Within the framework of this study, eight measurement stations (P1 to P8) were defined (Table 1).

Table 1. Sampling stations.

The survey was carried out in February 2025 (09:00 - 12:00, low tide). The parameters measured in situ comprised: transparency (Secchi disk), temperature, pH (WTW 340i), conductivity, salinity, turbidity, TDS, dissolved oxygen (DO) and the O₂ saturation rate (AQUAREAD AP-2000 multiparameter probe). Laboratory analyses covered BOD₅ (OxiTop respirometric method), COD (ISO 15705:2002, closed-tube method) and TSS (filtration and differential weighing). Phytoplankton was sampled using a 20 µm mesh plankton net (30 cm radius opening) by a single 10 m horizontal tow at each station. The tow distance was used to calculate the volume of water filtered, allowing phytoplankton abundance to be expressed as individuals/L. The concentrated sample (250 mL) collected in the cod end was preserved with 5% Lugol’s solution, and phytoplankton taxa were identified by microscopic examination using standard taxonomic keys.

Water quality was assessed against the thresholds of the SEQ-EAU (Water Quality Assessment System; [15]), presented in Table 2. Phytoplankton abundance (A, individual/L) was computed as:

where n is the number of individuals counted in 1 mL, V₁ the total volume filtered by the net (L), and V₂ the concentration volume (mL). The Shannon-Weaver index:

Pielou’s evenness:

and Simpson’s dominance index:

where S is the number of species, were calculated per station. The frequency of occurrenceF:

(where F_i is the number of samples containing species i and F_t the total number of samples taken) was calculated for each taxon. Following the scale proposed by [9], four species groups were distinguished according to the value of F: (75% - 100%) permanent species; (50% - 74.9%) frequent species; (25% - 49.9%) occasional species; (0% - 24.9%) rare species. The D/C (diatom/cyanobacteria) ratio served as a trophic indicator, with values > 1 indicating diatom dominance, whereas values < 1 suggest cyanobacterial dominance and conditions potentially associated with eutrophication ([13]). PCA was performed to explore the relationships between physico-chemical variables and the abundance of the main taxa.

Table 2. SEQ-EAU water-quality assessment grid ([15]).

4.1.1. Temperature, Salinity and TDS

Figure 2 illustrates the distribution of the four mineralisation parameters along the spatial gradient.

Source: Field data 2025.

Figure 2. Distribution of temperature, conductivity, salinity and TDS.

Temperature was relatively homogeneous in space, averaging 30.42 ± 0.69˚C. Values ranged from a minimum of 29.08˚C at P4 to a maximum of 31.2˚C at P2, reflecting stable thermal conditions. Conductivity averaged 20,661 ± 7598 µS/cm, with a minimum of 5660 µS/cm at P8 and a maximum of 30,760 µS/cm at P4, following a decreasing gradient P4 > P1/P2 > P7/P6 > P5 > P3 > P8. Salinity followed the same trend, averaging 12.80 ± 4.22 PSU, with a minimum of 5.66 PSU at P8 and a maximum of 19.33 PSU at P4, indicating a marked marine influence at the downstream stations (P1 - P4) and a predominance of fresh water at the upstream stations (P7 - P8). TDS confirmed this trend, averaging 10,285 ± 3781 mg/L, with a minimum of 2821 mg/L at P8 and a maximum of 15,311 mg/L at P4.

4.1.2. SEQ-EAU Quality Parameters: pH, Dissolved Oxygen, Organic Load, Turbidity, TSS and Transparency

Figure 3 presents the parameters assessed against the SEQ-EAU grid (pH, DO, saturation, transparency, turbidity, TSS, COD, BOD₅).

Source: Field data 2025.

Figure 3. Distribution of physico-chemical parameters with SEQ-EAU rating.

pH averaged 8.01 ± 0.32, with a minimum of 7.45 at P1 and a maximum of 8.50 at P8; values remained within the “very good to good” quality class according to SEQ-EAU. In contrast, the oxygenation status was of concern: DO averaged 2.62 ± 1.06 mg/L, with a minimum of 0.03 mg/L recorded at P3 and a maximum of 3.19 mg/L at P2. All values fell within the “critical” zone, except P3, P5, P6, and P8, which nevertheless remained very close to the critical threshold. The saturation rate followed the same trend (mean: 35.04 ± 12.22%; min: 12% at P1; max: 45% at P3), highlighting a generalised hypoxic stress liable to threaten the survival of aquatic fauna.

Transparency averaged 91.38 ± 33.41 cm (min: 37 cm at P6; max: 132 cm at P1), corresponding overall to a “fair” quality according to SEQ-EAU; the Secchi disk reached the bottom at stations P2, P3, P6 and P7. Turbidity averaged 6.94 ± 1.87 NTU (min: 5.50 at P2; max: 10.90 at P8) and TSS averaged 0.19 ± 0.15 mg/L (min: 0 at P1; max: 0.467 at P4), both of which were rated “very good”.

COD averaged 338.63 ± 55.70 mg/L (min: 248 mg/L at P2; max: 409 mg/L at P1) and BOD₅ averaged 263.88 ± 44.24 mg/L (min: 184 mg/L at P2; max: 318 mg/L at P1), both in the “critical” zone, with stations P1 (318 mg/L) and P4 (313 mg/L) being the most heavily impacted.

Table 3 presents the taxonomic composition, species richness, and occurrence of phytoplankton taxa.

Table 3. Taxonomic composition, species richness and occurrence of phytoplankton taxa.

Source: Field data 2025. (+) = presence; (−) = absence; N (%) = frequency of presence; PS: permanent species; FS: frequent species; OS: occasional species; RS: rare species.

The phytoplankton inventory (Table 3) records 46 species distributed across 4 classes and 24 families. The Diatomophyceae (diatoms) constituted the most diverse class, with 28 species belonging to 16 families (60.9% of total richness). The Cyanophyceae comprised 9 species in 3 families; the Chlorophyceae 6 species in 3 families; and the Dinophyceae 3 species in 2 families. Species richness per station ranged from 6 species at P1 and P2 to 29 species at P8.

The occurrence analysis distinguished four categories (Table 3). Rare species totalled 17: 6 Chlorophyceae, all confined to P8; 2 Cyanophyceae; and 9 Diatomophyceae. Occasional species numbered 8: 2 Cyanophyceae, 5 Diatomophyceae and 1 Dinophyceae. Frequent species formed the richest group, with 16 species: 4 Cyanophyceae, 11 Diatomophyceae and 1 Dinophyceae. Permanent species numbered 5, all of them diatoms.

Figure 4 illustrates the variations and distribution of phytoplankton abundance by class and station.

Analysis of the figure reveals a spatial heterogeneity of abundance. Total abundance amounted to 777 individuals/L, but this value masks an extreme concentration at station P8 which, with 576 individuals/L, accounted on its own for 74.1% of total abundance. At the opposite extreme, P1 (6 individuals/L) and P2 (8 individuals/L) displayed the lowest densities. The intermediate stations P3 - P7 showed moderate and relatively homogeneous abundances (19 to 50 individuals/L).

The overall distribution reveals a strongly unbalanced community structure: Cyanophyceae dominated with 558 individuals/L (71.8% of total abundance), driven essentially by the family Oscillatoriaceae and more particularly by Lyngbya sp. (481 individuals/L at P8 alone, i.e. 63.45% of total abundance). The Diatomophyceae constituted the second group, with 194 individuals/L (25.0%), concentrated mainly at stations P3 - P6. The Chlorophyceae (17 individuals/L, 2.2%) were present exclusively at P8, probably favoured by the low salinity and the nutrient richness of that site. The Dinophyceae, with only 8 individuals/L (1%), remained marginal and limited.

Source: Field data 2025.

Figure 4.Distribution of abundance by class and station.

Figure 5 presents the three ecological indices (Shannon H', Pielou J', Simpson D) calculated for each station to assess diversity.

Source: Field data 2025.

Figure 5.Shannon (H'), Pielou (J') and Simpson (D) indices by station.

Phytoplankton diversity decreased progressively from P1 to P8, revealing an increasing eutrophication gradient that culminated in extreme cyanophyte dominance and a severe ecological imbalance at the global scale. The Shannon-Weaver index (H') ranged from 1.22 bits at P8 to 4.01 bits at stations P5 and P6. Pielou’s evenness (J') varied from 0.22 (P8) to 0.73 (P5, P6). These values show that the downstream stations P1–P2 already exhibited a degree of imbalance, amplified to an extreme at P8 (J' = 0.22). Only stations P5 and P6 approached a relative equilibrium, benefiting from a functional community structure. Simpson’s dominance index (D) conveyed the same reality inversely: it rose from 0.25 (P2) to 0.80 (P8), indicating that 80% of the individuals at P8 belonged to a handful of dominant species.

Figure 6 presents the D/C ratio and the compared abundances by station.

Source: Field data 2025.

Figure 6.D/C ratio by station (left) and compared abundances (right).

The D/C (diatom/cyanobacteria) ratio indicates a progressive shift from diatom dominance at stations P1 and P2 (D/C = ∞) to cyanobacterial dominance from P5 onwards, reaching a minimum value of 0.07 at P8. This pattern suggests conditions potentially associated with eutrophication, although this inference is based solely on phytoplankton community composition, as nutrient concentrations and chlorophyll a were not measured. The observed trend is supported by the absolute abundances, where station P8 stands out with 521 individuals/L of Cyanophyceae against only 38 individuals/L of diatoms.

Figure 7 presents the results of the PCA performed on the full set of physico-chemical and biological variables of the eight stations.

Source: Field data 2025.

Figure 7. Principal component analysis (PCA).

The PCA explains 60.85% of the total variance and reveals two environmental gradients structuring the distribution of the stations. Axis 1 (40.63%) opposes a mineralisation pole (conductivity, salinity and TDS) associated with stations P1 and P2 to an algal-proliferation pole grouping pH, turbidity, Cyanophyceae and Chlorophyceae, confirming that salinity is the main abiotic factor controlling the structure of the phytoplankton assemblage. Axis 2 (20.22%) reflects a gradient of oxygenation and water clarity, distinguishing the life-favourable conditions (temperature, transparency, DO) of stations P1 and P2 from the more turbid conditions associated with Diatomophyceae, Dinophyceae and TSS that characterise stations P3 and P4. The proximity of the organic-load variables (BOD₅, COD) to the origin indicates a homogeneously high organic pollution across the whole section. In the individuals’ plane, P8 isolates itself radically at the far right of Axis 1 as a major cyanobacterial hotspot, while P1 and P2 occupy the opposite pole and P3 and P6 form an intermediate, diatom-structured group, confirming the existence of a continuous and progressive ecological gradient along the lagoon section.