Effects of Salinization on Lake

Metabolism

Effekter av förhöjd salthalt på sjöars

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 , since the lakes and rivers themselves are important parts in the global . In this study I have looked at what effects increased salinity levels have on different lakes metabolism, specifically oxygen concentration and . 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 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 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 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 , 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 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. All tests were made using IBM SPSS Statistics 25 for Windows (IBM Corporation, Armonk, NY). Gross primary productivity (GPP) was also calculated as a mean of two consecutive dates for salinities 0, 500, 1000, and 1500 mgCl/L, using the model developed by Richardson et al. (2015). The dates used were from one week into the experiment to allow for initial fluctuations, therefore they were not the same between the three lakes. When the experiment was conducted the sensors in mesocosms 5 and 9 in Lake Erken had to be restarted during the study, therefore they were not included in the calculations. In Lake Stortjärn mesocosms 3, 5 and 10 had some missing data, but were still included.

Results The results for the test made to ensure similarity between mesocosms generally showed that this was the case. In Lake Stortjärn there was no difference in the amount of photosynthetically active radiation (PAR) (ANOVA: F19,460=1.197, p=0.255) (figure 1A) or temperature between mesocosms (ANOVA: F19,460=0.482, p=0.969) (figure 2A). The same was true for Lake Feresjön, neither PAR (ANOVA: F19,460=0.074, p=1.000) (figure 1C) nor temperature (ANOVA: F19,460=0.029, p=0.954) (figure 2C) showed significance between mesocosms. Lake Erken showed no difference in PAR (ANOVA: F17,414=0.305, p=0.997)

3 (figure 1B), but it did with temperature (ANOVA: F17,414=76,738, p<0.001) especially at salinities 700 and 1000 mgCl/L (figure 2B). Results for the difference in dissolved oxygen between salinities were significant for all

three lakes, Lake Stortjärn (ANOVA: F19,460=38.747, p<0.001), Lake Erken (ANOVA: F17,414=44.762, p<0.001), and Lake Feresjön (ANOVA: F19,460=9.633, p<0.001). Although the pattern of DO was not identical between the lakes, DO appearing highly irregular without any linearity in Lake Stortjärn (figure 3), mainly irregular but tending towards higher amounts of dissolved oxygen at lower salinities in Lake Erken (figure 4), and with oxygen initially decreasing with increasing salinities but showing a slight rise towards maximum salinity in Lake Feresjön (figure 5).

A. C.

B.

Figure 1. The 95% confidence interval for the amount of photosynthetically active radiation (PAR) at the different salinities in Lake Stortjärn (A), Lake Erken (B), and Lake Feresjön (C).

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A. C.

B.

Figure 2. The 95% confidence interval for the temperatures at the different salinities in Lake Stortjärn (A), Lake Erken (B), and Lake Feresjön (C). Note the outlying values for salinities 700 and 1000 mgCl/L in Lake Erken.

Figure 3. Amount of dissolved oxygen (DO) at the different salinities in Lake Stortjärn, with bars representing 95% confidence interval.

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Figure 4. Amount of dissolved oxygen (DO) at the different salinities in Lake Erken, with bars representing 95% confidence interval.

Figure 5. Amount of dissolved oxygen (DO) at the different salinities in Lake Feresjön, with bars representing 95% confidence interval.

The three lakes also differed in their pattern of diurnal dissolved oxygen variation. In Lake Stortjärn dissolved oxygen was highest and similar at salinities 0 and 1000 mgCl/L, while lower at salinities 500 and 1500 mgCl/L (figure 6A). In Lake Erken the variation was higher at salinity 0 mgCl/L, while the other salinities showed similar values (figure 6B). Lake Feresjön also had a larger diurnal change in dissolved oxygen at lower salinities (figure 6C), albeit with maximum salinity (1500 mgCl/L) showing DO-values above that of salinity 1000 mgCl/L.

6 A. C.

B.

Figure 6. Mean daily variation from throughout the experiment in dissolved oxygen (DO) at four salinities in Lake Stortjärn (A), Lake Erken (B), and Lake Feresjön (C).

The values for gross primary production was similar to the ones for dissolved oxygen for Lake Erken and Lake Feresjön (table 2), with Lake Feresjön showing a more linear decline. Lake Stortjärn unlike previously showed a clear decline in gross primary production with increasing salinities (table 2), being more similar to Lake Feresjöns dissolved oxygen curve.

Table 2. Estimated gross primary production for the three lakes (in mg O2/L/day) as the mean of two dates, under four different salinities. Lake: Dates: 0 mgCl/L 500 mgCl/L 1000 mgCl/L 1500 mgCl/L

Stortjärn June 29th to July 1st 0.006 -0.103 -0.242 -0.235

Erken July 4th to July 7th 0.033 -0.177 -0.473 -0.099

Feresjön July 11th to July 13th 0.258 0.196 0.086 0.097

7 Discussion My hypothesis, that higher salinity levels has a negative impact on lake metabolism and even more so on sensitive lakes, appears to be at least partly correct. Oligotrophic Lake Feresjön seems to be highly affected by increased salinities, while the results for Lake Erken and especially Lake Stortjärn and are less clear cut. Lake Feresjöns pattern of decreasing dissolved oxygen with elevated salinity might be explained by its oligotrophic state, which means that it does not support as large species diversity as a lake of more intermediate nutrient status would (Brauer et al., 2012), thus there are less species available to adapt to higher salinities. Lake Feresjöns naturally low salinity also increases its sensitivity, both because the organisms present could be less adapted to higher salinities, and due to the fact that any increase in salinity is proportionally bigger compared to a lake with higher natural salinity. Yet there was a slight increase in gross primary productivity and dissolved oxygen at the highest salinities tested, likely because zooplankton is limited there by the salt, as noted by Van Meter et al. (2011), allowing phytoplankton to grow more freely, thereby showing higher rates of primary productivity and oxygen production. In Lake Stortjärn on the other hand there are two possible reasons to its random-looking dissolved oxygen pattern. Firstly there could simply be no effect of the added salt and the small variation (mainly between 86 and 88% oxygen saturation) is simply due to innate differences in the mesocosms. Or salinity has an effect on the lake but with complex interactions between salt and the organisms, with Lake Stortjärns dystrophic status meaning that its oxygen concentration is not only determined by primary producers but also influenced to a high degree by heterotrophic decomposers (Brönmark & Hansson, 2018). That Lake Stortjärn is also oligotrophic increase the importance of heterotrophic organisms over autotrophs, as the former can be supported by allochthonous nutrients (del Giorgio & Gasol, 1995). So if there are effects on either important primary producers or heterotrophic decomposers (which reduce oxygen) at different salinities it could lead to the irregular pattern seen. Though it is worth noting that the difference in oxygen saturation between salt treatments is only a couple of percentage points, so could be caused by even small differences in species salt sensitivity. In Lake Erken, it initially appears to be none or only a small effect of salinity on dissolved oxygen levels, but if the values for salinities 700 and 1000 mgCl/L are considered less accurate, and would benefit from recalibration, a stronger effect appears. Even though Lake Erken showed significance for temperature between multiple mesocosms (likely because of difference in amount of sunlight received as the mesocosms located in the middle of the experimental platform tended to show higher temperatures, or small calibration errors), the values generally only varied by about one degree Celsius, except for salinities 700 and 1000 mgCl/L which were deviating by approximately twice that amount. With oxygen saturation being temperature dependant, and temperature is expected to be equal between mesocosms within one lake, it is plausible that the dissolved oxygen value for salinity 700 mgCl/L should be higher while the one for 1000 mgCl/L should be lower. If that is correct then the curve for dissolved oxygen and salinity in Lake Erken becomes similar to the one from Lake Feresjön,

8 although with an even more pronounced dip around salinity 900 mgCl/L, likely for the same reason as in Lake Feresjön. Important to note is that the higher dissolved oxygen values for salinity 1000mgCl/L in Lake Erken will probably not affect the calculation of gross primary production. As long as the difference between day and night levels is unaffected whether these values are unusually high or low does not matter. Nevertheless, the values for GPP are still not without issues. The results for gross primary production are not realistic for two of the three lakes. Negative values for GPP as seen in both Lake Stortjärn and Lake Erken should not be possible since the main primary producers are cyanobacteria and different , both which produce oxygen during photosynthesis (Brönmark & Hansson, 2018). For Lake Stortjärn one reason for the negative GPP values could be that photosynthesis is overshadowed by the effect of heterotrophic organisms, disrupting the “normal” diurnal pattern. The GPP calculations are also made on just fraction of the total number of experimental days, the negative numbers could be caused by natural variation in weather for example, this is supported by the average graph over Lake Erken were the amount of dissolved oxygen rise over the day and decline during the night as expected. If the actual values for GPP were to be ignored for Lake Stortjärn and Lake Erken, focusing instead on the pattern, the lakes appear similar. The highest productivity in Lake Feresjön is without added salt, with salt added productivity decreases markedly until about a salinity of 1000 mgCl/L where it levels out and even shows a small increase at 1500 mgCl/L, as mentioned earlier likely because a decrease in grazing when zooplankton population goes down. This mirrors the dissolved oxygen pattern for Lake Feresjön, and to a lesser degree Lake Erken, but it is completely different from the one in Lake Stortjärn. Since the calculations for gross primary production only gives us primary production it indicates that while Lake Stortjärn, like the others, experience a decline in productivity with increased salinity, but this is not what determines the amount of dissolved oxygen. This is instead, as mentioned earlier, likely to be caused by the strong influence of its heterotrophic decomposers, because of Lake Stortjärns dystrophic and nutrient poor status (Brönmark & Hansson, 2018; del Giorgio & Gasol, 1995). There is also the question as to whether it is actually possible to accurately represent a both dystrophic and oligotrophic lake such as Lake Stortjärn using mesocosms, an ecosystem that relies heavily on input of resources from outside and with only low levels of stored nutrients might not behave naturally when kept in isolation. Perhaps a regular addition of material to the mesocosm that it would naturally receive from upstream, either similar to the nutrients given to the other two lakes or more natural like dead leaves, might have created a different result. This would also make for an interesting experiment in itself, and useful in order to accurately represent a dystrophic lake using mesocosms. Another point to consider in future studies is erroneous values. These could perhaps be reduced by for example switching the measuring probes between mesocosms at least once during the study, but this does increase the risk of damage from moving the equipment as well as the risk of cross-contamination of mesocosms. The effect of climate change also needs attention; with increased salinities reducing primary production and oxygenation in some lakes this could further increase atmospheric carbon dioxide. Still, the effects of climate change, such as shifts in weather, might also interact with

9 the effects of increased salinities, either reducing metabolism even further or by mitigating the effects due to increased precipitation (Trenberth, 2011). If only considering the effect of warming waters this appear to lead to increased carbon dioxide emissions (Kosten et al., 2010). Though judging by the results, especially those from Lake Feresjön, salinity is likely to remain an important factor for some time. To conclude, increased levels of salinity do affect lakes, with the expectedly sensitive oligotrophic lake low in natural salinity and with no influx of more nutrient rich water showing the clearest response. The effect of salt was, where seen, a lowering of the lakes metabolism; both its oxygenation and primary production. Although at the highest salinities used in the experiment a small rise in both dissolved oxygen and primary production could be seen, but not as high as the levels present without added salt. With salts reducing the lakes ability to produce oxygen, and thereby decreasing carbon dioxide, increasing salinities in a lake will cause more carbon in the atmosphere adding to global climate change by some degree. Still, if climate change progresses far enough de-icing salts might no longer be necessary.

Acknowledgements The data used in this study were supplied by Aqua-net team 2018, as a part of a larger project. I would like to thank my proof-readers who helped correct many an error, both relevant and aesthetic ones. And a special thanks to my supervisor Lovisa Lind Eirell for her continuous guidance, as well as quick and valuable feedback.

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