This study was conducted from July 2023 to July 2024. The water samples were collected from the surface layer of the pond (<0.5 m) at three different sampling locations: S1 (upstream), S2 (deepest point) and S3 (downstream) as shown in Figure 1.

One-litre water samples were collected using bottles that were thoroughly cleaned and rinsed. The samples were stored at 4 °C and transported to the laboratory for analysis on the same day. The selected water quality parameters included Temperature (T), pH (Potential of Hydrogen), EC (Electrical Conductivity), Dissolved Oxygen (DO), Secchi Disk Depth (SDD), Total Suspended Solids (TSS), Total Phosphorus (TP), Soluble Reactive Phosphorus (SRP), Ammonium (NH₄), Nitrates (NO₃), Sulphates (SO₄) and Chlorophyll-a (Chl-a). Temperature, pH, EC and DO were measured on site using a Multi-parameter analyser. A 30-cm metal disk was used to measure the SDD. The analytical procedures for TSS, SRP, TP, NH₄, NO₃, SO₄ and Chl-a were performed according to standard methods [26].

In this study, the WQI was used to assess the impact of both natural and anthropogenic factors on various key parameters of the surface water chemistry in El Oulfa Pond. This index was calculated using the weighted arithmetic index method, as described by [27]. This approach involves calculating a numerical value known as the relative weight (Wi), which is assigned to each physicochemical parameter, based on eq. (1):

Here, k represents the constant of proportionality, which can also be determined using eq. (2):

Where n denotes the number of parameters, and Si – the maximum value of each parameter’s standard Moroccan threshold of surface water quality [28], expressed in mg/L for BOD5, in µg/L for Chlorophyll-a, etc.

Next, a quality rating scale Qi is determined for each parameter by dividing the concentration by the standard for that parameter and multiplying the result by 100, as illustrated in eq. (3):

Where Ci denotes the concentration of each parameter in mg/L.

Finally, the overall WQI is computed using eq. (4):

The trophic state index of Carlson [21] was determined based on the values of water transparency measured by the Secchi disc (TSI_SD) and the values of surface concentrations of Chl-a (TSI_Chl-a) and total phosphorus (TSI_TP), according to eq. (5), eq. (6) and eq. (7):

Where SD denotes Secchi disk depth [m], TP – total phosphate [μg/L], and Chl-a – Chlorophyll-a [μg/L].

The overall index (TSI) is calculated by taking the average of the values TSI_SD, TSI_Chl-a and TSI_TP, according to eq. (8):

The WQI is a numerical index that integrates various water quality parameters into a single value. It classifies water as illustrated in 2 into five categories: excellent (WQI < 25), good (25 ≤ WQI ≤ 50), poor (50 < WQI ≤ 75), inferior (75 < WQI ≤ 100), and non-potable (WQI > 100) [29]. This classification offers a simplified yet comprehensive overview of the water’s suitability for various uses.

As shown in Figure 2, the pond water was generally of non-potable quality. The mean WQI values for stations S1, S2, and S3 were 1984, 2026, and 1928, respectively. Station S3 exhibited the highest variability with a standard deviation of 549, compared to 475 for S1 and 367 for S2, indicating significant fluctuations in water quality (Table 1).

Figure 2.

Comparative map of WQI and TSI on the three different stations during the 13 months; the pie chart’s content for each slice represents a WQI and TSIcolour category

Temporal analysis (Figure 3) revealed key trends. In July 2023, WQI values ranged from 1412.62 to 2013.34, indicating moderate levels of water quality. From September to January, WQI values stabilised across all stations, ranging between 1500 and 2000, suggesting more uniform water conditions. In December 2023, a notable spike was observed at S1 (WQI = 2691.69).

Figure 3.

Temporal variation of the WQI in S1 (a), S2 (b), and S3 (c)

In March 2024, S1 recorded its highest WQI value (2761.67), while the most extreme value in the entire dataset occurred in June 2024 at S3, reaching 3755.52. Throughout the study period, S3 appeared as the most variable station, while S1 and S2 showed more consistent trends. Clear seasonal patterns were evident, with higher WQI values in warmer months (June, July, August) and stabilisation during colder periods (September to February).

Table 2 shows the results of the WQI assessment in both summer and winter seasons. During summer, WQI values ranged from 1859.69 (S1) to 3755.52 (S3), indicating severe water quality deterioration. Winter values, although somewhat lower (1155.17 to 1987.6), remained within the non-potable category. Across both seasons, all stations exceeded drinking water safety thresholds. S3 consistently exhibited the poorest water quality, while S1 recorded relatively better values.

The traditional TSI criteria, based on TP, Chl-a, and SDD, were applied to assess eutrophication in the selected reservoirs. The typical TSI ranges are 30 to 40 for oligotrophic conditions, 40 to 50 for mesotrophic conditions, 50 to 70 for eutrophic conditions, and greater than 70 for hypereutrophic conditions.

Table 3 shows the descriptive statistics of the TSI values, revealing some interesting patterns in the nutrient levels at each sampling point. From these data, it is clear that all three sampling points generally fell within the eutrophic category throughout most of the year. However, the variability in nutrient levels was particularly pronounced at S3, suggesting a more unstable trophic state there.

At S1, TSI values ranged from a low of 49.00 in June 2024 to a high of 63.58 in December 2023. The average TSI of 55.26 indicates that this site mainly remained eutrophic, with the occasional dip towards mesotrophic conditions, notably in June 2024. The relatively low standard deviation of 4.23 suggests that the nutrient levels were relatively stable at this point.

In contrast, S2 exhibited TSI values ranging from 51.48 (April 2024) to 68.34 (August 2023), with an average of 58.33 and a StDev of 4.66. This point consistently showed elevated nutrient levels, especially during the late summer months. The slight increase in variability compared to S1 hints at localised factors that may influence nutrient dynamics, leading to occasional spikes in TSI, as shown in Figure 4, such as the fact that S2 is the deepest sampling point.

Sampling point S3 stood out with a mean TSI of 59.79, showcasing the highest variability among all sites. The TSI ranged from 52.86 in May 2024 to a concerning 83.49 in April 2024, indicating hypereutrophic conditions. The high standard deviation of 8.87 further emphasises the instability at this site.

Table 4 and Figure 4 show TSI values across all points, revealing distinct seasonal trends. Notably, nutrient concentrations peaked in August 2023 (S2: 68.34), December 2023 (S1: 63.58), and April 2024 (S3: 83.49). Interestingly, the warmer month of June 2024 saw a significant decrease in TSI values, particularly at S1, which reached mesotrophic status. Significant spatial variability was also observed. S3 recorded the highest TSI in April 2024, while S1 showed more stable nutrient levels, gradually decreasing towards the end of the study period. S2 experienced moderate fluctuations but consistently maintained eutrophic conditions.

Figure 4.

Temporal variation of the TSI in S1 (a), S2 (b), and S3 (c)

The higher variability observed at S3 may reflect localised nutrient loading or specific environmental pressures disproportionately affecting this site [27]. Notably, urban discharges have been recorded in this area, further contributing to fluctuations in water quality. The sharp increase in WQI values at S2 during summer 2023 likely suggests an influx of pollutants or nutrients, potentially from anthropogenic sources such as wastewater discharges from urban stormwater outlets [30]. It aligns with existing literature that highlights enhanced nutrient loading during the summer due to increased rainfall and urban runoff [31].

The stabilisation of WQI values from September to January may be attributed to more stable hydrological conditions and reduced biological activity during colder months [32]. However, the spike observed in December 2023 at S1 could indicate a localised pollution event or internal nutrient cycling, potentially driven by organic matter degradation or redox dynamics within the pond [33].

The peak at S1 in March 2024 likely corresponds to the onset of spring, when runoff intensifies, introducing new nutrient inputs [34]. The extremely high value recorded at S3 in June 2024 (3755.52) may point to a major pollution incident or nutrient surge, potentially associated with heavy rainfall events or unmanaged discharges [30]. The seasonal trends observed support the findings of Cloern [35], who noted that warmer months promote evaporation, pollutant concentration, and heightened biological activity, while cooler months favour dilution and natural flushing [36]. The spikes recorded in August 2023 at S2 and in June 2024 at S3 likely result from intense stormwater runoff, which is a well-known contributor to nutrient enrichment in aquatic environments [37].

Overall, water quality dynamics in El Oulfa Pond are strongly influenced by both seasonal variations (temperature, precipitation) and site-specific nutrient sources, which play a crucial role in shaping the overall water conditions. This influence is reflected in the WQI results, which indicate that all three monitoring stations consistently fall within the non-potable water classification throughout the year, as illustrated in Figure 2. The combination of external environmental factors and localised nutrient inputs significantly impacts the pond’s ecological balance.

The pronounced TSI variability at S3, especially the 83.49 value in April 2024, suggests significant nutrient overload, likely exacerbated by localised inflows or runoff [38]. Rain during this period might have washed polluted water from an upstream area, where there is a known pollution source, into the lower basin. This runoff could have picked up contaminants along the way, contributing to the problem.

The brief drop at S1 in June 2024, when it reached mesotrophic status, could indicate improved water quality, possibly due to increased nutrient uptake by aquatic plants or enhanced water exchange. Nevertheless, throughout the year, TSI values remained predominantly in the eutrophic range, emphasising the pond’s vulnerability to high nutrient levels.

Spatial trends were equally telling. S1’s relatively low variability and stable average suggest a site less affected by short-term nutrient inputs. However, the peak in December 2023 could reflect seasonal factors, such as increased productivity or internal nutrient cycling [39]. Meanwhile, S2, the deepest site, showed slightly greater variability and a clear peak in August 2023, consistent with seasonal nutrient loading [40]. This observation may be linked to stratification effects or delayed mixing at depth [41]. S3, by contrast, presented the most unstable trophic state, with wide fluctuations and the highest standard deviation. The hypereutrophic peak in April 2024 was particularly alarming and suggests hotspots of nutrient input, potentially from direct runoff or nearby pollution sources [42]. Even the minimum value at S3 in May still reflected eutrophic conditions, confirming that the site is consistently under stress. These results indicate that all sampling points generally remained within the eutrophic range throughout the year [43], with occasional excursions into mesotrophic or hypereutrophic conditions as shown in Figure 2.

Overall, seasonal dynamics played a key role, with peak nutrient levels aligning with months of heightened biological productivity (e.g., August, April, and December). The temporary improvement observed in June might be due to plant uptake or water turnover. Still, the persistent eutrophic status confirms the ongoing challenge of nutrient enrichment in El Oulfa Pond.