Cerrillos Reservoir is one of the largest reservoirs in south Puerto Rico. It is used for drinking water, recreational activities and provides flood control. Cerrillos Reservoir dam is situated in the municipality of Ponce in southern Puerto Rico (Figure 1). The construction of the Cerrillos dam began in 1984 and was

completed in 1991 by the US Army Corp of Engineers [19]. The dam is a rockfill structure with a height of 98.45 meters and a length of 473.96 meters [19]. The reservoir receives streamflow from the Cerrillos River and an unnamed creek that enters Cerrillos Reservoir [19]. Cerrillos Reservoir had an original storage capacity of 38.03 million cubic meters at maximum conservation pool elevation of 174.65 meters above mean sea level and a drainage area of 174.65 square kilometers [19]. The available capacity of this reservoir is rapidly diminishing due to sedimentation.

Water samples were collected in triplicate at surface from the inlet, center, and outlet of the Cerrillos Reservoir (Figure 1). The inlet corresponded to the entry point of the main river that provides water to the Cerrillos Reservoir. The center was located at a midpoint of the reservoir, at the deepest part of the reservoir. The outlet was located near the dam. Sampling occurred once or twice per month from May 2018 to January 2019. Phosphorus (P) (Soluble Reactive Phosphorus (SRP)), Nitrogen (N) (nitrate ( NO 3 − ), nitrite ( NO 2 − ) and ammonia (NH₃)), temperature (˚C), turbidity (cm), conductivity (µS/cm), pH and dissolved oxygen (DO, mg/L) were measured in the field for each sampling event. Instruments used to measure these parameters were a HACH SL1000 portable colorimeter for SRP, NO 3 − , NO 2 − , NH₃, and pH; HACH 2100Q for turbidity. Temperature, conductivity and DO were determined with a YSI 556 Water Quality Sonde. A Garmin GOS Oregon 650T was used for sample site identification.

Rainfall data were recorded from June 2018 to January 2019 with a rain gauge located at the Department of Natural and Environmental Resources office at the Cerrillos Reservoir. All samples for biological parameters (total coliforms and Escherichia coli) were collected in glass bottles and stored at a low temperature and analyzed within 24 hours using the Colilert test and Quanti-Tray System. The most probable number (MPN) was determined for total coliforms while E. coli was assessed as presence/absence.

Mean SRP, nitrate, nitrite, ammonia, temperature, turbidity, conductivity, pH and DO values were calculated for each sample (triplicate sampling, n = 3 for each sample). ANOVA (Analyses of Variance) was used to determine significant differences between SRP, nitrate, nitrite, ammonia, temperature, turbidity, conductivity, pH and DO for the three sampling points of Cerrillos Reservoir. Pearson Correlation analyses were used to evaluate the possible correlations of SRP, nitrate, nitrite, ammonia, total coliforms with rainfall one week and two weeks prior sample collection. We also used Pearson Correlation to evaluate the possible relation of total coliforms with nutrient concentrations. Results were considered significant when a p-value was less than 0.05.

Monthly rainfall (mm) for Cerrillos Reservoir recorded from June 2018 to January 2019 (Figure 2) indicated heavy rainfall during the month of August and allowed identification of specific rainfall events that occurred one and two weeks prior to water sample collection.

Physicochemical parameters (Table 1) at Cerrillos Reservoir showed significant differences between temperature and turbidity for the three sampling points at the Cerrillos Reservoir (One Way ANOVA: p = 0.000 and p = 0.005, respectively) (Table 2). The inlet had the highest turbidity (6.1 NTU) and the lowest

(^a) Temp: Temperature was measured with a YSI 556 Water Quality Sonde; (^b) Turbidity: The turbidity was determined with a HACH 2100Q; (^c) Conductivity: Conductivity was determined with a YSI 556 Water Quality Sonde; (^d) pH: pH was determined with a HACH SL1000; (^e) DO: Dissolved oxygen was determined with a YSI 556 Water Quality Sonde.

temperature (25.0˚C). For conductivity, pH and DO there were no significant differences between sampling points (One Way ANOVA: p = 0.423, p = 0.090 and p = 0.602, respectively). Values for pH are considered optimal (according to regulatory guidelines) on all but two occasions, when the pH at the inlet sampling point was extremely acid (5.7 (July 24, 2018) and 1.6 (August 22, 2018)). Mean DO for the three sampling points ranged from 5.0 to 8.9 mg/L. Mean temperatures for all locations ranged from 23.0˚C to 28.6˚C, and conductivity ranged from 172 to 271 (µS/cm).

SRP (Soluble Reactive Phosphorus) ranged between 37 µg/L and 243 µg/L (Figure 3). SRP showed significant differences for the three sampling points (One Way ANOVA: p = 3.94 and p = 0.028, respectively) (Table 3). Nitrite concentrations were below the HACH SL1000 range (0.005 - 0.6 mg/L or 5 - 600 µg/L), except for one occasion in the center of Cerrillos Reservoir (May 15, 0.334 µg/L). Nitrate and ammonia ranged between 0 µg/L to 6417 µg/L and 50 µg/L to 540 µg/L, respectively (Figure 4 and Figure 5). For 10 samples, ammonia concentrations were below the instrument’s (HACH SL1000) range (0.05 - 1.5 mg/L or 50 - 1500 µg/L). Neither nitrate nor ammonia showed significant differences for the three sampling points (One Way ANOVA: p = 0.14 and p = 0.057) (Table 3). We do not have nitrate data for May 15 and May 29.

Neither SRP nor ammonia were significantly correlated with rainfall events at Cerrillos Reservoir (inlet, center, outlet) (Table 4). Nitrate at the outlet was significantly correlated with rainfall both one week before sampling, and two weeks before sampling (r = 0.859, p = 0.000 for one week and r = 0.798, p = 0.001 for two weeks, respectively) (Table 4). Total coliforms at inlet, center and outlet were not significantly (p > 0.05) correlated with rainfall at one or two weeks prior to sample collection.

Mean concentrations of total coliforms range between 23 MPN/100 and 994

MPN/100 (Table 5). All water samples showed presence of E. coli (Table 5).

Total coliforms were not significantly correlated with SRP, nitrate or ammonia (p > 0.05 for each, Table 6).

Before hurricane Maria devasted Puerto Rico, the Puerto Rico 2016 305(b) and 303(d)Integrated Report indicated that the Cerrillos Reservoir was impaired by low dissolved oxygen and pH [16].

^aSoluble Reactive Phosphorus; ^bPearson correlation coefficient; ^cp-value for the correlation.

^aMost probable number.

^aSoluble Reactive Phosphorus; ^bPearson correlation coefficient; ^cp-value for the correlation.

During our study, Cerrillos Reservoir did not exceed the parameters of temperature (22.7˚C - 28.6˚C), turbidity (1.0 NTU - 10.2 NTU), pH (1.6 - 8.37) nor DO (5.0 to 8.9 mg/L), established by the Puerto Rico Department of Natural and Environmental Resources (PRDNER). Water Quality Standards Regulations set by the PRDNER establish limits for surface waters to not exceed 30˚C of temperature, not exceed 50 NTU for turbidity, maintain a pH range of 6.0 to 9.0 and a DO of 5 mg/L or more [20]. PRDNER does not establish limits for conductivity but if for TDS (not exceed 500 mg/L).) Conductivity increases with increasing ion content, which means that in most cases it provides a good approximation of TDS using the conversion factor of 1 mg/L (ppm) = 2 µS/cm [21]. Thus, the conversion of conductivity to TDS revealed a range of 121 mg/L (ppm) to 130 mg/L (ppm), which does not exceed the limit of 500 mg/L.

SRP (inorganic P) ranged between 37 µg/L and 243 µg/L in the reservoir’s surface water. Under natural conditions SRP concentrations usually are equivalent to about 5% - 20% of total P concentration [22]. Total phosphorus concentrations include organic and inorganic P. Total phosphorus shall not exceed 26 µg/L in any reservoir or lake according to PRDNER [20]. Thus, we conclude that Cerrillos Reservoir showed a general behavior of eutrophic/hypereutrophic because total phosphorus concentrations of eutrophic and hypereutrophic lakes are between 30 to 100 µg/L and > 100 µg/L, respectively [22].

Nitrate concentrations ranged between 0 µg/L to 6417 µg/L, and nitrite concentrations were below the instrument’s (0.005 - 0.6 mg/L or 5 - 600 µg/L), except for one occasion in the reservoir’s (May 15, 0.334 µg/L). Nitrite concentrations in the reservoir are low, and the presence of high dissolved oxygen (DO) may be oxidizing to nitrate. Ammonia range between 50 µg/L to 540 µg/L, respectively. In general, the level of lake productivity due to inorganic nitrogen includes ultra-oligotrophic (<200 µg/L), oligo-mesotrophic (200 - 400 µg/L), meso-eutrophic (300 - 650 µg/L), eutrophic (500 - 1500 µg/L) and hypereutrophic (>1500 µg/L) [22]. Cerrillos Reservoir showed a general behavior of meso-eutrophic/eutrophic in relation with inorganic nitrogen. However, on two occasions we observed concentrations above 1500 µg/L (or hypereutrophic) for inorganic nitrogen. Sources of nitrogen in surface water include: 1) precipitation falling directly onto the lake surface, 2) nitrogen fixation both in the water and the sediments, and 3) inputs from surface and groundwater surface [22].

N and P concentrations in Cerrillos Reservoir were not sufficient to disrupt the balance of this ecosystem. This lake does not experience algal blooms, excessive aquatic plant growth or oxygen depletion. The deep lakes are less productive [23] and the hydrodynamic allowing the transport of sediments, toxics, and nutrients through Cerrillos Reservoir Lake geometry, vertical stratification, hydrological conditions, and meteorological conditions are factors that can influence the balance of ecosystems [23].

Reservoirs can be classified as riverine, transition and lacustrine zones [23]. The riverine zone is narrow, well mixed and shallow allowing an aerobic environment [23]. Also, in this zone advective forces transport significant quantities of finer particles, sediments, toxic substances and nutrients and light penetration is minimal [23]. In our study, the inlet (or the riverine zone) receives the input of the main river that provides water to the Cerrillos Reservoir, so it is expected that the natural flow regime affects other environmental factors including pH, turbidity, temperature and SRP.

The total coliforms exceeded (23 MPN/100mL - 994 MPN/100mL) the parameter established for the Puerto Rico Water Quality Standards (PRWQS). For total coliforms, the PRWQS establishes that the median of the waters taken sequentially, shall not exceed 14MPN/100mL, and not more than 10 percent of the samples shall exceed 43 MPN/100mL [20]. Total coliform concentrations are considered to be hygiene indicators and in surface water imply the presence of bacterial genera Enterobacter, Escherichia, Klebsiella, and Citrobacter among others [24]. All water samples showed presence of E. coli, an indicator of recent fecal pollution [25] that can cause human diseases. Although, total coliforms were not significantly (p > 0.05) correlated with nutrients (SRP, nitrate and ammonia) it is important to note that the nutrients dissolved in the water help the microorganisms synthetize their main biomolecules, obtain energy and growth. After hurricane Maria, many reports presented an increase in conditions related to untreated water, including vomits, diarrhea, conjunctivitis, scabies, asthma, and leptospirosis [14].

SRP, ammonia and total coliforms were not significantly (p > 0.05) correlated with rainfall events at Cerrillos Reservoir (inlet, center, outlet). During the study, Cerrillos Reservoir and its surrounding did not receive extreme precipitation events (more than 78 mm of rain per 24 hours in Puerto Rico (3 inches/24hours) [26]. Nitrate at the outlet was significantly correlated with both rainfall at one week before sampling, and two weeks before sampling (r = 0.859, p = 0.000 for one week and r = 0.798, p = 0.001 for two weeks, respectively) due to nitrates are characterized by a high degree of mobility.

After hurricane Maria, the Puerto Rico 2018 305(b) and 303(d) Integrated Report presented that the causes of impairment at the Cerrillos Reservoir were low dissolved oxygen and Total Phosphorus [27]. The same report for 2020, added Total Nitrogen to the list of causes of impairment at the reservoir [17]. We conclude that after hurricane Maria, nutrient (phosphorus and nitrogen) concentrations at Cerrillos Reservoir have increased.