Extreme droughts drive tropical semi-arid eutrophic reservoirs towards CO 2 sub-saturation

Aim: This study aimed to evaluate the carbon dioxide (CO2) dynamics in tropical semi-arid reservoirs during a prolonged drought period as well as to test if the trophic state affects the CO2 saturation. Methods: This study was performed in four reservoirs located in the tropical semi-arid region in the northeast of Brazil. All samplings were performed between 9 and 12 am using a Van Dorn Bottle. Samples for partial pressure of carbon dioxide (pCO2) measurements were taken in the sub-surface as well as samples for total phosphorus and chlorophyll-a. Correlation analysis and linear regression were used to detect relations among the calculated pCO2, water volume and chlorophyll-a. Results: The water level reduction due to atypical droughts caused chlorophyll-a concentrations to increase, which in turn, led to CO2 reduction in the water. However, CO2 concentrations were very variable and an alternation between CO2 sub-saturation and super-saturation conditions was observed. This paper showed that water volume and chlorophyll-a were important regulators of CO2 in the water, as well as important carbon balance predictors in the tropical semiarid reservoirs. Conclusions: The results of this paper indicate that the eutrophication allied to drastic water level reductions lead to a tendency of autotrophic metabolism of these systems.

due to nutrients input to inland ecosystems) into a global issue in the last century (Smith & Schindler, 2009) and several aquatic ecosystems (freshwater and marine) have been suffering its consequences (Smith et al., 2006).For instance, eutrophication process (nutrient input) has been causing phytoplankton blooms and cyanobacterial dominance in several tropical reservoirs (e.g.Soares et al., 2012;Medeiros et al., 2015).Due to high primary production, it could be expected that tropical eutrophic reservoirs would present autotrophic metabolism (Finlay et al., 2009;Lazzarino et al., 2009;Pacheco et al., 2013).However, a recent study demonstrated CO 2 supersaturation in 5 tropical eutrophic reservoirs (Roland et al., 2010).In addition, there is rising evidence that the sediment metabolism can play important roles in the carbon balance, either as sink or source of carbon to the water and atmosphere (e.g.Cardoso et al., 2013;Forbes et al., 2012;Mendonça et al., 2012).Thus, it is not yet clear what the role of the tropical reservoirs in the carbon cycle is.
The tropical semi-arid region in Brazil presents high temperatures year-round, long periods of drought and torrential rains in a short time.This region is dominated by reservoirs that have been turning eutrophic due to nutrient inputs from land and concentration of the nutrients in the water because of high evaporation rates and long hydraulic retention times (Chellappa et al., 2009;Barbosa et al., 2012;Brasil et al., 2016;Costa et al., 2016).
Taking the important role of freshwater ecosystems for the global carbon budget into consideration, the investigation of man-made lakes is poorly understood especially in tropical semi-arid reservoirs with high levels of eutrophication allied to prolonged drought as previously studied (Braga et al., 2015).The aim of this study was to evaluate the partial pressure of carbon dioxide (pCO 2 ) over time in tropical semi-arid eutrophic

Introduction
Inland aquatic ecosystems play an important role in the Global carbon cycle because they are responsible for processing great amounts of organic matter from terrestrial ecosystems and for emitting carbon dioxide (CO 2 ) to the atmosphere (Cole et al., 2007;Tranvik et al., 2009;Raymond et al., 2013).Man-made reservoirs are globally numerous (built for water supply, energy production, recreation, navigation and irrigation) and cause significant changes in the landscape by flooding terrestrial habitats and biomass; they usually have large watersheds which result in great availability of organic matter for decomposition (Rosenberg et al., 2000;St. Louis et al., 2000;Friedl & Wüest, 2002;Humborg et al., 2002;Roland et al. 2010).These ecosystems are particularly relevant to the global carbon balance because they are responsible for an important fraction of the CO 2 emission to the atmosphere from inland waters (Oki & Kanae, 2006;Barros et al., 2011).Furthermore, most of the CO 2 emission (up to 70%) through reservoirs occurs in tropical regions (St. Louis et al., 2000).
The global ecosystem metabolism results from the balance between production and decomposition of organic matter (as reviewed in Dodds & Cole, 2007).When production exceeds decomposition, the ecosystems become sub-saturated in CO 2 and they are considered autotrophic; however, when consumption exceeds production (based on accumulated autochthonous and allochthonous organic matter) the ecosystems become super-saturated in CO 2 , so they are considered heterotrophic.Thus, environmental factors such as nutrients (e.g.nitrogen and phosphorous), light, and organic matter availability play a central role in aquatic metabolism regulation (Dodds & Cole, 2007).For instance, nutrient input can stimulate phytoplankton primary production and CO 2 uptake from the water.
Extreme droughts drive... reservoirs.Additionally, it aims to evaluate how water quality (trophic state) and water level fluctuations affect the CO 2 metabolism.The authors believe that water fluctuations allied to eutrophication are regionally key factors in carbon dioxide metabolism during a prolonged drought as part of climate change scenarios.Due to the fact that climate change is reducing the rainfall in the tropical semi-arid as well as enhancing the frequency of extreme droughts, and probably increases eutrophication (Roland et al., 2012;Sarmento et al., 2013), it is extremely important to figure out the role of the eutrophic semi-arid reservoirs on the carbon balance and how they behave facing these changes in climate.

Sampling and data collection
The reservoirs were sampled monthly: Boqueirão to represent their spatial variability.All samplings were performed between 9 and 12 am.
Samples for pH and alkalinity measurements and further partial pressure of carbon dioxide (pCO 2 ) estimations were taken from the sub-surface into a polyethylene flask using a proper device that allowed a three times overflow leaving no headspace.These samples were kept refrigerated in the dark to reduce metabolic activity until they arrived at the laboratory for analysis (c.a. 4 hours).This procedure was previously evaluated and did not affect CO 2 concentrations in the water.
Additionally, water samples were taken with a Van Dorn bottle for total phosphorous (TP) and chlorophyll-a (Chl-a) determinations.Water transparency was measured through a Secchi disk.Chemical measurements (pH, dissolved oxygen, temperature) were conducted with a sonde (Hydrolab DS5).In the laboratory, the alkalinity was determined (Golterman et al., 1978) and subsequently calculated through the Alcagram program (Carmouze, 1994).TP and Chl-a were determined by colorimetric methods for spectrophotometer, according to Valderrama (1981) and Jespersen & Christoffersen (1988) methods, respectively.pCO 2 concentrations were calculated from pH and alkalinity measurements (according to Stumm & Morgan 1996) with corrections for temperature, altitude and ionic strength following Cole et al. (1994) and Weiss (1974).pH was measured with a precision of 0.01 pH units with a calibrated pH meter (Tecnal Tec-3MP) and alkalinity was measured by Gran's titration (APHA 1998).In this study, we considered the atmospheric CO 2 concentration at 390 µatm.It was recently shown that there is an over-estimation of pCO 2 from the alkalinity method (the method used here) when water pH is below 6.5-7.0, when compared to direct measurements with "in situ" infrared gas analyzers [IRGA] (Abril et al., 2015).However, the pCO 2 estimation from alkalinity presents no deviation from pCO 2 estimated by direct measurements in waters with pH above 7.0 (Abril et al., 2015).As the great majority of the pH measured in our samples were above 8.0 (see Table 1) and just very few pH measurements were between 6.0 and 7.0, we are confident that our CO 2 data is very precise and robust.

Statistics
Correlation analysis was used to detect relations among the calculated pCO 2 , water volume and Chl-a.Only the most influential variables were showed with a variable retention criterion of p = 0.05.Data of Chl-a, volume and pCO 2 were log transformed to fulfill the normality requirements.All statistical tests were performed using Sigmaplot software (Systat software Inc., California, U.S.A.).

Results
During the period of study all reservoirs drastically diminished their volumes and Dourado, Boqueirão and Passagem das Traíras reservoirs were close to 0% of the total volume at the end of the sampling periods (Figure 2).It was not possible to observe any annual pattern for any of the measured limnological parameters (data not shown).Temperature was on average above 26°C in all four aquatic ecosystems, reaching up to 34ºC in Passagem das Trairas.All the other physical, chemical and biological measured limnological parameters presented high variability during the study and are summarized in Table 1.We highlight the average TP concentrations above 100 µg.L -1 , except for Boqueirão and all maximum TP concentrations above 300 µg.L -1 .Moreover, T = temperature, OD = dissolved oxygen, Zmax = maximum depth, TP = total phosphorus, Chl-a = chlorophyll-a, pCO 2 = partial pressure of carbon dioxide.

Discussion
The current study was performed in four man-made reservoirs located in a tropical semi-arid region during a prolonged drought period from 2012 to 2014 (Mendonça Júnior et al., 2014;Costa et al., 2016).It is already documented that the regular annual dry periods in this area increase the trophic state of the reservoirs and they are usually mostly in eutrophic conditions (Brasil et al., 2016).In fact, our data corroborate that the studied reservoirs were under eutrophic conditions during the current study, according to the classification proposed for arid and semi-arid aquatic ecosystems by Thornton & Rast (1993).Besides, this study also corroborated the reduction of the water volume of the lakes intensified eutrophication, by concentrating nutrients, as revealed by the negative correlations between % of volume and Chl-a concentration.
The global ecosystem metabolism is based on a balance between the gross primary production (GPP) and the organic matter consumption (e.g.net ecosystem respiration [R]), which is known as net ecosystem production (NEP).Both production and consumption are positively affected by temperature, but GPP is also influenced by nutrient availability (e.g.phosphorus), while R is affected by the organic matter concentration and availability (Dodds & Cole, 2007).Therefore, the interaction between these components directly affects the carbon balance  in ecosystems and, consequently, the concentrations of CO 2 in the water.
As primary production increases with eutrophication state, one could expect that these eutrophic ecosystems should present predominantly autotrophic conditions (i.e.prevalence of sub-saturation of CO 2 in the water) and function as carbon sinks from the atmosphere (Cole et al., 2001) as in other eutrophic aquatic ecosystems in higher latitudes (Balmer & Downing, 2011;Pacheco et al., 2013).In fact, in this paper, extremely high Chl-a concentrations were observed (a proxy for primary production) and consistent negative relations between pCO 2 and Chl-a, which suggestedphytoplankton CO 2 uptake (via photosynthesis) from the water.Furthermore, the prolonged drought during this study, besides favoring the CO 2 uptake by phytoplankton due to eutrophication and high primary production, resulted in negligible input of allochthonous organic matter into the reservoirs (Mendonça Junior et al., 2014;Costa et al., 2016), which probably did not stimulate ecosystem respiration favoring autotrophy (Dodds & Cole, 2007).However, we did not record the prevalence of CO 2 sub-saturation, but similar overall frequencies of super and sub-saturation.Thus, we did not register consistent autotrophic conditions in the studied reservoirs.
A recent study performed in a tropical eutrophic reservoir in the semi-arid demonstrated net heterotrophy due to high temperatures (Almeida et al., 2016).In fact, the pCO 2 was previously positively correlated to temperature in tropical ecosystems (Marotta et al., 2009) once higher temperatures increases respiration rates (Amado et al., 2013).In agreement, other tropical eutrophic reservoirs located in wetter areas (such as Cerrado and Atlantic Forest biomes) were also classified as predominantly super-saturated in CO 2 (Roland et al., 2010).Thus, it is not yet clear what drives heterotrophic and autotrophic conditions in the semi-arid eutrophic reservoirs, but prolonged droughts seem to stimulate primary production driving the metabolism towards the autotrophic state.
It was recently demonstrated that pCO 2 can be overestimated when it is estimated using the alkalinity method in organic-rich and acidic waters (Abril et al., 2015).The reservoirs studied here presented predominantly basic pH and almost no input of organic matter from the watershed (Table 1, Mendonça Junior et al., 2014).Thus, it is reasonable to believe that this methodological bias did not affect the CO 2 pattern discussed here.
The Earth is currently facing changes in the global climate.Current predictions suggest temperature increase and rainfall reduction (until the end of this century) in the tropical semi-arid region is negatively affecting the water supply to the reservoirs (Marengo et al., 2009).Consequently, these ecosystems would turn into a more eutrophic state or dry out (Roland et al., 2012).On one hand, the temperature increase could lead to increasing heterotrophic conditions by stimulating respiration.On the other hand, it could lead to increasing autotrophic conditions due to eutrophication (Moss et al., 2011).Taking into account that the low volume could result in inorganic turbidity from the sediment resuspension (Costa et al., 2016) and sediment respiration relevance to the net metabolism in reservoirs (Cardoso et al., 2013), predicting the effect of climate changes to carbon balance in the tropical semi-arid reservoirs is not straightforward and should be done with caution.Nonetheless, the current study suggests that the eutrophic reservoirs have a fundamental role in the regional carbon cycle and can potentially increase the absorption of atmospheric CO 2 in the current century.It is important to point out that further research is required for better understanding the regulation of CO 2 balance, regarding the physical and chemical characteristics of the man-made systems, to quantify the contribution of these systems to the global carbon cycle.

Conclusions
• Semi-arid eutrophic reservoirs are not consistently autotrophic because of high temperatures and intense heterotrophic processes; • The prolonged droughts favor autotrophic process and consequent CO 2 sub-saturation in the water of the semi-arid tropical eutrophic reservoirs; • The tropical semi-arid eutrophic reservoirs present great regional relevance to the carbon dioxide balance; • Changes in the climate will directly affect how the tropical semi-arid reservoirs function in the carbon cycling.
Figure 1.Location of the studied water supply reservoirs in the semi-arid region, Brazil.The sampling stations are indicated in the maps.

Figure 3 .
Figure 3. Linear regression between chlorophyll-a and volume of all reservoirs over the period of study.(a) Boqueirão Reservoir; (b) Passagem das Traíras Reservoir; (c) Gargalheiras Reservoir; (d) Dourado Reservoir.All data were log transformed.

Figure 4 .
Figure 4. Linear regression analysis of variation in pCO 2 vs. % of accumulated volume (a-d) and pCO 2 vs. chlorophyll-a (e-h).Boqueirão Reservoir (a and e), Passagem das Traíras Reservoir (b and f ), Gargalheiras Reservoir (c and g) and Dourado Reservoir (d and h).All data were log transformed.

Table 1 .
Median values, maximum and minimum (between parentheses)of limnological variables of each semi-arid reservoir studied during the period 2010-2013.