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Effects of Salinization on Lake Metabolism

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Effects of Salinization on Lake Metabolism Effects of Salinization on Lake Metabolism Effekter av förhöjd salthalt på sjöars metabolism Emil Nordström Faculty of Health, Science and Technology Biology Bachelor’s thesis, 15 hp Supervisor: Lovisa Lind Eirell Examiner: Larry Greenberg 2020-06-05 Series number: 20:173 Abstract With rising salinity levels in many freshwaters across the globe caused by for example sea- level rise and de-icing salts, it becomes important to understand what effect it has on freshwater ecosystems, since the lakes and rivers themselves are important parts in the global carbon cycle. In this study I have looked at what effects increased salinity levels have on different lakes metabolism, specifically oxygen concentration and primary production. The experiment was conducted using mesocosms in three different lakes in Sweden, separated both geographically and by nutrient status (eutrophic, oligotrophic, and dystrophic as well as oligotrophic). The response to increased salinity differed between the lakes; the increased salinity had a strong negative effect on the oligotrophic lake. In general, increased salinity caused a decline in oxygen content, both the maximum value and the amount of diurnal variation, as well as primary production. Therefore, the conclusion is that a rise in salinity will affect lake metabolism in a detrimental way, with a stronger effect on more sensitive lakes. Sammanfattning Med stigande saltnivåer i många sötvatten världen över, orsakade av exempelvis ökande havsnivåer och applicering av vägsalt, blir det viktigt att förstå vilken påverkan detta har på ekosystem i sötvatten då de utgör en viktig del I den globala kolcykeln. I den här studien har jag tittat på vilken effekt förhöjda salthalter har på olika sjöars metabolism, specifikt syrehalt och primärproduktion. Experimentet utfördes med hjälp av mesokosmer i tre olika svenska sjöar, skilda både geografiskt och trofiskt (eutrof, oligotrof samt dystrof och oligotrof). Sjöarnas respons till den ökade salthalten varierade; saltet hade en starkt negativ effekt på den oligotrofa sjön. På en generell nivå så sjönk syrehalten, både maximum värden och dygnsvariationen, samt primärproduktionen vid högre salthalter. Slutsatsen blir därför att ökade saltnivåer kommer att påverka sjöars metabolism negativt, med en starkare effekt på mer känsliga sjöar. Introduction Humans are well known for affecting the planet in multiple ways, maybe the most recognised issues today are climate change and plastic waste, but also through salinization of freshwater. Salinization of freshwater habitats is a growing problem, with causes ranging from diversion of water for irrigation, sea level rise and de-icing salts among others (Herbert et al., 2015). A small rise in the salt concentration might seem trivial but it will have a negative impact on both on the environment and on humans, by for example affecting agriculture and the production of drinking water (Canedo-Arguelles et al., 2016). Therefore it is important to create such a complete understanding as possible of the effects of salinization on freshwaters. So far studies of increasing salinity levels and their effects have mainly focused on either specific freshwater organisms (e.g., Castillo et al., 2018), or their interactions; for example between predator and prey (e.g., Hintz & Relyea, 2017), or community structure (e.g., Hintz et al., 2017). The impact of salinization on the metabolism of the entire limnic ecosystem on the other hand is much less explored. Exploring the influence of salt on lake metabolism is essential since lakes are an important factor in the global carbon dioxide cycle, especially with regards to the remineralization of carbon derived from allochthonous material (Del Giorgio & Williams, 2005). Although a lake’s metabolism is naturally not static, but will vary with seasons and weather over the year. In a study by Staehr and Sand-Jensen (2007) gross primary production in a Danish lake underwent three to four maximums over the summer and a longer minimum over the winter. Still, the lake was overall balanced, with net autotrophy during the summer offsetting the net heterotrophy occurring in winter (Staehr & Sand-Jensen, 2007) making it carbon neutral. Although not all lakes are carbon neutral (e.g., Yang et al., 2008), the poorly understood effects of salinization could potentially shift a lake to release more or less oxygen. In one study it was found that increasing salinity levels affected zooplankton more strongly than phytoplankton, thereby removing grazing pressure at higher salinities (Van Meter et al., 2011), which would enable the waterbody to sequester more carbon dioxide. On the contrary, some zooplankton species such as Daphnia can relatively quickly adapt to increased salinity up to a certain level (Coldsnow et al., 2017), and thereby reduce the likelihood that salinization causes an “algal bloom”. In order to estimate the metabolism, whether or not it is subject to salinization, the following formula can be used: ΔO2/Δt = GPP – R – F – A (Staehr et al., 2010). In the formula ΔO2/Δt represent a change in dissolved oxygen over time, such as between night and day. GPP is gross primary production, which can be assumed to be close to zero at night. R is system respiration, which should be relatively stable regardless of time of day. Respiration and gross primary production is tightly coupled, since respiration relies on the production of autotrophs R can not be higher than GPP for any substantial time (Solomon et al., 2013). F is the interchange of oxygen between the water surface and the atmosphere, something that needs another estimation including wind and surface area. And finally A is any other factor affecting the amount of dissolved oxygen, such as mixing between water 1 layers, however it is usually considered to be negligible (Staehr et al., 2010). Although in reality it is even more complicated and time series considerably longer than a single day needs to be gathered to account for variation in factors such as sunlight, amount of wind, and mixing depth where the more oxygen rich surface layer meets deeper water (Coloso et al., 2011). With so many factors affecting lake metabolism the effects of salinization can become more important and thus, the aim of this study is to get a better understanding of the broader picture. My hypothesis is that a higher salinity levels has a negative impact on lake metabolism. Specifically, there will be a greater negative effects on more sensitive lakes, like those naturally low in dissolved minerals, since salinization is already known to have a negative effect on many freshwater taxa (Castillo et al., 2018), which could reduce species diversity and the productivity of the entire community. Earlier studies have also shown that periphyton suffers in higher salinities (Van Meter & Swan, 2014) and even though phytoplankton might be experiencing some reduction in grazing pressure, it could further reduce the productivity in lakes with increased salinity levels. Material and methods The study was conducted in three Swedish lakes: Lake Stortjärn, Lake Erken, and Lake Feresjön. Lake Stortjärn (64°15'41.0"N 19°45'46.3"E) is a both oligotrophic and dystrophic lake located in the northern part of Sweden surrounded by boreal forest, Lake Erken (59°50'42.3"N 18°35'22.6"E) is an eutrophic lake located in central Sweden, while Lake Feresjön (57°10'49.4"N 14°48'18.9"E) is an oligotrophic lake located in southern Sweden (SITES, 2020). Natural chloride levels for the lakes were 0.63 mg/L for Lake Stortjärn, 6.52 mg/L for Lake Feresjön and 11 mg/L at Lake Erken. The experiment was performed using in-lake mesocosm in the three lakes. The mesocosms consisted of 20 cylindrical containers of polyethylene attached to a common floating platform within each lake to ensure that the mesocosms received the same amount of heat and light as the subject lakes. Mesocosms measured 0.8 meters in diameter and were 1.5 meters deep (approximately 700 L) (SITES, 2017). At the setup of the experiment the mesocosms were filled with 550 L of lake water, inoculated with collected zooplankton, and then allowed to sit for a few days to stabilize before adding the salt (table 1). Salinity levels were later checked by sampling for chloride and by taking regular conductivity measurements. There were also small nutrient additions into the mesocosms of Lake Erken and Lake Feresjön to allow nutrient levels to remain stable and consistent with the parent lake throughout the experiment. Table 1. Salinity as amount of chloride (Cl) in the mesocosms, same order and concentration for all three lakes. Mesocosm Salinity (mgCl/l) Mesocosm Salinity (mgCl/l) 1 0 11 20 2 100 12 150 3 1500 13 1300 2 4 700 14 300 5 60 15 80 6 1100 16 1000 7 800 17 200 8 250 18 900 9 400 19 500 10 600 20 40 Measurements were taken hourly in the mesocosms in Lake Stortjärn and Lake Erken, and once every minute for Lake Feresjön, with sensors placed in the central water column. An Apogee SQ-500 (Apogee Instruments, Logan, UT) measuring the amount of photosynthetically active radiation (PAR) at wavelengths between 400 and 700 nm, as µmol/m2/s, and an Optode 4531 (Xylem inc., Rye Brook, NY) for dissolved oxygen (DO) and temperature. These measurements were taken between June 21th and September 13th in Lake Stortjärn, June 27th and August 10th in Lake Erken, and from July 4th to October 17th in Lake Feresjön. Although only dissolved oxygen is relevant for comparing the lake metabolism among the salt treatments, as long as there is no significant difference for PAR and temperature when comparing mesocosms within one lake. For data analysis, the data were summarized into daily hourly means (1-24 hours), to limit the effect of fluctuations. The tests used for all three lakes were ANOVAs, for the temperature and amount of PAR to ensure the similarity among the mesocosms, and for DO with post-hoc test (Tukey) to establish which cases differed.
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