There are still gaps in information about the exact location of the recharge sites for the water that emerges in the park. García-Becerra and García-Estrada (2009) suggest the injection of trace elements in possible sites as an alternative to specify the trajectories of groundwater flows. But the use of this method is controversial when it comes to water for urban use (Kreye et al., 1996) so other approaches should be used. These studies would also be useful to quantify the contribution percentages of the different infiltration sites and therefore to establish priorities in the establishment of protection and maintenance programs.
Physicochemical of water
Temperature. The water temperature variations during the study months were not statistically significant (P = 0.1128). Even so, a trend similar to what had been hypothesized was observed, but with a subsequent lag of one to two months: the maximum temperature of the stream occurred in July. The meteorological records of maximum atmospheric temperature in the GMA of the last 50 years indicate that 80% of the cases occurred in the months of April and May.
Temperature & pH. The inverse behavior of the temperature and pH showed at Figs. 2 and 3 respectively, is in accordance with the hypothesis of increase of the metabolism at the warmer season (Benjumena-Hoyos et al., 2014); the increase of the metabolism of benthic microorganisms, mainly decomposers, liberate carbon dioxide that transforms to carbonic acid. A consequence of the trend to acidify the sediments and the rest of the aquatic environment is the availability of nutrients, as will be shown in the section related to the algal growth in control site water.
Dissolved oxygen & limiting nutrient. The results of temperature and dissolved oxygen showed a parallel behavior during the study months (Figs. 2 & 4), which is striking, because from the physical point of view the dissolved oxygen levels must decrease with an increase in temperature (Goldman & Horne, 1983). Taking into account that throughout the study the limiting nutrient was phosphorus, the explanation for the increase in O2 levels was due to the increase in algal productivity, propitiated by the interaction of the decrease in pH (Fig. 3), and the phosphorus bioavailability (Boyd, 1979). The lack of correspondence between dissolved oxygen levels and water temperature, contrary to what had been hypothesized according to Henry's Law, is another indicator of biological dynamism in the stream. Due to the geographical location of the stream in the intertropical zone, we believed that the limiting factor would be nitrogen (Ramos-Higuera et al., 2008), which could be true under natural conditions; but the constant dumping of the excess volume of the “lago de aves” during the rainy season causes nitrogen to be present in abundance in the excreta of the ducks, giving rise to phosphorus behaving as the element that governs the dependent primary production of photosynthesis.
Availability of nutrients. The behavior obeys the aerobic model of consumption and degradation of organic matter, followed by the conversion of ammoniacal nitrogen to its nitrate form (Gaudy and Gaudy, 1980); these processes are mediated by invertebrates and microorganisms in the sediment. The monthly averages of NH4+ (0.024 mg/L) were lower than those of NO2− (0.026 mg/L) and in turn these were lower than those of NO3− (6.516 mg/L) which reveals an efficient microbial action of oxidation of nitrogen and explains the reason why this nutrient was limiting in none of the months. The low values of ammoniacal nitrogen and high nitrites, which were detected in the months of May and June, can be explained by the alkaline pH values that occurred in those months, which favored the bacterial activity of Nitrosomonas in ammonium oxidation (Rivera-Gutierrez, 2016). Regarding the availability of nutrients in general, it had been hypothesized that it would be greater in the warm season due to the greater metabolic activity in the system (Benjumea-Hoyos et al., 2014). The discussion will focus on phosphorus, which turned out to be the limiting nutrient and therefore the one that regulates the production based on photosynthesis: the nutrient had its highest concentrations in the first two months of sampling (0.246 mg/L) and from then on it showed a gradual decrease until October (0.137 mg/L), which is explained by the dilution effect that occurs during the rainy season. The concentrations of nitrogenous nutrients and phosphate in La Culebra stream are within national and international standards for the use of clean water (see Table 5).
Table 5
Summary of nutrients and coliforms of the present study with reference to national official documents.
Water uses | Nitrates (as N) mg/L | Nitrites (as N) mg/L | NH4+ mg/L | Phosphates mg/L | Fecal Coliforms MPN/100 mL | Total Coliforms MPN/100 mL | Source |
“La Culebra” stream | 1.35 1.58 | 0.003 0.018 | 0.001 0.039 | 0.11 0.26 | ≥ 1800 | ≥ 1800 | This study |
National Standards | 10 | 1 * | 0.5 | - | Not detectable | Not detectable | CONAGUA, 2016 |
Water for use and human consumption | 10 | 0.05 | 0.5 | - | Not detectable | 2 | NOM-127, 1994 |
Water for urban public use | 5 | 0.05 | - | 0.1 ** | 1000 | - | CDHCU, 2018 |
Water for agricultural irrigation | - | - | - | - | 1000 | - | CDHCU, 2018 |
Clean water supply | 5 | 0.05 | 6 | 0.1 | 100 | - | CCA, 1989 |
Recreational with primary contact | 6 | - | 6 | 6 | 1 | - | CCA, 1989 |
Agricultural irrigation | 6 | - | 6 | 6 | 1000 | - | CCA, 1989 |
For livestock | 90 | - | 6 | 6 | 6 | - | CCA, 1989 |
Clean water | 50 | 3 | - | - | Not detectable | Not detectable | WHO, 2017 |
Algal growth in control site water. The bioassay with the microalgae allowed the evaluation of the productive potential of the waters in the stream. As shown in Table 4, in group 2 the site water was evaluated each month. Although the production levels, indirectly measured by fluorescence units (FU), presented the lowest values in relation to the other experimental groups, the results are sufficient to evaluate the behavior through the months. The working hypothesis indicated that the growth of the microalgae would be greater in the warm season due to the increase in the metabolism of the degrading organisms and the greater availability of nutrients (Benjumea-Hoyos et al., 2014). The results are compatible with the hypothesis: there was an oscillatory increase starting in May and reaching the maximum value in September. In the month of May it began with a value of 66 FU followed by a decrease in June to 53 FU. In July it rose to 78 FU and in August it fell to 68 FU. In September the maximum value of 97 FU was achieved and from there was a decrease to 51 FU in October. In Fig. 2 it can be seen that the maximum temperature of the water in the stream was maintained in the interval from June to September, favoring the processes that lead to algal growth. In October there was a decrease in temperature and in turn a proportional drop in algal growth.
Inhibiting factors. The evaluation of inhibition factors through the use of EDTA as a chelating agent was a decision based on the protocol of Miller et al., (1978) and we really did not expect to find this type of evidence, considering the apparently good state of the ecosystem. In the results of Group 6 (site water + EDTA) in Table 4, the favorable and proportional effect of algal growth is observed in time: in May a value of 92 FU was obtained followed by a value of 101 FU in the month of July and finally a value of 123 FU in the month of September. Since the additions of EDTA were always constant (1.00 mg/L of Na2EDTA), it can be assumed that the inhibition agent decays over time, possibly due to the action of dilution through the rainy season. This same line of thought suggests that the agent causing the inhibition is not caused by the runoff, but rather an underground contribution. The Miller et al. (1978) protocol is not designed to characterize the agent causing the inhibition; The most that could be suggested is that it is an ionic agent, possibly metallic, which is why the present study sets the guidelines for subsequent analyzes that allow elucidating both the identity and the concentration of the inhibiting agent. The same study should be of interest to the local authorities in charge of extracting clean water from the Colomos Forest and distributing it to the surrounding neighborhoods.
Crustaceans. The higher mortality of H. azteca found in the group with natural sediment from the stream was contrary to what had been hypothesized, but it is explained by the respiration of the benthos community: during the determinations of the % survival of the amphipods, the presence of of a community of macroinvertebrates such as: annelids, gastropods, copepods and flatworms. Benthic community respiration is evident by lower oxygen levels in the sediment group (3.93 mg/L) compared to the sand substrate group (5.23 mg/L). The discovery of a dynamic community of macroinvertebrates in the sediment opens a line of research that had not been proposed before for La Culebra stream. Rodríguez-Barrios et al., (2011) in their study on functional groups point out the need to integrate information to obtain ecological models on the structure of the community and the role of macroinvertebrates in fluvial systems. The role of macroinvertebrates in the sediment is associated with that of the mineralization of organic matter (Oseguera et al., 2016).
Coliforms. The standards for evaluating the sanitary quality of water in Mexico require the quantification of total and fecal coliforms (CONAGUA, 2016a, Barrera-Escorcia et al., 2013), and the Federal Law of Rights (CDHCU, 2018) establishes as permissible limits the values of 1000 NMP/100 mL, of fecal coliforms for urban public use and for agricultural irrigation. The rules are different in each country; In Europe, intestinal enterococci are also evaluated (DOUE, 2006), which are associated with warm-blooded organisms and therefore help to discriminate the origin of the contamination. According to the guide for the assessment of the quality of drinking water of the World Health Organization (WHO, 2017), coliform organisms are excreted by both humans and animals and have the ability to live and multiply in natural waters, especially when there is presence of biofilms; For this reason, it is indicated that they are not useful to evaluate contamination by fecal pathogens and that their use as indicators is more valuable to evaluate the cleanliness and integrity of water distribution systems.
Due to the fact that there is usually a positive correlation between sewage discharges and nutrient concentrations (Burford et al., 2012), and to the low values of nitrogen and phosphorus in the water, we believe that the origin of the coliforms detected in La Culebra stream is originated by the fauna present in the park, especially by birds, and by the presence of biological conditions that favor their permanence, such as the benthic community of the periphyton, especially present in the upper part of the stream.
To complete the sanitary characterization of the stream, it is suggested to make specific studies of enteric bacteria of importance, such as Salmonella and Shigella. Studies should be done on groundwater to rule out leachates originated from broken drainage pipes and also from biofilm scrapings to confirm the new hypothesis about the natural origin of coliforms.