACT 34 35 16 Measures to reduce lake phosphorus concentrations have been encouragingly successful in many 36 17 parts of the world. After significant eutrophication in the 20th century, nutrient concentrations 37 38 18 have declined in many natural settings. In addition to these direct anthropogenic impacts, 39 19 however, climate change is also altering various processes in lakes. Its effects on lacustrine 40 41 20 nutrient budget remain poorly understood. Here we investigate the total phosphorus (TP) 42 21 concentrations in the epilimnion of the meromictic Lake Zug under present and future climatic 43 22 conditions. Results are compared with those of other deep lakes. Data showed that TP transported 44 45 23 from the hypolimnion by convective winter mixing was the most important source of TP for the 46 24 epilimnion, reaching values more than ten times higher than the external input from the 47 48 25 catchment. We found a logarithmic relationship between winter mixing depth (WMD) and 49 26 epilimnetic TP content in spring. Warming climate affects WMD mainly due to its dependence 50 27 on autumn stratification. Model simulations predict a reduction of average WMD from 78 m 51 52 28 (current) to 65 m in 2085 assuming IPCC scenario A2. Other scenarios show similar but smaller 53 29 changes in the future. In scenario A2, climate change is predicted to reduce epilimnetic TP 54 55 30 concentrations by up to 24 % during warm winters and may consequently introduce significant 56 31 year-to-year variability in primary productivity. 57 58 32 Keywords: Limnology, Climate Change, Winter Mixing, Phosphorus, Lake Zug 59 60 61 This document is the accepted manuscript version of the following article: 62 Schwefel, R., Müller, B., Boisgontier, H., & Wüest, A. (2019). Global warming affects 63 nutrient upwelling in deep lakes. Aquatic Sciences, 81(3), 50 (11 pp.). 64 https://doi.org/10.1007/s00027-019-0637-0 65 1 2 3 4 33 1. INTRODUCTION 5 6 7 34 Excessive primary productivity fuelled by high nutrient input represents a major concern in many 8 9 35 lakes around the world (Vollenweider 1968; Dillon and Rigler 1974; Schindler 2006). Efforts to 10 11 36 limit nutrient input from agriculture and urban wastewater led to substantial input reductions in 12 37 phosphorus (P) and nitrogen concentrations and to reestablishment of oligotrophic nutrient 13 14 38 conditions in many formerly eutrophic lakes (De Pinto et al. 1986; Makarewicz 1993; Finger et 15 16 39 al. 2013; Müller et al. 2014). However, input reduction has not always resulted in a return to 17 18 40 oligotrophic conditions, oxic deepwater, or lower rates of primary production. Overall system 19 20 41 response to such drastic changes on relatively short timescales remain unclear. Biomass 21 22 42 concentrations (Anneville et al. 2005; Jeppesen et al. 2005) and oxygen depletion rates (Müller et 23 43 al. 2012a; Schwefel et al. 2018) do not always react in tandem with nutrient concentrations or 24 25 44 inputs. 26 27 28 45 Recent studies have recognized that rising air temperatures also affect lake ecosystems by 29 30 46 increasing water temperatures (Schmid et al. 2014; O’Reilly et al. 2015), which in turn affect 31 32 47 water column stability as well as nutrient and oxygen budgets of lakes (Sahoo et al. 2013; North 33 48 et al. 2014; Schwefel et al. 2016; Ficker et al. 2017; Salmaso et al. 2018; Lepori et al. 2018; 34 35 49 Rogora et al. 2018). Many studies indicate that climate warming affects eutrophication through 36 37 50 increased plankton growth rates, increased internal P loading (North et al. 2014), higher nutrient 38 39 51 inputs from the catchments (Moss et al. 2011), or shifts in phytoplankton community structure 40 41 52 (Anneville et al. 2002). Few studies, however, have directly queried the relationship between 42 water column structure and hypolimnetic nutrient upwelling into the euphotic zone of lakes 43 53 44 54 during winter mixing. While primary productivity in shallow lakes is mainly controlled by 45 46 55 inflows from the catchment, deep lakes have considerable quantities of nutrients stored in the 47 48 56 deep hypolimnion. These can increase productivity when penetrative convective events or strong 49 50 57 winds allow mixing with the euphotic zone. The deep lakes of the East African rift, Lake Kivu 51 52 58 and Lake Tanganyika, are classic examples of settings where deep mixing is the main driver of 53 59 interannual changes in productivity. Lakes Constance and Garda offer mid-latitude examples of 54 55 60 settings, where nutrient availability depends to a substantial degree on winter mixing depth 56 57 61 (WMD) (Straile et al. 2003; Salmaso et al. 2018). In oligomictic Lake Lugano, Simona (2003) 58 59 62 found that nutrient availability and phytoplankton composition depended on WMD. North et al. 60 61 63 (2014) observed no reduction in P transport from the hypolimnion to the epilimnion of Lake 62 63 64 65 1 2 3 4 64 Zürich, in spite of reduced convective mixing in winter. Yankova et al. (2017) meanwhile 5 6 65 reported decreased upwelling of nutrient-rich deep-water in Lake Zürich with potential impacts 7 8 66 on primary productivity. 9 10 67 This study interprets long-term monitoring data from the deep, meromictic Lake Zug (Fig. 1) for 11 12 68 evidence of diminished deep convective mixing during winter and associated decline in 13 14 69 epilimnetic P, which is considered the limiting nutrient as in many lakes worldwide. A detailed 15 16 70 dataset spanning 35 years allows for quantitative analysis of the dependency of epilimnetic total 17 18 71 phosphorus (TP) concentrations on WMD and in turn variation in WMD with changing 19 72 meteorological conditions (Fig. 1b). A one-dimensional model provides predictions on mixing 20 21 73 behaviour under different IPCC climate scenarios and their impact on Lake Zug’s P budget. 22 23 24 74 The goals of this study were to (1) understand the epilimnetic P budget of Lake Zug and its 25 26 75 dependency on WMD, (2) estimate WMD using climate data, (3) evaluate predicted changes in 27 28 76 winter deep mixing under conditions of a warming climate, and (4) interpret the impact of WMD 29 77 on the TP input to the epilimnion. To address these goals, we analysed monthly temperature, 30 31 78 salinity, and TP profiles at the deepest location of the lake, along with estimates for external P 32 33 79 load, TP outflow, and P net sedimentation (Müller et al. 2012b). Lake Zug provides an ideal 34 35 80 dataset for evaluating the effect of global warming on the P budget in the productive surface layer 36 37 81 of deep lakes. 38 39 82 2. METHODS 40 41 42 83 Background on Lake Zug 43 44 45 84 Lake Zug is a meromictic, eutrophic lake resting at 413 m elevation in central Switzerland. The 46 47 85 lake has a surface area of 38.4 km², a volume of 3.18 km³, a mean depth of ~83 m and a 48 86 maximum depth of 198 m (Fig. 1). Due to low inflow relative to lake volume, the mean water 49 50 87 residence time of 14.2 years is long for a lake of this size. Surrounding mountains shield the 51 52 88 dominant western winds, hence, wind speeds rarely exceed 2 m s-1 (mean wind speed: 1.47 m s-1; 53 54 89 Fig. S1; Wüest and Gloor 1998). These conditions contribute to a slight salinity gradient and 55 56 90 form weak but permanent water column stratification in the hypolimnion. Complete deep mixing 57 58 91 during winter occurs only rarely. Since the beginning of continuous monitoring in 1950, Lake 59 92 Zug has mixed to maximum depth only once, in 2006 when temperatures completely 60 61 93 homogenized, and oxygen concentrations increased from anoxic conditions to ~5 mg l-1. During 62 63 64 65 1 2 3 4 94 this particular winter, other deep Swiss lakes such as Lake Geneva or meromictic Lake Lugano 5 6 95 also experienced complete mixing (Holzner et al. 2009; Schwefel et al. 2016). As a consequence 7 8 96 of the weak mixing, the water column below ~100 m is anoxic and largely separated from the 9 10 97 productive surface layer. Even rare, severe local storm events, referred to as Föhn, do not result 11 in oxygenation of the deep hypolimnion (Imboden et al. 1988). 12 98 13 14 99 Nestled in an agricultural region, Lake Zug has a long history of eutrophication caused by urban 15 16 100 waste water and agricultural runoff.