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Introduction
River ecosystems provide humans with ecosystem services, such as drinking water, food, and recreational activities (Ullah et al., 2018). Changes in land use and some landscape patterns significantly influence the management of river ecosystems and are reflected in water quality, structure, and functionality (Malacarne et al., 2016; Rossi et al., 2018; Liu et al., 2022). River ecosystems have been used by humans as a source of resources and as a pathway for waste disposal, which historically originated a gradual degradation (Alonso & Camargo, 2005). For example, in Mexico, the occupation of the deltaic plain formed by the Mezcalapa-Grijalva-Usumacinta rivers dates back 3500 years with the presence of the Olmec culture and the impact of deforestation, as well as the modification of these rivers, has affected the morphodynamics of deltaic system (Sandoval-Rivera et al., 2022).
Ecosystems deterioration was accentuated after the Industrial Revolution, due to the increased production of waste materials, the introduction of new pollutants, and the increase in population in cities, which in turn generated more waste (Oscoz et al., 2006; De los Santos et al., 2022). Indeed, deterioration is such that currently, these ecosystems constitute one of the most degraded worldwide (Reid et al., 2019, Albert et al., 2021).
In recent years, climate change and human disturbances such as dam construction and deforestation have caused severe impacts on the ecological environment of large river basins, significantly altering the structure and functionality of the ecosystems, and enhancing vulnerability (Varis et al., 2012; Pan et al., 2022). Currently, worldwide and particularly in Mexico, there are several causes of water quality alteration and the degradation of biological communities. These include organic matter pollution, nutrient enrichment, elimination or degradation of riparian forests (Escalona-Domenech et al., 2022), rectification and channelization of rivers (Tovilla, 2005), channel regulation, increase of inorganic and persistent organic pollutants, and mining activities (Zhou et al., 2019; De los Santos et al., 2022; Golin et al., 2022). These causes have led to significant modifications in the ecological status of rivers in Mexico (Díaz-Pascacio et al., 2018; Ortiz, 2019).
Ecological status is the measure of the quality of the structure and functioning of ecosystems (Ferreira, 2012). Good ecological status becomes visible when the biological communities in an aquatic system are equal or very close to those that can be found in unaltered conditions (Volonté et al., 2015). In a good ecological state, the physicochemical conditions and also the configuration of the environment (hydromorphological conditions) should allow the development of these communities (Martínez et al., 2004; Ferreira, 2012).
Particularly in a river, a worthy ecological status is defined by aspects such as water quality, habitat, aquatic organisms, ecological processes, or hydrology, acting at different scales of interaction (Deegan et al., 2010; Pinto & Maheshwari 2011; Poole et al., 2013). Therefore, the assessment of physicochemical, hydromorphological, and biological quality (based mainly on the composition of aquatic flora, invertebrate fauna, and fish) determines the ecological status of water bodies (Ferreira, 2012). Specifically, physicochemical parameters provide adequate information on the nature of the physical properties and chemical species of water, allowing an assessment of its quality for different types of use, unlike other biological methods (Samboni et al., 2007).
The study of the ecological condition of rivers in Chiapas, Mexico is of singular importance due to the extensive process of land cover and land use change that the watersheds of the state are undergoing (Tovilla, 2005). These processes cause an increase in sediments that are naturally carried by the rivers downstream, and reach the coastal lagoons (where most of the coastal rivers flow), and cause siltation problems in the lagoons (Carbajal-Evaristo et al., 2015).
The Margaritas River basin in southeastern Chiapas is an important source of recharge and water supply that supplies coastal lagoons and low-lying communities (Tovilla, 2005). Some authors have found a very close relationship between the ecological condition of the habitat and abiotic factors assessed in the river (habitat quality, water temperature, pH) and nutrient concentrations such as phosphorus and nitrogen (Stevenson, 2014; Charles et al., 2019; Tang, 2020).
Thirty-seven percent of the extension of the Margaritas watershed is formed by induced grasslands in which livestock activity predominates, which has led to a decrease in the ecological condition of the banks and the fragmentation of riparian vegetation (Escalona-Domenech et al., 2022). For this reason, it is of utmost importance to make a diagnosis of the current state and condition of this river ecosystem. From this perspective, this study aimed to evaluate the ecological condition present in an altitudinal gradient of the Margaritas River, based on the physicochemical parameters of the water and the evaluation of the physical aquatic habitat.
Material and Methods
Description of the study area
The Margaritas river basin is located within the municipality of Pijijiapan, Chiapas, Mexico, between coordinates 93° 07’ 57” and 92° 59’ 06” W and 15° 25’ 01” and 15° 41’ 40” N. The watershed is located within the slope formed by the Sierra Madre de Chiapas and the Pacific Ocean, and is part of the hydrological region No. 23 (RH 23) Costa de Chiapas (CONAGUA, 2009) and its total land area is 19,475.81 ha (Figure 1).
The Margaritas River basin belongs to the physiographic region of the Pacific Coastal Plains of Chiapas, located in the Central American Cordillera (INEGI, 2002). The predominant climate in the basin is warm humid Am (w), which represents 60.95 % (11,871.25 ha) of the total surface area of the basin, while 39.05 % (7,604.56) has a warm subhumid climate Aw2(w) (in the middle and lower part of the basin) and a semi-warm humid climate ACm(W) (in the upper part of the basin) (INEGI, 2008). An average annual temperature of 27.6 °C and an average precipitation of 2,596 mm are reported for the basin according to data from meteorological station No. 23018 of the Comisión Nacional del Agua (CONAGUA-MEXICO) (Escalona-Domenech et al., 2022), although in the higher areas of the basin it can reach 2,600 mm per year. The flows of this river follow a behavior according to two marked seasons of the year, rainy and dry. The rainy season includes the months from May to October, while the dry season includes the months from November to April. The historical maximum precipitation values occur during the month of September and the minimum in January (Figure 2).
Figure 2 Walter and Lieth climate diagrams of the Margaritas River basin for the period 1951 to 2021.
Throughout the Margaritas River basin, the edaphology is formed by seven soil units, where cambisol, lithosol, and regosol units predominate, representing 37.58, 37.21 and 17.18 % respectively of the total area of the basin (INEGI, 2016).
Location of sampling sites
Three sampling sites, located at different altitudes of the Margaritas River, were located: 1) Margaritas 1 established between coordinates 15°32’14.37” N and 93° 4’50. 12” W at an altitude of 15 masl, 2) Margaritas 2 installed between 15°35’31.97” N and 93°3’23.53” W at 70 masl and 3) Río Ramón demarcated between coordinates 15°39’46.05” N and 93°1’45.39” W at an altitude of 386 masl (Figure 1).
Habitat evaluation
At each sampling site, the visual evaluation of the physical habitat was carried out following the Barbour et al. (1999) protocol for wadeable rivers, which consists of ten variables that are assigned a value from 0 to 20 points. With the sum of all the variables, a final score is given, giving the habitat condition a rating of optimal, suboptimal, marginal, and poor (Table 1).
Table 1 Variables for the evaluation of the condition of the habitat and its score
| Parameter | Condition | ||||
|---|---|---|---|---|---|
| Optimal | Suboptimal | Marginal | Poor | ||
| Channel alteration | 20-16 | 15-11 | 10-6 | 5-0 | |
| Channel status | 20-16 | 15-11 | 10-6 | 5-0 | |
| Covering of edges by sediments | 20-16 | 15-11 | 10-6 | 5-0 | |
| Bank stability | Right | 10-9 | 8-6 | 5-3 | 2-0 |
| Left | 10-9 | 8-6 | 5-3 | 2-0 | |
| Speed and depth regimes | 20-16 | 15-11 | 10-6 | 5-0 | |
| Substrate for epifauna | 20-16 | 15-11 | 10-6 | 5-0 | |
| Rapids frecuencies | 20-16 | 15-11 | 10-6 | 5-0 | |
| Width of the riparian zone of the bank | Right | 10-9 | 8-6 | 5-3 | 2-0 |
| Left | 10-9 | 8-6 | 5-3 | 2-0 | |
| Sediment deposition | 20-16 | 15-11 | 10-6 | 5-0 | |
| Vegetal protection of the bank | Right | 10-9 | 8-6 | 5-3 | 2-0 |
| Left | 10-9 | 8-6 | 5-3 | 2-0 | |
| Total | (200-166) | (156-113) | (100-60) | (47-0) | |
Bibliographical source: Barbour et al. (1999)
Hydrology and physicochemical water parameters
At each site, the parameters of depth (Prof), dissolved oxygen (Od), electrical conductivity (EC), temperature (T), and salinity (Sal) were determined directly in the field in triplicate using a YSI model 85 portable multiparameter equipment. Hydrogen potential (pH) was measured with an eco-Test pH sensor model pH2. In addition, ammonium (NH4 +), nitrite (NO2 -), nitrate (NO3 -), and phosphate (PO4 -) concentrations were determined using a Hach portable Case equipment model DR/890.
Statistical analysis
A completely randomized design was applied. The sample size included a total of three replicates for each experimental analysis performed (n ≥ 3). For the analysis of the results of each of the variables evaluated in the study sites, an analysis of variance and comparison of means by Tukey-Kramer (p ≥ 0.05) was applied using jmp pro 15 software (Statistical Analysis System [SAS], 2020).
Resultados
Physical habitat
In the three sites evaluated, ratings were obtained corresponding to two of the states for habitat conditions mentioned by Barbour et al. (1999), which were: marginal and suboptimal. Sites Margaritas 1 and Margaritas 2 showed the highest score corresponding to a suboptimal habitat condition (Table 2).
Table 2 Values obtained in the evaluation of the physical aquatic habitat
| Site | Altitude msnm | Coordinates | Score | Qualification |
|---|---|---|---|---|
| Margaritas 1 | 56 | 15°32’14.37”N, 93°4’50.12”O | 141 | Sub-optime |
| Margaritas 2 | 68 | 15°35’31.97”N, 93°3’23.53”O | 143 | Sub-optime |
| Ramón river | 386 | 15°39’46.05”N 93°1’45.39”O | 92 | Marginal |
The variables that most influenced this rating were those related to bank protection and riparian vegetation width (Table 3). The variable that showed the least variation among sites was channel alteration (Figure 3).
Table 3 Values were obtained for each variable by sites of the physical aquatic habitat evaluation according to Barbour et al. (1999)
| Variable | Ramón river | Margaritas 1 | Margaritas 2 |
|---|---|---|---|
| Bottom substrate for epifauna | 10 | 14 | 13 |
| Embedment (covering of edges by sediments) | 8 | 12 | 15 |
| Speed/depth regime | 10 | 13 | 20 |
| Sediment deposition | 10 | 12 | 10 |
| State of the flow in the channel | 14 | 19 | 14 |
| Alteration of the channel | 16 | 19 | 16 |
| Rapids frequencies | 14 | 20 | 15 |
| Margin stability (right margin) | 2 | 9 | 9 |
| Margin stability (left margin) | 2 | 10 | 9 |
| Margin protection (right margin) | 2 | 2 | 6 |
| Margin protection (left margin) | 2 | 5 | 6 |
| Riparian vegetation width (right margin) | 1 | 2 | 5 |
| Riparian vegetation width (left margin) | 1 | 4 | 5 |
| Total | 92 | 141 | 143 |
Physical-chemical parameters
The results obtained from the measurement of physicochemical parameters of the waters of the Margaritas River, taking into account each of the replicates, are shown in Table 4.Od (%) = dissolved oxygen in percent, Od (mg/L) = dissolved oxygen in milligrams per liter, EC 1 (µS/cm) = site specific electrical conductivity expressed in micro siemens/centimeters, EC 2 (µS/cm) = electrical conductivity at 25°C expressed in micro siemens/centimeters, Sal (ppm) = salinity in parts per thousand, T (°C) = emperature in degrees centigrade, pH = Hydrogen potential.
The mean values obtained for each parameter evaluated are shown graphically in Figure 4, while the mean comparison test and ANOVA applied to each parameter are shown in Table 5.
Figure 3 Status of each one of the attributes of the physical habitat of the protocol of Barbour et al, 1999 in the 3 sampling sites of the Margaritas River, Chiapas
Table 4. Depth, oxygen, electrical conductivity, salinity, temperature and pH of the water of the Margaritas River at the sampling sites
| Site | Repetition | Depth (cm) | Od (%) | Od (mg/L) | EC 1 (µS/cm) | EC 2 (µS/cm) | Sal (ppm) | T (0C) | pH |
|---|---|---|---|---|---|---|---|---|---|
| Margaritas 1 | 1- Left | 28 | 95.2 | 7.86 | 80.5 | 82.1 | 0.00 | 24.1 | 8.2 |
| Margaritas 1 | 2- Center | 28 | 96.0 | 8.07 | 83.8 | 85.2 | 0.00 | 24.2 | 8.1 |
| Margaritas 1 | 3- Right | 20 | 91.6 | 7.64 | 83.6 | 85.2 | 0.00 | 24.1 | 8.0 |
| Margaritas 2 | 1- Left | 34 | 92.4 | 7.59 | 86.6 | 86.4 | 0.00 | 25.1 | 8.3 |
| Margaritas 2 | 2- Center | 62 | 98.5 | 8.14 | 86.6 | 86.4 | 0.00 | 25.1 | 8.0 |
| Margaritas 2 | 3- Right | 46 | 94.7 | 7.70 | 86.7 | 86.4 | 0.00 | 25.2 | 8.1 |
| Ramón river | 1- Left | 14 | 81.2 | 6.79 | 76.10 | 77.4 | 0.00 | 24.2 | 8 |
| Ramón river | 2- Center | 29 | 90.4 | 7.44 | 74.20 | 75.7 | 0.00 | 24 | 7.7 |
| Ramón river | 3- Left | 56 | 92.8 | 7.83 | 74.00 | 75.4 | 0.00 | 24 | 7.6 |
Figure 4 Graphic interpretation of each of the physical-chemical parameters evaluated at each study site and their respective repetitions with respect to the mean
Table 5 Average values of depth, oxygen, electrical conductivity (EC), salinity, temperature and Hydrogen potential (pH) in Río Margaritas ± D.E. Tukey-Kramer test, p ≤ 0.05
| Site | Average | S.D. | ||
|---|---|---|---|---|
| Depth (cm) | ||||
| Margaritas 1 | 25.33 | 4.61 | F2,6=1.67 | |
| Margaritas 2 | 47.33 | 14.04 | n.s. | p = 0.26 |
| Ramón river | 33 | 21.28 | ||
| Oxygen (%) | ||||
| Margaritas 1 | 94.26 | 2.34 | F2,6=2.52 | |
| Margaritas 2 | 95.2 | 3.08 | n.s. | p = 0.15 |
| Ramón river | 88.13 | 6.12 | ||
| Oxygen mg/L | ||||
| Margaritas 1 | 7.85 | 0.21 | F2,6=1.71 | |
| Margaritas 2 | 7.81 | 0.29 | n.s | p = 0.25 |
| Ramón river | 7.35 | 0.52 | ||
| EC 1 (µS/cm) | ||||
| Margaritas 1 | 82.63 | 1.85 | b | F2,6=68.77 |
| Margaritas 2 | 86.63 | 0.05 | a | p ≤ 0.0001 |
| Ramón river | 74.76 | 1.15 | c | |
| EC 2 (µS/cm) | ||||
| Margaritas 1 | 84.16 | 1.78 | a | F2,6=59.67 |
| Margaritas 2 | 86.4 | 1.74e-14 | a | p = 0.0001 |
| Ramón river | 76.16 | 1.07 | b | |
| Salinity (ppm) | ||||
| Margaritas 1 | 0 | |||
| Margaritas 2 | 0 | |||
| Ramón river | 0 | |||
| T (0C) | ||||
| Margaritas 1 | 24.13 | 0.05 | b | F2,6=160.66 |
| Margaritas 2 | 25.13 | 0.05 | a | p ≤ 0.0001 |
| Ramón river | 24.06 | 0.11 | b | |
| pH | ||||
| Margaritas 1 | 8.1 | 0.1 | F2,6=4.82 | |
| Margaritas 2 | 8.13 | 0.15 | n.s. | p = 0.06 |
| Ramón river | 7.76 | 0.2 | ||
According to the average values obtained in each of the sites, there were significant differences in electrical conductivity, which was evaluated under two conditions: site-specific electrical conductivity (EC1) and electrical conductivity at 25 ºC (EC2). The results show differences between the three sites evaluated for both conditions. For EC1, the highest value corresponds to the Margaritas 2 site (86.63 ± 0.05 µS/cm), while, in EC2, the Margaritas 1 (84.16 ± 1.78 µS/cm) and Margaritas 2 (86.4 ± 1.74e-14 µS/cm) sites were much higher. For both EC1 and EC2, the results obtained for the Río Ramón site were significantly lower (Table 5).
Water temperature also presented significant statistical differences, having a higher value at the Margaritas 2 site (25.13 ± 0.05 ºC) with respect to the Margaritas 1 site (24.13 ± 0.05 ºC) and the Río Ramón site (24.06 ± 0.11 °C), as shown in Table 5. Depth (Prof.), dissolved oxygen (Od) and pH values did not present significant differences in the three evaluated zones within the Margaritas River (Table 5).
As for the chemical parameters, ammonium only presented one value for the Margaritas 2 site, which was very low (Table 6). The amount of nitrate was higher at the Margaritas 2 site with an average value of 6.83 ± 0.55 mg/l while the Río Ramón site only presented 0.2 ± 3.40E-17 mg/l (Table 6).
Table 6 Results of the physicochemical parameters of the water of the Margaritas River at three sampling sites during the rainy season with standard deviation values
| Site | Parameter | |||
|---|---|---|---|---|
| NH4 + (mg/L) | NO3 - (mg/L) | NO2 - (mg/L) | PO4 - (mg/L) | |
| Margaritas 1 | 0 ± 0 | 2.57 ± 2.60 | 0 ± 0 | 0.02 ± 0.02 |
| Margaritas 2 | 0.01 ± 0.02 | 6.83 ± 0.55 | 3.67 ± 1.15 | 0.05 ± 0.12 |
| Ramón river | 0 ± 0 | 0.2 ± 3.40E-17 | 0.67 ± 0.58 | 0.12 ± 0.05 |
NH4 + = ammonium, NO2 - = nitrites, NO3 - = nitrates, PO4 - = phosphates
Nitrites had similar behavior to nitrates at the Margaritas 2 site, obtaining the highest values (3.67 ± 1.15 mg/l) with respect to the Río Ramón and Margaritas 1 sites (Table 6). For phosphate analysis, the site that presented the highest values was Río Ramón (0.12 ± 0.05 mg/l) as shown in Table 6. Thus, it is considered that ammonium and phosphate concentrations were low in the three sites analyzed.
Discusion
The results of the habitat assessment showed that in the Margaritas River, human activities such as cattle ranching have caused a decrease and/or change in riparian vegetation towards other types of vegetation or its disappearance. This may be accompanied by a human disturbance in the landscape, geological aspects, and precipitation that tend to naturally affect stream conductivity (Vander Laan et al., 2013) and could have contributed to the destabilization of the banks, which was observed in the case of the Río Ramón site.
Precipitation, soil fertility, watershed slope, and river size affect the location and intensity of agricultural and urban land use because they regulate crop growth, erosion, transport, and water supply (Ramankutty et al., 2006). These regional-scale natural factors also determine land use and cover changes at the watershed level (Dodds et al., 2015). The habitat condition, structure, and functionality of ecosystems determine the potential for the existence of their vulnerability (Micheli et al., 2014). Therefore, riparian ecosystems such as the Margaritas River that present suboptimal (Margaritas 1 and 2 sites) and marginal (Ramon River site) conditions in their tributaries, give the first indication of habitat degradation. So, from a management perspective, as Tang (2020) points out, human activities can be managed to reduce pollutants and ecosystem alterations in the evaluated sites.
The physical-chemical parameters Prof, Od and pH show similarity in the three zones evaluated within the Margaritas River. Water pH showed a certain level of alkalinity in the waters of the Margaritas River in the three sites evaluated, which is very similar to that reported by Garcia et al. (2019) applying linear regression models in which no differences were presented in the alkaline pH of the Chimbo River, Ecuador. On the other hand, the EC shows significant differences in which the Río Ramón site has low values with respect to the other study sites, however, the obtained values at study sites turn out to be higher than those reported by Arroyo and Encalada (2009) in the Guajalito (58.8 μS/cm), Palmeras (55 μS/cm) and Brincador (30 μS/cm) rivers which are within the permissible standards as established by Ríos and Prat (2004). Thus, some physicochemical conditions in the stream such as high nutrient concentrations have been widely associated with natural and human factors at the watershed scale (Golden et al., 2016), which makes it more evident a higher concentration of dissolved solids at site Margaritas 2. The above could be derived from a higher emission of organic waste, fertilizers, or materials from agricultural and livestock practices that increase these values.
In the case of chemical parameters such as phosphates, it was observed that these showed a decrease as the altitude decreased and the river flow increased, also increasing due to seasonal rainfall. Similar results were obtained by Hernández (2014) for the Cacaluta River. This decrease in phosphates could be explained due to the dilution effect that water has on this compound, this is because the Río Ramón site is located in a headwater river, while the Margarita 1 and Margarita 2 sites are located on the main channel, much wider and with greater flow. Marcarelli and Wurtsbaugh (2007) note that high phosphorus concentrations benefit the abundance of nitrogen-fixing taxa and an increase in their rate of fixation as nitrogen input into the waters increases; however, nitrate concentrations at the Río Ramón site were lower concerning the Margaritas 1 and Margaritas 2 sites. This low nitrogen fixation at the Río Ramón site, in addition to the altitude, may be due to a very limited light energy whose fixation rate in shaded rivers is lower (Marcarelli et al., 2008) as is the case in this higher altitude zone within the Margaritas River.
Ammonium concentrations only showed very low values for the Margaritas River 2 site but related to the higher EC obtained for this site, they are indicative of greater contamination in its waters. This result is also influenced by the concentration of oxygen and pH, which causes it to oxidize rapidly to nitrite (Hernández, 2014), which could be related to the fact that the Ramón 2 site obtained the highest results in nitrites. Agricultural activities (nitrogen fertilizers and cattle manure) that are carried out in this watershed could be influencing the concentration of these parameters (Auquilla et al. 2005; Hernández, 2014).
Conclusions
In the Margaritas River, the ecological condition reported for the three sections evaluated is favorable according to the information obtained. Habitat conditions and dissolved oxygen values generate suboptimal conditions for the development of aquatic life. However, it is necessary to implement management strategies at the Margaritas 2 site, since this site has a higher electrical conductivity and a greater presence of nitrites, which indicate the degradation of this area of the river.
Obtained data provide an overview of the ecological condition of the Margaritas River in the three zones evaluated. However, it is recommended that a more extensive and intensive study be carried out, increasing the number of sites at different altitudes. It will also be important to include climatic conditions (rainfall and dryness) to complete the information generated in this research so that more concrete management strategies can be proposed for each zone of the Margaritas River.
