Table 1 shows water temperature, pH, electrical conductivity, dissolved oxygen, alkalinity and salinity. Water temperature ranged from 16.9 to 23.7°C, pH was between 6.7 and 8.3, electrical conductivity ranged from 2364 to 3282 µs/cm, dissolved oxygen ranged from 0.6 to 3.7 mg/L, alkalinity was between 11.2 and 296 mgCaCO3/L, and salinity ranged between 0.7 and 1.7‰. Water pH values are the results of the buffer capacity of limestone (Razo et al., 2004), while electrical conductivity values are the result of calcite and gypsum dissolution in the aquifer (Gómez-Hernández et al., 2020). Small variations on water temperature and dissolved oxygen were observed, however they are typical of groundwater at tropical altitudes (Caspers et al., 1981), suggesting typical environmental conditions. A high transparency, as well as a low amount of algae or aquatic vegetation and low biomass were observed in the studied water bodies, which allowed their classification as "oligotrophic systems" according to Roldán and Ramírez, (2008).
Table 1
Water temperature (T), pH, electric conductivity (EC), dissolved oxygen (DO), alkalinity (Alk), and salinity (Sal) measured in situ at two sampling campaigns (S1 and S2) at the different sampling sites. S.D.= Standard deviation.
Site | Sampling | T (°C) | pH | EC (µs/cm) | DO (mg/L) | Alk (mgCaCO3/L) | Sal (‰) |
Club de Tiro | S1 | 22.5 | 6.7 | 2762.0 | 1.3 | 219.0 | 1.4 |
S2 | 21.5 | 6.7 | 3209.0 | 0.6 | 296.0 | 1.7 |
Mean | 22.0 | 6.7 | 2985.5 | 0.9 | 257.5 | 1.5 |
S.D. | 0.7 | 0.0 | 316.0 | 0.5 | 54.4 | 0.2 |
Abrevadero | S1 | 21.1 | 7.0 | 2364.0 | 3.4 | 121.0 | 0.7 |
S2 | 20.5 | 7.5 | 2376.0 | 0.7 | 193.0 | 1.3 |
Mean | 20.8 | 7.2 | 2370.0 | 2.0 | 157.0 | 1.0 |
S.D. | 0.4 | 0.3 | 8.4 | 1.9 | 50.9 | 0.4 |
Laguna | S1 | 20.9 | 7.5 | 2423.0 | 3.6 | 90.0 | 1.2 |
S2 | 20.4 | 8.3 | 2591.0 | 0.9 | 11.2 | 1.3 |
Mean | 20.6 | 7.9 | 2507.0 | 2.3 | 50.6 | 1.2 |
S.D. | 0.3 | 0.6 | 118.8 | 1.8 | 55.7 | 0.1 |
Presa | S1 | 23.7 | 7.3 | 2892.0 | 2.6 | 103.0 | 1.6 |
S2 | 18.7 | 7.7 | 3282.0 | 0.9 | 142.0 | 1.7 |
Mean | 21.2 | 7.5 | 3087.0 | 1.7 | 122.5 | 1.6 |
S.D. | 3.5 | 0.3 | 275.7 | 1.2 | 27.6 | 0.1 |
Canal | S1 | 18.8 | 7.4 | 2528.0 | 3.7 | 127.0 | 1.2 |
S2 | 16.9 | 7.8 | 2888.0 | 1.3 | 169.5 | 1.5 |
Mean | 17.8 | 7.6 | 2708.0 | 2.5 | 148.2 | 1.3 |
S.D. | 1.3 | 0.3 | 254.5 | 1.7 | 30.1 | 0.2 |
Figure 2 shows As concentrations in water samples, which ranged between < 0.01 mg/L and 53.23 mg/L. At Club de Tiro, As concentrations were 38.98 mg/L and 53.23 mg/L, during S1 and S2, respectively. In Abrevadero, As concentrations were 1.62 mg/L and 5.91 mg/L, during S1 and S2, respectively. In Laguna, As concentrations were 1.9 mg/L and 5.3 mg/L, during S1 and S2, respectively. Canal and Presa showed the lowest concentrations of As in water with values between < 0.01 mg/L and 0.2 mg/L, respectively. More importantly, 6 out of the 10 water samples were above the maximum permissible levels of As in water for irrigation, recreation and protection of aquatic life (CCME, 2001; DOF, 1989). All contaminated water samples were collected from Club de Tiro, Abrevadero and Laguna. Furthermore, some of these samples were also above the As lethal concentration reported for zooplankton (3 mg/L) (Chen et al., 1999), highlighting a likely risk for microcrustacean life in the impacted water bodies.
As concentrations in water simples, in squares the data of sampling 1 and in triangles the data of sampling 2, additionally the maximum permissible As concentration in natural waters (0.2 mg/L) in Mexico (DOF, 1989) as well as the As lethal concentration for zooplankton (3 mg/L) are presented in dotted lines (Chen et al., 1999).
Figure 3 shows total As concentration in sediment samples for contaminated and non-contaminated sampling sites. Total As concentrations in sediment were 2472.84 ± 611.48 mg/kg at Club de Tiro, 553.48 ± 9.11 mg/kg at Laguna, and 10.37 ± 5.39 mg/kg at Presa, which largely exceed the international guideline values for As in sediments for quality purposes (5.9 mg/kg) (CCME, 1999) as well as the probable effect level of As for freshwater aquatic organisms (17 mg/kg) (CCME, 1999; EPA, 2002a). So far, up to date, no regulations are available in Mexico for sediment quality. Additionally, a high mobility of As, from the sediments, was found in this study, which accounted for 47.20% of the total As at Club de Tiro, 52.19% of the total As at Laguna, and 100% at Presa as determined by the recovery of As using 1% HNO3. Considering that As recoveries in acid digestions using concentrated acids (> 65%) are between 85 and 99.9% (Davidowski and Sarojam, 2012), As mobility at Club de Tiro and Laguna using mild acid might have been limited by the experimental conditions (solid to solution ratio and reaction time), highlighting the possibility of being higher than the values obtained in this study and the risk of the samples. We also found that the mobile As was actually As(V). No methylated As (III) species were found in sediments. This, likely due to the minerals that were suggested to control the mobility of As in the study area, which accounted for calcium arsenates (Martínez-Villegas et al., 2013). Similarly, in this study, the oxidation state of As was + 5.
Total As concentration in sediment samples, recovery percentages of As in sediment digestions using 1% HNO3 and the maximum permissible concentration of As for the protection of healthy aquatic systems (17 mg/kg) is presented in red dotted line (CCME, 1999).
Table 2 shows the ecological indicators for each sampling site and time. Abundance ranged from 0 to 25.17 org/L, richness was between 0 and 4, and Shannon index ranged from 0 to 1.24, showing that the site with the greatest species richness was Laguna. For Canal, we found absence of zooplankton likely due to the water flow that may complicate the survival of the organism in the stream of the channel. Typically, Shannon index values are between 2 and 3, values less than 2 are considered low in diversity and greater than 3 are high in species diversity (Margalef, 1972). Therefore, the sampling sites studied here exhibited low diversity, with Club de Tiro exhibiting the lowest diversity, with Shannon indexes of zero at two sampling campaigns.
Table 2
Ecological indicators obtained from microcrustaceans samples.
Site [As mg/L] | Sampling | Abundance (org/L) | Richness | Shannon Index |
Club de Tiro 38.98–53.23 | S1 | 0.32 | 1 | 0 |
S2 | 25.17 | 1 | 0 |
Abrevadero 1.62–5.91 | S1 | 1.45 | 3 | 0.49 |
S2 | 0.25 | 1 | 0 |
Laguna 1.9–5.3 | S1 | 3.91 | 4 | 0.67 |
S2 | 2.49 | 4 | 0.89 |
Presa 0.01–0.2 | S1 | 11.54 | 4 | 1.24 |
S2 | 7.88 | 2 | 1.12 |
Canal < 0.01 | S1 | 0.21 | 2 | 0.19 |
S2 | 0.00 | 0 | ND |
Table 3 shows the cladoceran (Crustacea; Branchiopoda: Ctenopoda, Anomopoda) and copepod (Crustacea: Copepoda, Cyclopoida) species inhabiting the surveyed systems in both sampling periods.
In non-contaminated sampling points (0.0-0.2 mg/L), 5 species of microcrustacean were found (Simocephalus punctatus, Alona glabra, Eucyclops leptacanthus, Macrocyclops albidus and Pleuroxus (Picripleuroxus) quasidenticulatus). In the moderately contaminated sampling points (1.61–5.91 mg/L), 4 species were found (Latonopsis australis, Pleuroxus (Picripleuroxus) quasidenticulatus, Eucyclops chihuahuensis, Acanthocyclops americanus Paracyclops chiltoni), while in the most contaminated sampling point (up to 53.23 mg/L) only one species was found (Pacacyclops chiltoni).
Additionally, two species were found to cohabite As free and contaminated water, namely Pleuroxus (Picripleuroxus) quasidenticulatus and Macrocyclops albidus. One species was found cohabiting As moderately contaminated and highly contaminated water, while 4 species were found only in non contaminated water. Large species like Simocephalus punctatus (Cladocera: Anomopoda) were the most abundant in absence of As (6.06 org/L), whereas smaller microcrustacean, like the copepod Paracyclops chiltoni (Copepoda: Cyclopoida), inhabiting the most As contaminated water, showed an abundance up to 25.17 org/L, which was, in turn, the highest abundances found among all sampling points.
Table 3
Microcrustacean species inhabiting non-contaminated and contaminated freshwater in Matehuala, San Luis Potosi, Mexico.
Level of As (mg/L) | Sampling point | Specie | Distribution* | Highlights |
0.00–0.02 | Presa | Simocephalus punctatus (Orlova-Bienkowskaja, 1998) | Canada, U.S.A., and North of Mexico (Nearctic species). | Intermediate alkalinity ranged from 100–170 mgCaCO3/L; zooplankton of longer body size with abundances ranged from 10–100 org/L. |
Presa | Alona glabra (Sars, 1901) | South America and some localities in Central Mexico. |
Presa | Eucyclops leptacanthus (Kiefer, 1956) | South and Central America, and North of Mexico. |
Presa and Canal | Pleuroxus (Picripleuroxus) quasidenticulatus (Smirnov, 1996)− | Cosmopolitan, recorded in Europe, Asia, North, Central and South America. |
Canal | Macrocyclops albidus albidus (Jurine, 1820)+ | Cosmopolitan, in North, Central and Southeastern of Mexico. |
1.61–5.91 | Laguna | Latonopsis australis (Sars, 1888) | Presumably cosmopolitan, probably a species complex. | Low alkalinity from 11 to 190 mgCaCO3/L; zooplankton of medium body size, with abundance ranged from 0 to < 10 org/L. |
Laguna | Macrocyclops albidus albidus (Jurine, 1820)+ | Cosmopolitan, in North, Central and Southeastern of Mexico. |
Laguna | Acanthocyclops americanus (Marsh, 1893) | Considered as a cosmopolitan species. |
Abrevadero and Laguna | Pleuroxus (Picripleuroxus) quasidenticulatus (Smirnov, 1996)− | Cosmopolitan, recorded in Europe, Asia, North, Central and South America. |
Abrevadero and Laguna | Eucyclops chihuahuensis (Suárez-Morales and Walsh, 2009) | Possibly endemic to a few localities in the North of Mexico. |
5.91–53.23 | Abrevadero and Club de Tiro | Paracyclops chiltoni (Thomson, 1883) | Cosmopolitan, widely distributed in North and South America. | High alkalinity from 220 to 300 mgCaCO3/L; zooplankton of short body size, with abundance ranged from 1 to 60 org/L. |
* Distribution and body size data found in the Barcode of Life Data System v4 database (BOLD, 2021). |
−,+ Species found in two sites and different concentrations of As. |
Figure 4 shows the PCA. PC1 (38.31%) combined electrical conductivity, salinity, alkalinity, As and abundance, while the PC2 (28.39%) combined pH, richness and Shannon index. These two main components explained 66.70% of the total variability of the data. Table 3 shows the parameter coefficients to each principal component.
Figure 4 and Table 4 suggest that PC1 was a factor that combined abiotic and biotic aspects related to mineral solubility and abundance of zooplankton, which was supported by a positive correlation between the level of electrical conductivity and salinity, as well as salinity and abundance and alkalinity and As (Table 5). Salinity is considered one of the most important environmental factors that shapes the biodiversity and abundance of zooplankton (Ojaveer et al., 2010; Perumal et al., 2009; Yuan et al., 2020). It has been reported that the groups of zooplankton most affected by salinity are rotifers and cladocerans, showing a decrease in their abundances and diversities (Ojaveer et al., 2010; Yuan et al., 2020). On the other hand, copepods show little effect on abundance with increasing salinity, and even positive correlations have been reported between salinity and abundance of copepods in freshwater (Perumal et al., 2009), as in the case of this work. In this study, electrical conductivity showed a relationship with salinity as a result of calcite and gypsum dissolution. In the study area, water evaporation and mineral dissolution explain the positive correlation between the concentration of As and alkalinity as well (Table 5).
PC2 was a biotic factor related to pH, which was supported by a positive correlation between pH and richness. It has been reported that, although zooplankton organisms show a high degree of tolerance to changes in pH, negative effects in abundance and richness have been related to moving away from neutrality (Echaniz et al., 2012).
PCA biplot showing PC1 and PC2 as well as the contribution variables of each component. The physicochemical and ecological variables are presented in blue and the sampling sites in red.
Table 4
| Coefficients of PC1 | Coefficients of PC2 |
Abundance | 0.36559 | 0.26023 |
Shannon | 0.21621 | 0.36870 |
Richness | -0.07763 | 0.46492 |
T | 0.05217 | -0.21034 |
pH | -0.20972 | 0.47342 |
EC | 0.44508 | 0.17796 |
DO | -0.34911 | -0.13026 |
Alk | 0.38742 | -0.31489 |
Sal | 0.41569 | 0.21547 |
As | 0.35213 | -0.34292 |
Table 5
Pearson’s correlation matrix between ecological and physicochemical parameters, showing values > 0.5 in red and <-0.5 in blue.
| Abundance | | | | | | | | |
Shannon | 0.5326 | Shannon | | | | | | | |
Richness | 0.2623 | 0.5277 | Richness | | | | | | |
T | -0.1504 | 0.0847 | 0.1367 | T | | | | | |
pH | -0.0706 | 0.1243 | 0.5687 | -0.5386 | pH | | | | |
EC | 0.8075 | 0.4491 | 0.0299 | -0.0823 | -0.1309 | EC | | | |
DO | -0.4005 | -0.2664 | -0.0354 | 0.1027 | -0.1218 | -0.5328 | DO | | |
Alk | 0.3017 | 0.1005 | -0.6239 | 0.0647 | -0.7276 | 0.4316 | -0.4161 | Alk | |
Sal | 0.5963 | 0.5067 | 0.0745 | -0.0208 | 0.0398 | 0.8536 | -0.6225 | 0.3780 | Sal |
As | 0.1966 | -0.1773 | -0.3765 | 0.3923 | -0.6856 | 0.3941 | -0.4567 | 0.7549 | 0.3429 |
In this survey, the cosmopolitan P. chiltoni probed to be a copepod highly tolerant to extremely high concentrations of As. The presence of P. chiltoni has been recorded in systems with different trophic states and in the littoral or planktonic habitats in worldwide freshwater systems (Lansac-Tôha et al., 2002). In this study, P. chiltoni was the only species inhabiting a system with more than 50 mg/L of As with Shannon Indexes of zero. That is, P. chiltoni seems to be resistant to severe stress and thrive in extreme conditions, providing additional evidence as an extremophile crustacean as previously reported in Mendoza-Chávez et al., (2021).
Based on the results from this study, 4 species inhabiting the Canal and Presa (Simocephalus punctatus, Alona glabra, Eucyclops leptacanthus and Macrocyclops albidus) may exhibit potential as likely bioindicators vulnerable to As contamination and alkalinity. Species of the genus Simocephalus, S. mixtus and S. serrulatus have been reported (Martínez-Jerónimo et al., 2008; Nogueira et al., 2008) to show great sensitivity in toxicological tests and bioassays, even with better sensitivities than the commonly used tested Daphnia. Therefore, we suggest Simocephalus puntactus as a likely bioindicador for As contamination in freshwater, genus that meet the characteristics of bioindicators, such as short life cycles, diverse feeding habits, small sizes and high reproductive rates (Chakri et al., 2014; Murugan and Sivaramakrishnann, 1973; Sharma and Pant, 1982). Especially because, unlike Daphnia, the species recommended for toxicity tests in the country's regulations, this species is distributed in Mexico (Elias-Gutierrez et al., 2008; Young et al., 2012).
On the other hand, species inhabiting water contaminated with As, such as Latonopsis australis, Pleuroxus (Picripleuroxus) quasidenticulatus, Eucyclops chihuahuensis, Acanthocyclops americanus and Paracyclops chiltoni, could be studied in the future to better understand As methylation in freshwater organisms, as well as their possible adaptations to survive.