1. Introduction
The contribution of pollutants from point and diffuse emission sources in hydrological basins causes an accumulation of nutrients and eutrophication of fresh water bodies [
1]. Eutrophication causes changes in the ecological structure in water bodies due to the accelerated growth of photosynthetic organisms, which increases the contents of organic matter in water and sediment and the resulting oxygen demands, thereby affecting negatively water quality [
2]. The increase in organic matter is due to the capture and transformation of CO
2 to autochthonous organic carbon (C) [
3] that, together with external or allochthonous C loads, build up in sediment, where heterotrophic organisms degrade and mineralize accumulated organic matter, generating greenhouse carbon gases (GHCG).
The generation of GHCG in freshwater bodies is the result of respiration processes, and is related to the concentrations of oxygen and oxidized nitrate, iron oxy-hydroxide, manganese dioxide, and sulfate, that participate in metabolic processes and mineralization reactions [
4]. In the presence of dissolved oxygen, GHCG generation processes are aerobic, with O
2(aq) acting as the electron acceptor, oxidizing organic C to CO
2 [
5]. Dissolved oxygen is the principal electron acceptor in the sequence of metabolic mineralization reactions that generate GHCG in water bodies. However, the availability of O
2(aq) may vary both temporally and spatially in lakes and reservoirs. Dissolved oxygen usually decreases with water depth, and is often depleted at the water-sediment interface as microorganisms use oxygen to carry out respiration processes and obtain energy [
6,
7]. When O
2(aq) is depleted, the other oxygen-containing compounds take over as electron acceptors in the following order: manganese dioxide, nitrate, iron oxy-hydroxide, and sulfate [
8,
9]. These acceptors are subsequently consumed to oxidize organic C under suboxic and anoxic conditions [
10]. Redox potentials (Eh) depend on the amounts of these compounds that vary as a function of the microorganisms’ energy requirements [
11]. Respiration reactions are named according to the corresponding electron acceptors (
Table 1).
Fermentation of organic C is another respiration process, where microorganisms generate energy through the exchange of electrons amongst different C atoms in organic matter. Fermentation occurs parallel to other organic C degradation processes, forming less complex organic compounds, which can be mineralized by iron- and sulfate-reducing microorganisms, and by methanogenic bacteria [
13,
14,
15]. When electron acceptors are depleted, methanogenesis becomes the predominant biodegradation process that generates CO
2 and CH
4 in equimolar amounts [
16].
Different methods have been used to determine GHCG emission rates, such as in situ static chambers that capture ebullient gases at the water surface [
17], continuous sampling or monitoring of water samples, and separation of headspace gas [
18]. Captured gases are then characterized, and GHCG generation and/or emission rates determined [
19]. Laboratory incubation experiments are another method to determine GHCG generation rates. The convenience of such incubation experiments is that the effects of temperature, nutrient loadings, and gas supplies can be readily evaluated [
20,
21]. Finally, a third method to determine GHCG generation rates in water bodies is through mathematical modeling based on experimental results, and simulating the reactions where carbon is involved [
22,
23]. Laboratory incubation experiments and mathematical modelling may also allow evaluating the relative importance of each mineralization process involved in the generation of GHCG, whereas field sampling does not allow to distinguish such processes.
Sánchez-Carrillo et al. [
24] evaluated the GHCG emissions from freshwater bodies in Mexico. Analyzing their, data it can deduced that lakes and reservoirs represent 97% of the greenhouse carbon effect of all the GHCG emitted from Mexican freshwater bodies. One of these water bodies is the Valle de Bravo reservoir, located in central Mexico (
Figure 1), this lake is being used for tourism and is an important water supply for the Valley of Mexico Metropolitan Area (VMMA). It provides approximately 25% of the 60 m
3 s
−1 water supply to nearly 22 million inhabitants in the VMMA [
25,
26]. The Valle de Bravo reservoir is warm monomictic with eutrophic water and thermal stratification during winter [
27]. Due to the eutrophic state of water in the reservoir and consequent deterioration of water quality, algae blooms and cyanobacteria growths are commonly observed [
28]. There is an interest in improving water quality in this reservoir [
29]; however, it is unknown how eutrophication control methods may affect the generation of GHCG in this water body.
The application of eutrophication control methods may improve water quality although the effects on the mineralization velocities of organic matter and generation of GHCG have not yet been established. The selection of treatments for this study was based on reported uses to control eutrophication in freshwater bodies around the World [
31,
32,
33,
34,
35]. It is expected that oxygenation in the hypolimnion, would increase the generation of GHCG due to aerobic degradation of organic matter, while application of a selective PO
43−-adsorbing material would cause decreased generation of GHCG due to limitation of available PO
43− for growth of degrading organisms. To determine the effects of eutrophication control methods on the production of GHCG in sediment samples from a eutrophied reservoir, in this paper we evaluate the generation of GHCG and the corresponding respiration processes.
In freshwater bodies PO
43− is often the limiting nutrient [
36]; therefore, reducing the available PO
43− concentrations may regulate the production of autochthonous organic matter. To reduce available PO
43− concentrations in lakes and reservoirs, both external and internal contaminant loads must be taken into account [
37], since internal loads may continue to cause eutrophication of lakes and reservoirs for several decades [
38].
The reduction of available PO
43− may be achieved by the application of PO
43−-adsorbing materials or by oxygenation to produce adsorbing substrates of iron and manganese oxides. Methods used for PO
43− removal in water bodies include (1) multi-charged iron and aluminum salts that coagulate PO
43−-containing particles; (2) calcite and dolomite that react with dissolved PO
43− precipitating this nutrient as phosphate-containing minerals; and (3) clay materials or other substrates that have been modified to obtain positive surface charges able to remove anionic PO
43− by adsorption [
31,
39,
40].
The former two methods are pH or redox-sensitive and therefore not suitable for application in natural waters. Phoslock is a bentonite-type clay where cations have been substituted with lanthanum (La
3+) producing positive surface charges, which attract and selectively adsorb aqueous PO
43− [
41]. This material has been found to be highly efficient for controlling concentrations of PO
43−, reducing algae blooms and cyanobacteria growths in freshwater bodies [
32,
42,
43].
Hypolimnetic oxygenation allows controlling the effects of eutrophication in water bodies, supplying O
2(aq) to deeper parts of lakes and reservoirs, where lower temperatures and higher hydrostatic pressures allow more efficient dissolution of the gas without affecting thermal stratification. Dissolved oxygen is very reactive, effectively decreasing oxygen demand, and, when applied in sufficient amounts, reversing anoxic conditions in water bodies. Dissolved oxygen also forms PO
43−-adsorbing substrates of oxidized iron and manganese [
34]. However, a Fe:P molar ratio of at least 2:1 is required and this relation may increase due to the competition of other anions in the adsorption of PO
43− on these substrates. However, a sustained supply of oxygen is required to avoid reductive dissolution of oxidized iron and manganese at Eh lower than −100 mV, and consequent re-dissolution of PO
43− [
44].
2. Materials and Methods
Water and sediment samples were obtained from three stations located in the deeper parts of the Valle de Bravo reservoir where sediments richer in organic matter accumulate [
45]. Water samples were obtained with high-density polyethylene flasks and sediment samples with an Ekman dredge. Water and sediment samples were combined in equal amounts and then characterized. Redox-sensitive compounds and mayor ions were analyzed in the combined water sample according to standard methods (
Table 2). Carbonate (TIC) and organic matter (OM) were determined in the combined sediment sample by weight difference following acidification and calcination, according to Walthert et al. [
46]. Contents of organic matter were converted to total organic carbon (COT) by stoichiometric calculation.
Respiration experiments were set up in four reactors (
Table 3) with characterized water and sediment, not allowing gaseous headspace in order to assure that the emitted GHCG were released into the gas-trapping systems. Reactors were maintained in the dark with a blackout cover and water was slowly agitated with a mechanical paddle stirrer (RCF = 0.3 g) just above the water-sediment interface to allow mixing of the aqueous phase without resuspension of sediment. Four-hole stainless steel lids connected tubing for gas applications to porous rock diffusers located in the bottom of the reactors, allowing the release of gases trapped in sediments. Non-return valves allowed the GHCGs to be released from the gas-trapping systems. Half-hourly monitoring and automatic data storage of Eh and pH were carried out (Hanna model 1005 and 2001 sensors,
Figure 2), and distributions of Eh and pH values were analyzed using the statistical software InfoStat [
62], eliminating the outliers beyond ±2 standard deviations (σ), and determining the resulting median (μ), and σ.
Gases and nutrients were added to all reactors in amounts equivalent to two oxygenation systems [
63] and gas applications proportional to external loads reported by Hansen and Márquez-Pacheco [
64], and Phoslock was added in the relation Phoslock:P 100:1 [
65] to the PHOS and HOS + PHOS reactors. Four gas- and nutrient application events were carried out (1) initially, (2) and (3) when at least one of the gas-trapping systems was full, and (4) at the end of the experiment:
Reactor lids were equipped with non-return valves to avoid alkaline solutions from subsequent traps to flow back into the reactors as gas supplies were paused. Between gas- and nutrient application events, non-return valves were connected to 100 mL graduated high-density polypropylene probes (Sarstedt), which were immersed upside-down in water. The volumes of emitted gases were recorded periodically by measuring the volume of displaced water in the probes. Gas samples were obtained through silicon septa in the probes, and characterized by gas chromatography (SRI 8610c CA, USA), according to EPA Method 25 GC [
66], with 10 mL min
−1 He(g) (Infra 99.997%) as the carrier, corresponding to a 10-min retention time in a carbon column (Restek ShinCarbon) that was coupled to a helium ionization detector (HID). Four-fold carrier gas applications were performed between analyses to assure that previous GHCG had been rejected. A mixture of 30% CO
2 and 70% CH
4 (Infra P-191613) was used as calibration standard.
The O
2 (g) and N
2 (g) application systems included previous alkaline traps (NaOH) to eliminate eventual impurities of CO
2 before entering the reactors. The CO
2 emitted from the reactors were stoichiometrically determined by modification of electrical conductivities of the Ba(OH)
2 solutions in the subsequent alkaline traps according to van Afferden et al. [
21].
During gas- and nutrient application events, the gases emitted from the subsequent alkaline traps were captured in 400-mL Tedlar bags (Sigma Aldrich, St. Louis, MO, USA) from which duplicate samples were obtained with 1-mL SampleLock syringes (Hamilton) for characterization by gas chromatography, as described above.
The amounts of generated GHCG for each gas and nutrient application event were determined by totaling the following:
CO2 and CH4 collected in inverted probes
CO2 captured in alkaline traps
CH4 (and eventual traces of CO2) collected in Tedlar bags
Water samples obtained prior to each gas- and nutrient-application were characterization for PO
43−, alkalinity, redox-sensitive ions as described above, and major ions only in the beginning of the experiment (
Table 2). Stoichiometric variations in concentrations of redox-sensitive ions and emitted GHCG were analyzed to identify the mineralization reactions (
Table 1). The decrease of ions with higher oxidation state (Fe(III), N(V), S(VI), and C(IV)), and increase in reduced ions (Mn(II), Fe(II), N(Ø), N(-IV), S(-II), and C(-IV)), were stoichiometrically evaluated and the magnitude of the different reactions, validated by comparing to amounts of emitted CO
2. Surplus emitted CO
2 was considered to be produced by oxic respiration.
Carbon mass balances were determined with results from each reactor for quality control purposes and for experimental error estimation, by comparing final and initial masses in water, sediment, and emitted GHCG (
Figure 3).
4. Conclusions
The hypothesis of the present work was that applications of eutrophication control methods that include oxygenation in the hypolimnion would increase the generation of GHCG due to the aerobic degradation of organic matter, while the application of selective PO43 -adsorbing material would cause a decreased generation of GHCG due to the limitations of available PO43− for growth of the degrading organisms. To determine the effects of these eutrophication control methods on the generation of GHCG in sediment samples from a eutrophied reservoir, in this paper we evaluated the generation of GHCG and the corresponding respiration processes. This evaluation was implemented in an innovative laboratory experimental system, where the C budgets in the experimental reactors allowed accounting for over 99% of total C.
Generations of GHCG were higher at lower redox potentials, mainly related to increased sulfate-reduction in sediments as other electron acceptors were depleted. Eutrophication control methods reduced between 20 and 40% the generation rates of GHCG, mainly due to lower sulfate reduction as a result of limited available PO
43− when the selective PO
43−-adsorbing material was added, and inhibited metabolism of sulfate-reducing bacteria, when oxygen was applied. Despite O
2 (g) having been added to the reactors in amounts equivalent to two oxygenation systems (Hansen et al. [
63]), redox potentials decreased more rapidly after gas and nutrient applications. This caused oxic conditions to be shorter and did not allow HOS to generate more GHCG as hypothesized for this eutrophication control method. The proportions of emitted CO
2 and CH
4 were constant throughout the experiments despite the redox conditions varying from oxic to anoxic, and methanogenesis occurred most likely within anoxic sediments even under oxic and suboxic conditions.
The good news is that the application of eutrophication control methods in the Valle de Bravo reservoir would most probably result in lower GHCG generation and emission rates. This is due to the repression of sulfate-reduction in water-sediment systems where HOS and PHOS were applied both individually and combined.