Ecological Quality Assessment of Mediterranean Wadis in the Bizerte Lagoon Catchment (Northern Tunisia) Using Benthic Diatoms

Abstract

The study presents an integrated assessment of water quality in wadis of the Bizerte Lagoon catchment (Northern Tunisia), using benthic diatom assemblages. Seasonal sampling was conducted between winter 2023 and winter 2024 in three wadis exposed to varying anthropogenic pressures. In each wadi, upstream and downstream stations were surveyed to assess spatial variability. Diatom assemblages showed clear responses to both environmental gradient and seasonal variation. Pollution-tolerant taxa, particularly Navicula veneta, Gomphonema parvulum, and Nitzschia inconspicua, were mainly recorded during winter in downstream stations of wadis most impacted by nutrient enrichment and organic pollution. halophilic and marine-related taxa, including Gyrosigma kuetzingii, Navicula salinarum, Navicula simulata, Halamphora coffeaeformis and Haslea ostrearia, dominated during warmer seasons in the high-conductivity wadi. Pollution-sensitive taxa, such as Encyonema minutum, Cymbella affinis and Gomphonema affine, were restricted to the least disturbed wadi. Diatom-based indices confirmed the predominance of hypertrophic conditions in all wadis and revealed a clear gradient of environmental degradation, with ecological status ranging from moderate to poor and bad quality. These findings highlight the strong potential of benthic diatoms and their ecological indices as valuable tools for monitoring eutrophication and salinization gradients in Mediterranean coastal wadis, although their adaptation and calibration to Tunisian freshwater ecosystems still require further validation.

1. Introduction

Freshwater ecosystems are among the most productive environments on Earth, providing essential ecosystem services, such as biodiversity support, nutrient cycling and water resources for human activities [1,2]. Unfortunately, these ecosystems are increasingly threatened by rapid population growth, overexploitation of surface and groundwater reserves, and the discharge of untreated pollutants into the environment [3,4]. These pressures have significantly altered the physical, chemical, and biological properties of freshwater systems, often resulting in eutrophication, oxygen depletion, and major changes in aquatic community structure [3]. Moreover, climate change is exacerbating the existing pressures on freshwater resources through shifts in precipitation regimes and the increasing frequency of extreme events such as droughts and floods [5]. Consequently, freshwater availability is expected to decline further, intensifying global water scarcity. It is projected that, from 2025 onward, nearly two-thirds of the world’s population could experience conditions of water stress [6,7]. In this context, the assessment of water quality has become essential for detecting ecological alteration, identifying the sources of pollution, and supporting the development of sustainable management and restoration strategies [3,8]. To address these growing challenges, a wide range of tools has been developed for freshwater monitoring and ecological assessment, including physical and chemical analyses and biological indicators.

Benthic diatoms are among the most widely used bioindicators for water quality assessment due to their high sensitivity to environmental changes and their rapid response to variations in nutrient levels, organic pollution, salinity, conductivity, pH, and hydrological conditions [9,10,11,12,13]. Their short life cycle, high species diversity, and close attachment to substrates allow them to reflect local ecological conditions with great precision [9,10]. Moreover, diatom communities reflect the cumulative effects of multiple environmental stressors over time, which makes them particularly effective indicators of eutrophication, organic enrichment, and anthropogenic disturbance. Consequently, the analysis of benthic diatom assemblages provides a reliable tool for evaluating the ecological status of freshwater ecosystems and supporting water management strategies [10,13,14]. Furthermore, several diatom-based ecological indices can be calculated from species composition and relative abundances. Among the most widely used are the Specific Pollution Sensitivity Index (SPI), the Biological Diatom Index (BDI), and the Trophic Diatom Index (TDI). These indices integrate species-specific ecological preferences and sensitivities into standardized metrics of water quality, trophic conditions, and anthropogenic pressures [15,16]. Hence, they are widely applied in freshwater biomonitoring programs and constitute key components of ecological assessment frameworks, particularly within the context of the European Water Framework Directive (WFD, 2000/60/EC, European Union 2000). The SPI was originally developed in France by Coste (1982) [17] to assess ecological quality and pollution levels in freshwater ecosystems using benthic diatoms. The BDI was later proposed by Coste [18] as a standardized diatom-based tool for river monitoring and ecological status assessment. In the United Kingdom, the TDI was developed by Kelly and Whitton (1995) [9] specifically to evaluate eutrophication and nutrient enrichment in freshwater systems through the analysis of benthic diatom assemblages. Since their development, these indices have been widely validated and successfully applied across several aquatic ecosystems in the Northern Mediterranean, demonstrating their effectiveness for ecological quality assessment and water management purposes [19,20].

Although much interest has been given to diatom diversity and diatom-based indices in Northern Mediterranean regions, their use in water quality assessment remains limited in southern regions, particularly in North Africa. Paradoxically, these regions are among the most water-stressed areas worldwide [21,22] and are increasingly affected by water scarcity and the degradation of freshwater resources, reflecting the broader environmental challenges across North Africa [21,23]. Indeed, only a few studies have applied diatom assemblages for environmental assessment. In Morocco, diatoms have been used in recent research to evaluate ecological status and anthropogenic pressures in rivers, reservoirs and wadis, including the Sidi Chahed, Mikkès and Mellah systems [16]. Diatom communities have also been employed as indicators of organic pollution and nutrient enrichment in Algerian wadis, such as Kebir-East, and in Egypt, particularly in the Nile River system [24,25].

In contrast, diatom-based approaches remain largely underexplored in Tunisian freshwater ecosystems. There, ecological assessments have predominantly relied on physicochemical parameters and macroinvertebrate communities [26,27], with little integration of diatom assemblages despite their well-established sensitivity to environmental changes. This represents a significant knowledge gap in biomonitoring approaches across Southern Mediterranean freshwater systems, particularly in North Africa, where aquatic ecosystems are increasingly affected by water scarcity and growing anthropogenic pressures. Wadis, as intermittent and highly variable freshwater systems typical of arid and semi-arid Mediterranean landscapes, play a key hydrological and ecological role by ensuring groundwater recharge, sustaining specialized aquatic communities, and connecting inland catchments to downstream coastal ecosystems [23,27]. Despite this ecological significance and their strong socio-economic value for water supply and agriculture, wadis remain among the least monitored freshwater ecosystems in the Southern Mediterranean region, leaving their ecological status largely unassessed.

In North Tunisia, several wadis, such as Ben Hessine, Guenniche and Haima, drain into the Bizerte Lagoon, a coastal ecosystem of major ecological and socio-economic importance. This Lagoon supports high biodiversity and essential human activities, particularly fisheries and aquaculture [28]. These tributary systems are increasingly affected by environmental degradation resulting from urban discharges, agricultural runoff and industrial effluents [26,29], contributing to ecological imbalance and environmental degradation of the Bizerte Lagoon [8,30,31,32]. In the context of water scarcity and freshwater resource degradation in Tunisia, understanding and monitoring the ecological status of these wadis is therefore essential for the sustainable management and conservation of the lagoon ecosystem.

This study aims to evaluate the ecological quality of freshwater systems within the catchment area of the Bizerte Lagoon through a multi-scale approach, integrating both spatial and seasonal variability. The assessment is based on benthic diatom assemblages and diatom-based ecological indices, recognized as sensitive bioindicators of environmental change and water quality degradation. By combining biological indicators with spatio-temporal analysis, this study provides a more comprehensive understanding of ecological dynamics within these tributary systems and their potential influence on the lagoon ecosystem. To our knowledge, this is the first study in Tunisia, and one of the few in North Africa, to apply a diatom-based biomonitoring approach at this scale under Mediterranean environmental conditions.

2. Materials and Methods

2.1. Study Area

The studied wadis are located within the catchment area of the Bizerte Lagoon in Northern Tunisia (Figure 1). They constitute major freshwater inputs to the Lagoon, particularly during the wet season (from October to April), with average annual inflows estimated at 4.27, 4.21, and 16.5 mm³ for Ben Hessine, Guenniche and Haima wadis, respectively [26,33]. The watersheds cover approximately 45 km² for Ben Hessine, 86 km² for Guenniche and 104 km² for Haima. Ben Hessine and Haima wadis are primarily exposed to urban discharges and agricultural runoff, while Guenniche wadi is subject to multiple sources of pollution, including agricultural practices, industrial effluents, domestic discharge and wastewater treatment plant releases [26,33,34]. These anthropogenic pressures differ among the three wadis according to land use and human activities within their respective catchments, resulting in varying degrees and types of environmental disturbance. The main sources of pollution affecting each investigated wadi are summarized in

Table 1. Geographical, hydrological and environmental characteristics of stations in the three studied wadis.

	Code	GPS Coordinates	Hydrological Regime	Max. Depth (cm)	Flow Velocity (cm s−1)	Anthropogenic Pressure	Shading	Sampling Seasons
Ben Hessine (upstream)	B1	37°07′21.98″ N 9°54′17.34″ E	Permanent	20	25–75	Agricultural runoff (Cereal cultivation)	Open	Winter 2023, spring 2023, summer 2023, autumn 2023, winter 2024
Ben Hessine (downstream)	B2	37.165° N, 9.870° E	Permanent	40	5–25	Direct domestic discharge	Open	Winter 2023, spring 2023, summer 2023, autumn 2023, winter 2024
Guenniche (upstream)	G1	37°10′9.94″ N 9°58′4.90″ E	Intermittent	15	5–25	Direct domestic discharge	Semi-open	Winter 2023, spring 2023, summer 2023, autumn 2023, winter 2024
Guenniche (downstream)	G2	37°10′11″ N, 9°57′2″ E	Permanent	35	25–75	Wastewater agriculture runoff Industry discharge	Semi-open	Winter 2023, spring 2023, summer 2023, autumn 2023, winter 2024
Haima (upstream)	H1	37°13′27″ N, 9°44′43″ E	Intermittent	30	5–25	Agricultural runoff (mixed crops)	Semi-open	Winter 2023, spring 2023, summer 2023
Haima (downstream)	H2	37.225° N, 9.735° E	Permanent	60	5–25	Agricultural runoff (mixed crops)	Open	Winter 2023, spring 2023, summer 2023, autumn 2023, winter 2024

. As a result, these wadis receive substantial inputs of nutrients, organic matter, and various contaminants [29,35], leading to the deterioration of water quality and potentially impairing the ecological integrity of these freshwater ecosystems.

Two sampling stations were selected along each wadi, one upstream and one downstream, to assess spatial variability and the influence of cumulative anthropogenic pressures along the watercourse. This approach enabled the comparison of physicochemical and biological parameters between sites. The nomenclature of the stations and their hydrological and environmental characteristics are presented in Table 1.

2.2. Water Sampling for Chemical Analyses

Seasonal sampling was conducted at all stations in winter 2023 (February), spring 2023 (May), summer 2023 (July), autumn 2023 (November), and winter 2024 (January), in order to investigate the seasonal variability. The upstream station of Haima wadi was excluded from the autumn 2024 and winter 2024 campaigns due to restricted access to the private property where the site is located.

Due to the shallow water depth of all sampling stations (15–60 cm, Table 1), water samples (in triplicate) were collected from the surface in 2 L polyethylene bottles previously acid-washed (10% HCl) and vigorously rinsed with ultrapure water. The 2 L sample volume was selected to provide sufficient water for all planned physicochemical analyses while ensuring an adequate reserve for sample handling and processing. Samples were stored in isothermal containers, transported to the laboratory and processed within 24 h of collection. Subsamples (1 L) for inorganic nutrient analyses were filtered on 0.2 µm polycarbonate membranes and the filtrates were used to determine nitrate (NO₃), nitrite (NO₂) and ammonium (NH₄) according to the standard spectrophotometric procedures described by Parsons et al. (1984) [36]. The detection limits of the analytical methods were 4.6 × 10⁻⁴ mg L⁻¹ for nitrite and 9 × 10⁻⁴ mg L⁻¹ for both nitrate and ammonium.

Total nitrogen (TN) and total phosphorus (TP) were determined following the persulfate digestion method proposed by Valderrama (1981) [37]. Subsamples (500 mL) were digested using potassium persulfate under high-temperature and pressure conditions to convert nitrogen and phosphorus compounds into nitrate and orthophosphate forms, respectively. Following digestion, nutrient concentrations were determined using a spectrophotometer. Detection limits for TN and TP analyses were 0.02 mg L⁻¹ and 0.01 mg L⁻¹, respectively.

The determination of biological oxygen demand over five days (BOD₅) and chemical oxygen demand (COD) was made on 500 mL subsamples. BOD₅ was determined following the incubation method, based on the difference in dissolved oxygen concentrations before and after a 5-day incubation period at 20 °C in darkness [38]. COD was determined using the dichromate oxidation method under acidic conditions following standard methods for water analysis [38].

For chlorophyll a (Chl a) analysis, subsamples (500 mL) were filtered through Whatman GF/F glass fiber filters (0.7 µm pore size, 47 mm diameter). Chlorophyll pigments were extracted in a 90% acetone solution (v/v) for 30 h in darkness at 4 °C, and pigment concentrations were subsequently determined according to the spectrophotometric method described by Parsons et al. (1984) [36].

In situ measurements of physicochemical parameters were performed at each station. Water temperature, conductivity, and pH were recorded using a multi-parameter probe (WTW Multi 1970i, WTW, Weilheim, Germany), whereas dissolved oxygen concentrations were measured with an oximeter (WTW Oxi 330/set, WTW, Weilheim, Germany ).

2.3. Diatom Sampling, Preparation and Identification

During seasonal sampling campaigns, benthic diatom assemblages were collected from epilithic substrates in all study stations. A composite sample was obtained by gently brushing biofilms from 7 to 10 submerged stones selected in flowing sections of each wadi. This multi-substrate approach, consistent with standard biomonitoring practices, ensures a representative assessment of epilithic diatom assemblages [39,40] and follows the European standard protocol for diatom sampling in running waters [40]. The collected materials were immediately preserved in 95% ethanol and subsequently cleaned using hydrogen peroxide following the method described by Krammer and Lange-Bertalot (1986) [41]. After digestion, the samples were rinsed several times with distilled water to remove residual organic matter and chemical reagents. The cleaned materials were then mounted in Naphrax® (Brunel Microscopes Ltd., Wiltshire, UK) by placing a drop of the cleaned suspension onto a coverslip prior to slide preparation. At least 400 valves were identified and counted on each slide under light microscopy using a Di-Li 2025-20 microscope (Di-Li, Kaiserslautern, Germany) equipped with a 20 MP USB 3.0 camera, as well as a Bioimager microscope (BUM360FLL) (Bioimager Inc., Vancouver, BC, Canada) fitted with a USB 3.0 BIC-E3S CMOS 20 MP camera.

2.4. Calculation of Diatom-Based Ecological Indices

Diatom species relative abundances were integrated into the OMNIDIA database and processed using the OMNIDIA software (version 5.3) for index calculation [42]. Each identified taxon was assigned its corresponding species-specific code within the software database. Based on the taxonomic composition and relative abundance of benthic diatom assemblages, the BDI, SPI and TDI indices (

Table 2. Ecological and trophic classification of surface waters based on the Biological Diatom Index (BDI), the Specific Pollution Sensitivity Index (SPI), and the Trophic Diatom Index (TDI) (Kelly and Whitton, 1995; Eloranta and Soininen, 2002) [9,43].

Water Quality Class	Ecological Status	BDI/SPI	Trophic Level	TDI
Class I	High	>17	Oligotrophic	<35
Class II	Good	15–17	Oligo-mesotrophic	35–50
Class III	Moderate	12–15	Mesotrophic	50–60
Class IV	Poor	8–12	Eutrophic	60–75
Class V	Bad	<8	Hypertrophic	>75

) were calculated and used as standardized tools for freshwater biomonitoring and ecological assessment. To our knowledge, these indices have not yet been applied in the environmental assessment of Tunisian aquatic ecosystems.

2.5. Statistical Analyses

Seasonal and spatial variations in physicochemical parameters, nutrient concentrations and diatom-based indices were evaluated using two-way analysis of variance (ANOVA) [43]. Prior to the analyses, data normality and homogeneity of variances were verified using the Shapiro–Wilk and Levene tests, respectively [44]. Relationships between biological metrics and environmental variables were assessed using Spearman’s rank correlation analysis [45,46]. All statistical analyses were performed using SPSS Statistics version 18.0 for Windows.

3. Results

3.1. Physical and Chemical Variability

Physical and chemical parameters were expressed as seasonal minimum, mean and maximum values (

Table 3. Physical and chemical parameters recorded in upstream and downstream stations of the studied wadis, presented as seasonal maximum, mean and minimum values.

Stations	Code		T (°C)	DO (mg O2 L−1)	BOD5 (mg O2 L−1)	COD (mg O2 L−1)	pH	Conductivity (µS cm−1)
Ben Hessine Wadi	Max		21.3 Sp	9.7 Wi1	31.3 Su	129.0 Su	8.0 Wi2	4180 Wi1
	B1	Mean	19.2	6.8	12.6	106.6	7.5	2375
	Min		14.5 Wi2	2.0 Su	2.0 Wi1	103.0 Wi1	7.3 Sp	1377 Wi2
	Max		23.7 Su	7.0 Wi1	31.3 Su	120 Sp	7.8 Su	3885 Wi1
	B2	Mean	18.2	5.8	13.0	100.0	7.5	2212
	Min		12.8 Wi1	3.1 Su	1.0 Wi1	78.7 Wi1	7.4 Wi1	1183 Su
Guenniche Wadi	Max		24.0 Su	8.8 Wi1	44 Su	115.0 Su	7.9 Sp	4900 Wi2
	G1	Mean	18.0	5.5	24.0	120.5	7.6	1965
	Min		12.0 Wi1	1.1 Su	2 Wi1	125.0 Wi2	7.3 Su	828 Sp
	Max		23.6 Su	4.5 Wi	29.0 Su	130 Su	8.2 Su	3250 Wi1
	G2	Mean	19.6	2.8	26.4	116.6	7.9	2190
	Min		15.5 W1	2.0 Sp	22.0 Sp	98.0 Sp	7.6 Au	1420 Au
Haima Wadi	Max		23.7 Su	9.7 Wi1	25 Su	100.0 Sp	7.7 Wi1	7630 Wi1
	H1	Mean	17.6	7.5	15.0	94.7	7.8	6523
	Min		10.0 Wi1	6.0 Su	3.0 Wi1	89.0 Wi1	8.0 Su	4500 Sp
	Max		29.0 Su	9.0 Wi1	29.0 Au	110.0 Su	8.0 Wi2	7060 Au
	H2	Mean	19.2	6.0	20.1	103.0	7.6	4917
	Min		11.0 Wi1	3.6 Su	12.0 Wi1	87.0 Wi2	7.7 Wi1	1363 Wi1

Notes: T: water temperature; DO: dissolved oxygen; BOD₅: biological oxygen demand; COD: chemical oxygen demand; Wi1: winter 2023; Sp = spring 2023; Su = summer 2023; Au = autumn 2023; Wi2 = winter 2024.

). Most parameters exhibited significant spatial and seasonal fluctuations (ANOVA, p < 0.001), whereas pH remained relatively stable throughout the study period, varying between 7.3 and 8.0, which indicated predominantly neutral to alkaline conditions. No significant spatial or seasonal variation in pH was observed among the studied stations.

For most stations, water temperature peaked in summer and reached lower values in winter. The highest temperature (29 °C) was measured at station H2 (Haima downstream) during summer 2023, while the lowest (10 °C) was observed at station H1 (Haima upstream) in winter 2023.

In Ben Hessine (i.e., stations B1 and B2) and Haima (i.e., stations H1 and H2) wadis, seasonal mean dissolved oxygen concentrations ranged between 5.8 and 7.5 mg O₂ L⁻¹. In contrast, Guenniche wadi showed episodes of severe oxygen depletion, particularly at the downstream station (i.e., G2), where seasonal mean concentrations approached the critical threshold of 2 mg O₂ L⁻¹. The lowest dissolved oxygen concentrations were consistently recorded at Guenniche, indicating poorer oxygenation conditions compared with the other wadis. In all stations, the maxima of dissolved oxygen were consistently registered in winter.

In most stations, seasonal mean BOD₅ values exceeded 10 mg O₂ L⁻¹, indicating elevated organic pollution. The highest seasonal mean values (24–26.4 mg O₂ L⁻¹) were observed in Guenniche upstream and downstream, whereas the lowest levels (12.6–13 mg O₂ L⁻¹) were recorded in Ben Hessine wadi. Overall, Guenniche exhibited the highest organic pollution levels among the investigated wadis. For most wadis, the maximum BDO₅ values were measured in summer. Seasonal mean COD concentrations consistently remained above 80 mg O₂ L⁻¹, reflecting persistently high levels of oxidizable pollutants. The peak concentration (130 mg O₂ L⁻¹) was observed in station G2 (Guenniche downstream) in summer 2023. Similarly, COD values were generally higher at Guenniche than at Ben Hessine and Haima.

A wide range of conductivity values was observed, with the minimum (1183 µS cm⁻¹) recorded in station B1 (Ben Hessine upstream) in summer 2023. The highest values were measured in stations H1 and H2 (Haima upstream and downstream) in winter and autumn 2023, where conductivity exceeded 7000 µS cm⁻¹.

3.2. Nutrients and Chl a Variability

Nutrients displayed seasonal fluctuations (ANOVA, p < 0.001, Figure 2 and Figure 3), with higher levels registered mainly in winter and sometimes in spring. Nutrient concentrations also significantly varied across wadis and stations (ANOVA, p < 0.001, Figure 2 and Figure 3).

Inorganic nitrogen concentrations (i.e., NO₂ + NO₃ + NH₄) were higher in Ben Hessine (3.63 –12.10 mg L⁻¹) and Guenniche (2.29–15.62 mg L⁻¹) wadis than in Haima wadi (0.82–8.95 mg L⁻¹). In most seasons, higher concentrations were recorded in downstream stations (B2 and G2, particularly in winter 2024) than in upstream stations (B1 and G1) of Ben Hessine and Guenniche wadis. In contrast, Haima wadi exhibited higher concentrations in the upstream station (H1), particularly during spring 2023 (Figure 2A–C).

Regarding total nitrogen, Guenniche wadi clearly differed from the other two systems, with markedly higher concentrations throughout the study period. The downstream levels (G2: 13.20–89 mg L⁻¹) greatly exceeded those measured in the upstream station (G1: 1–23.78 mg L⁻¹). The highest values were observed in winter 2024. In the other wadis, TN levels were a little higher in downstream stations. Values ranged from 1 to 25.47 mg L⁻¹ in Ben Hessine wadi, and from 1.60 to 12.1 mg L⁻¹ in Haima wadi (Figure 2D–F).

Guenniche wadi also showed higher TP concentrations (0.16–2.93 mg L⁻¹) compared with the other wadis (Ben Hessine: 0.05- 0.21 mg L⁻¹; Haima: 0.05–0.22 mg L⁻¹), particularly in the downstream station (G2), where the maximum value was recorded in winter 2024 (Figure 3A). In Ben Hessine and Haima wadis, TP concentrations were generally higher in downstream stations (B2 and H2) than in upstream stations (B1 and H1) across most seasons. In both systems, the highest concentrations were recorded in winter 2023 (Figure 3B,C).

Chl a concentrations varied significantly both spatially and seasonally (ANOVA, p < 0.001; Figure 4). Ben Hessine wadi displayed lower concentrations compared with the other wadis. Values ranged from 1 µg L⁻¹ (B1, autumn 2023) to 4.6 µg L⁻¹ (B2, winter 2023), with generally higher concentrations observed downstream than upstream (Figure 4A). Guenniche wadi was characterized by the highest Chl a concentrations across most seasons (2.3–18 µg L⁻¹), with consistently higher values in the downstream station relative to the upstream station (Figure 4B). During winter 2023, a pronounced Chl a peak (51 µg L⁻¹) was observed upstream of Haima wadi (station H1), while a lower level (15.3 µg L⁻¹) was recorded at station H2. In the remaining seasons, concentrations varied between 1.5 µg L⁻¹ (H1, spring 2023) and 12.6 µg L⁻¹ (H1, summer 2023). Chl a concentrations were positively correlated with TP (r = 0.475, p < 0.001) and TN (r = 0.305, p < 0.05).

3.3. Benthic Diatom Community Structure

The relative abundances of benthic diatoms showed significant spatial and seasonal variations (ANOVA, p < 0.001), indicating marked temporal dynamics in community structure. Overall, a total of 134 species belonging to 42 genera were identified in the epilithic samples collected at upstream and downstream stations of the three wadis during the five sampling seasons. The genera Nitzschia (30 species) and Navicula (21 species) displayed the highest species richness (

Table A1. Complete list of diatom taxa recorded in the studied wadis, including their OMNIDIA codes, number of occurrences (number of samples in which each taxon was recorded across all sampling sites and seasons), and maximum relative abundance.

Omnidia Code	Species Names and Their Authors	Number of Occurrences	Maximum Relative Abundance (%)
ABAN	Achnanthes brevipes Cleve 1895	4	1.24
ACEL	Achnanthes coarctata Brebisson 1880	3	0.83
ADAM	Achnanthidium saprophilum Round & Bukhtiyarova 1996	3	9.96
ADAM	Achnanthidium atomoides Lange-Bertalot 2004	2	7.80
ADMO	Achnanthidium delmontii Pérès et al. 2012	2	5.99
AERT	Achnanthidium ertzii Lange-Bertalot 2011	1	1.95
ADEG	Achnanthidium exiguum Grunow 1994	1	0.43
ADJK	Achnanthidium jackii Rabenhorst 1861	6	7.32
ACLI	Achnanthidium lineare W. Smith 1855	16	15.85
ADMI	Achnanthidium minutissimum Kützing 1994	2	5.21
ADMS	Adlafia minuscula Lange-Bertalot 1999	8	1.60
ACOP	Amphora copulata Kützing 1986	3	1.43
AMMC	Amphora micra Levkov 2009	1	0.64
APED	Amphora pediculus 1885	9	11.18
CAMP	Caloneis amphisbaena 1894	1	0.25
CCBI	Campylodiscus clypeus Ehrenberg 1844	1	1.66
CEUG	Cocconeis lineata Ehrenberg 1849	2	2.37
CPLA	Cocconeis placentula Ehrenberg 1838	6	7.74
CTWE	Conticribra weissflogii Grunow 2009	5	10.36
CRBU	Craticula buderi Lange-Bertalot 2000	2	3.17
CMNO	Craticula minusculoides Lange-Bertalot 2001	2	2.55
CRML	Craticula molestiformis Lange-Bertalot 1996	1	2.93
ESBM	Craticula subminuscula Manguin 2015	2	1.90
CCRE	Cyclotella cretica John 1996	1	2.48
CAFF	Cymbella affinis Kützing var.affinis 1844	2	1.95
DTEN	Diatoma tenuis C. Agardh 1812	6	0.64
DBLC	Diploneis balcanica K. Buczkó 2010	2	3.45
DOCU	Diploneis oculata Cleve 1894	2	1.75
ELBI	Encyonema lange-bertalotii Krammer 1997	2	2.55
EMNT	Encyonema minutum (Hilse) D.G. Mann 1990	3	2.43
ENVE	Encyonema ventricosum (C. Agardh) Grunow 1875	1	2.47
ENVA	Encyonema ventricosum Agardh 1875	1	2.43
EALA	Entomoneis alata cf Ehrenberg 1845	2	6.5
FPSP	Fallacia pygmaea Lange-Bertalot 1990	10	1.92
FPEL	Fistulifera pelliculosa Lange-Bertalot 1997	1	7.82
FVAU	Fragilaria vaucheriae Kützing 2012	1	0.23
FVAS	Frustulia vulgaris (Thwaites) De Toni 1891	3	2.37
	Gogorevia exilis Kützing 2020	4	4.5
GAFF	Gomphonema affine Kützing 1880	2	14.45
GCVT	Gomphonema clavatulum E. Reichardt 1999	1	1.5
GGCL	Gomphonema gracile Ehrenberg 1838	5	9.56
GMIS	Gomphonema minusculum Krasske 1932	1	16.43
GPAR	Gomphonema parvulum var. Lange-Bert. 1983	21	28.2
GPAT	Gomphonema parvulum Kützing 1990 abnormal form	5	2.53
GPSA	Gomphonema pseudoaugur Lange-Bertalot 1979	1	2.68
GPAS	Gomphonema saprophilum (Lange-Bertalot & E. Reichardt) Abarca, R. Jahn, J. Zimmermann & Enke 2014	6	7.37
GYKU	Gyrosigma kuetzingii Grunow 1894	17	19.4
HACO	Halamphora coffeaeformis Levkov 1903	7	15.25
HPVE	Halamphora paraveneta Lange-Bertalot	1	0.32
HVEN	Halamphora veneta (Kützing) Levkov	4	5.63
HAAB	Hantzschia amphioxys Ehrenberg 1880	2	1.71
HOST	Haslea ostrearia Simonsen 1974	6	5
HHUN	Hippodonta hungarica Lange-Bertalot 1996	4	3.33
-	Humidophila perpusilla Lange-Bertalot 2014	1	1.48
MAAT	Mayamaea atomus Lange-Bertalot 1997	5	2.93
MAPE	Mayamaea permitis Lange-Bertalot 1997	8	4.5
NANT	Navicula antonii Lange-Bertalot 2000	3	9.23
NCRO	Navicula cari Ehrenberg 1861	2	3.91
NCRY	Navicula cryptocephala Kützing 1844	4	5.25
NCTO	Navicula cryptotenelloides Lange-Bertalot 1993	1	1.43
NERI	Navicula erifuga Lange-Bertalot 1985	2	3
NESC	Navicula escambia (Patrick) Lange-Bertalot 2007	3	3
NEXI	Navicula exilis Kützing 1844	1	5.23
NPHY	Navicula phyllepta Kützing 1885	7	2.97
NPHP	Navicula phylleptosoma Lange-Bertalot 1999	3	1.42
NPUT	Navicula punctulata W. Smith 1871	3	2.5
NSAL	Navicula salinarum Grunow var.salinarum	12	17.5
NSAR	Navicula salinarum var. rostrata Lange-Bertalot	1	6.8
NSIA	Navicula simulata Manguin	18	18.3
-	Navicula teratorogenic	1	0.45
NTPA	Navicula tripunctata Bory 1822	13	34.4
NVDA	Navicula vandamii Schoeman 2000	2	15.2
NVTG	Navicula veneta Kützing 1844 abnormal fo.	2	1.73
NVEN	Navicula veneta Kützing 1844	25	24.19
NCIH	Navicula cincta Ehrenberg 1861	3	2.15
NVIT	Navicula vitrea Hustedt 1943	1	0.74
-	Navicula cf.	1	2
NZAR	Nitzschia amphibia Grunow 1862	11	6.2
NACA	Nitzschia acicularis Kützing 1853	3	0.43
NCPL	Nitzschia capitellata Hustedt 1952	14	4.5
NCPL	Nitzschia capitellata Hustedt in A. Schmidt	2	0.73
NDIS	Nitzschia dissipata Kützing 1860	17	11.5
NFIC	Nitzschia filiformis (W. Smith) Van Heurck 1896	3	26.6
NUMB	Nitzschia umbonata Lange-Bertalot 1978	1	0.95
NFOM	Nitzschia fonticola (Grunow) Grunow 1881	1	0.37
NIFI	Nitzschia frustulum (Kützing) Grunow 1880	1	22
NFTE	Nitzschia frustulum (Kützing)Grunow abnormal fo. 1880	1	1.24
NINC	Nitzschia inconspicua Grunow 1862	28	35.78
NZIT	Nitzschia inconspicua Grunow abnormal form 1862	6	2.36
NZLR	Nitzschia longissima Grunow 1862	2	0.5
NLBA	Nitzschia lorenziana Grunow 1880	2	0.86
NMEL	Nitzschia microcephala Grunow 1880	2	1
NNAN	Nitzschia nana Grunow 1881	9	20
NPHS	Nitzschia palea Kützing W. Smith 1856	26	14
NPAT	Nitzschia palea Kützing var. tenuirostris 1881	3	7.9
NPAD	Nitzschia palea var. debilis	1	1.26
NIPM	Nitzschia perminuta (Grunow) M. Peragallo 1903	11	11.26
NIPM	Nitzschia perminuta (Grunow) M. Peragallo Abnormal 1903	2	1.35
NZSA	Nitzschia salinarum Grunow 1881	1	0.5
-	Nitzschia saporotolerans	11	2.38
NSTS	Nitzschia soratensis E.A.Morales & M.L.Vis 2007	12	7.84
NISC	Nitzschia scalpelliformis Grunow 1880	1	2
NSIG	Nitzschia sigma (Kützing) W.M.Smith 1878	7	2.81
NSIG	Nitzschia sigma (Kützing) Teratorogenic 1878	1	0.39
NISO	Nitzschia solita Hustedt 1953	6	2.43
NSUA	Nitzschia subacicularis Hustedt 1938	1	1.9
-	Nitzschia cf.	1	3
PFMA	Planothidium frequentissimum Lange-Bertalot 1999	6	3.3
PTLA	Planothidium lanceolatum Lange-Bertalot 1999	5	3.48
PTLA	Planothidium pumilum cf	1	3.3
-	Planothidium victorii, J.Braidwood & C.Kilroy 2012	1	1.42
PSAT	Psammothidium subatomoides Hustedt 1996 cf 1	1	1.95
PSAT	Psammothidium subatomoides Hustedt 1996 cf 2	1	0.86
-	Psammothidium sp 2 cf.	1	0.81
PSPE	Pseudostaurosira perminuta Sabbe & Vyverman	1	1
RABB	Rhoicosphenia abbreviata (C. Agardh) Lange-Bertalot 1980	2	30.3
NMTO	Sellaphora atomoides Wetzel & Van de Vijver 2015	7	23
EOMI	Sellaphora nigri (De Notaris) Wetzel & Ector 2015	3	17
SNIN	Stauronella cf. indubitabilis Lange-Bertalot & Genkal	1	0.5
SNAR	Stauronella arctica (Hustedt) Cf Lange-Bertalot	2	3.5
-	Stephanocyclus meneghinianus (Kützing) Kulikovskiy, Genkal & Kociolek 2022	2	3.43
SACO	Surirella angusta Kützing 1844	1	0.9
SBKU	Surirella brebissonii Krammer & Lange-Bertalot 1987	21	7.9
SBKU	Surirella lacrimula J.D.English 2012	2	12.46
SOAP	Surirella ovalis Kütz. var.apiculata O.Muller	6	10.5
SSGL	Surirella striatula Turpin 1827	1	1.2
-	Surirella teratorogenic	2	1.6
TFAS	Tabularia fasciculata (Agardh)Williams et Round 1986	7	5.9
TTAB	Tabularia tabulata (C.A. Agardh) Snoeijs	4	24.3
TAPI	Tryblionella apiculata W.Gregory 1857	20	17.6
THLI	Tryblionella hungarica (Grunow) Frenguelli 1942	22	18.8
TLEV	Tryblionella levidensis Wm. Smith 1880	2	2.9
TLIT	Tryblionella littoralis (Grunow in Cl. & Grun.) 1881	2	0.92
-	Tryblionella tryblio Cantonati & Lange-Bertalot. 2017	1	0.5
UUAC	Ulnaria ulna Compère 2001	4	2.5
UACU	Ulnaria acus Kützing 2003	3	1.5

). Several species were characterized by high occurrence frequencies and substantial relative abundances, whereas others were rare or restricted to particular sites. Based on their ecological affinities, the diatom assemblages could be classified into distinct groups, reflecting their differences in environmental conditions and habitat preferences (

Table 4. Ecological profiles of the recorded diatom species, including the number of occurrences (N.O.), mean relative abundance (M.R.A), and ecological preferences according to Levkov [47] and the ecological classification proposed by Rimet and Bouchez [48].

		N.O.	M.R.A. (%)	Ecological Preferences	Ecological Classification
Species associated with hypertrophic conditions	Gomphonema parvulum Kützing	21	28.20	Eutraphentic	3
	Navicula veneta Kützing	25	24.19	Halophilic	2
	Nitzschia inconspicua Grunow	28	35.78	Halophilic	1
	Nitzschia palea Kützing W. Smith	26	14.00	Eutraphentic	3
	Surirella brebissonii Krammer	21	7.90	Halophilic	4
	Tryblionella hungarica Grunow	22	18.80	Eutraphentic	4
	Gyrosigma kuetzingii Grunow	17	19.40	Mesotraphentic	-
Halophilic species	Conticribra weissflogii Grunow	5	10.36	Eutraphentic	-
	Halamphora coffeaeformis C. Agardh	7	15.25	Eutraphentic	3
	Haslea ostrearia Simonsen	6	5.00	-	4
	Navicula salinarum Grunow	12	17.50	-	4
	Navicula simulata Manguin	18	18.30	Halophile
	Entomoneis alata Ehrenberg	2	6.50	-	3
	Nitzschia acicularis Kützing	3	0.43	Eutraphentic	2
Species with lower tolerance to eutrophic conditions	Cymbella affinis Kützing	2	2.00	-	3
	Encyonema minutum Hilse	3	2.97	Oligotraphentic	2
	Gomphonema affine Kützing	2	14.45	-	4

Notes: N.O. (Number of Occurrences) = number of samples in which each species was recorded across all sampling stations and seasons. M.R.A. (maximum relative abundance, %) = highest relative abundance (%) recorded for each species across all sampling stations and seasons. Ecological profile according to Rimet and Bouchez (2012) [48]: 1 = oligotrophic, 2 = oligo-mesotrophic, 3 = mesotrophic, 4 = eutrophic=; - = not defined.

and

Table 5. Spearman correlation between relative abundance of benthic diatoms and environmental parameters across sampling stations. Significance levels: ** 0.001 < p < 0.01; * 0.01 < p < 0.05; - non significatif; N = 84.

		T	DO	BOD5	COD	NO3	NO2	NH4	TN	TP	Conductivity	Chl a
Species associated with hypertrophic conditions	Gomphonema parvulum	-	−0.25 *	-	-	-	0.26 *	-	0.23 *	0.77 **	-	0.22 *
	Navicula veneta	-	−0.30 **	-	0.21 *	-	0.40 **	0.31 **	0.34 **	0.64 **	-	-
	Nitzschia inconspicua	0.23 *	-	-	-	-	0.36 **	-	-	-	-	-
	Nitzschia palea	-	-	-	0.37 **	-	-	-	-	0.26 *	-	-
	Surirella brebissonii	-	-	-	-	-	-	-	-	-	-	-
	Tryblionella hungarica	−0.35 **	−0.26 *	-	-	-	-	-	-	0.40 **	-	0.27 *
Halophilic species	Gyrosigma kuetzingii	-	0.36 **	-	-	-	-	-	-	-	0.30 **	−0.21 *
	Conticribra weissflogii	-	-	-	-	-	-	-	-	-	0.37 **	-
	Halamphora coffeaeformis	-	-	-	-	-	-	-	-	-	0.27 **	-
	Haslea ostrearia	-	0.38 **	-	-	-	-	-	-	-	0.46 **	-
	Navicula salinarum	-	-	-	-	-	-	-	-	-	-	-
	Navicula simulata	-	0.28 **	-	-	-	-	-	-	-	0.39 **	−0.45 **
	Nitzschia acicularis	0.32 **	0.24 *	-	-	-	-	-	-	-	0.23 *	-
Species with lower tolerance to eutrophic conditions	Cymbella affinis	0.34 **	-	-	-	-	-	-	-	−0.21 *	−0.19 *	−0.30 **
	Encyonema minutum	0.27 **	-	-	-	-	-	-	-	−0.23 *	-	−0.24 *
	Gomphonema affine	−0.23 *	-	−0.39 **	-	-	-	-	-	-	-	-

).

3.3.1. Dominant Diatom Taxa Associated with Eutrophication and Organic Pollution

Gomphonema parvulum (Figure 5C; Table 4) was recorded at high frequency during the study period, occurring in 21 out of the 28 analyzed samples (i.e., 75% of samples) across most sampling stations. The species was particularly present in Guenniche wadi, contributing 2–28.2% of the total diatom assemblage. Its highest contribution was observed in station G2 (Guenniche downstream) during winter 2024. In Ben Hessine wadi, the relative abundance of G. parvulum ranged from complete absence in station B1 in spring 2023 to 26% in station B2 in autumn 2023. In contrast, the species was only weakly represented in both stations of Haima wadi, where its relative abundance did not exceed 3%. According to the ecological classification proposed by Levkov 2026 [47] (Table 4), G. parvulum is considered an eutraphentic species, typically associated with nutrient-rich environments and eutrophic conditions. Correlation analysis further revealed a significant positive relationship of the species relative abundance with concentrations of NO₂ (r = 0.26, p < 0.05), TN (r = 0.23; p < 0.05), TP (r = 0.77, p < 0.001) and Chl a (r = 0.22, p < 0.05), but a negative correlation with DO (r = −0.25 p < 0.05) (Table 5).

Navicula veneta (Figure 5G; Table 4) also exhibited a high frequency of occurrence throughout the study, being detected in 25 of the 28 analyzed samples (i.e., 89% of samples). Similar to G. parvulum, this species reached particularly high relative abundance in station G2 (Guenniche downstream), ranging from 9 to 24.2%, with a maximum recorded in winter 2024. In station G1 (Guenniche upstream), N. veneta was continuously present but at comparatively lower abundances around 11%. This species was also recorded in Ben Hessine wadi throughout all sampling seasons, with higher mean relative abundance (12.5%) in the downstream station B2 compared to the upstream station B1 (6.8%). In contrast, N. veneta was either absent or poorly represented in Haima wadi, both upstream and downstream. The highest relative abundance recorded did not exceed 5% in station H2 during spring 2023. Significant positive relationships were found between N. veneta and TP (r = 0.646, p < 0.001), TN (r = 0.34, p < 0.001), NH₄ (r = 0.31, p < 0.05), NO₂ (r = 0.40, p < 0.001) and COD values (r = 0.21, p < 0.05). The species was, however, negatively related to DO (r = −0.30, p < 0.01) (Table 5). According to the ecological classification proposed by Levkov 2026 [47] (Table 4), this species is considered halophilic, reflecting its affinity for mineralized and nutrient-enriched environments.

Nitzschia inconspicua (Figure 5F; Table 4) was the most recorded species across all seasons and stations, with an occurrence frequency of 28/28 (i.e., 100% of samples). However, its relative abundance exhibited marked spatial and seasonal variations. The highest value (35.8%) was registered in Ben Hessine wadi at the downstream station (B2) during spring 2023. The upstream station (B1) of this wadi also showed an important contribution of the species (15.6%). In Guenniche wadi, the relative abundances of N. inconspicua ranged from 3.5% in station G1 (summer 2023) to 33.3% in G2 (autumn 2023); however, lower values were recorded for Haima wadi (3–20.7%) in both stations. This species showed a significant positive correlation with NO₂⁻ concentration (r = 0.36, p < 0.01) and temperature (r = 0.36, p < 0.05) (Table 5). Ecologically, N. inconspicua was classified within mesotrophic conditions [49] and as a halophilictaxon (Levkov 2026 [47]) (Table 4).

Nitzschia palea (Figure 5E; Table 4) was also widely distributed across the investigated wadis, with an occurrence frequency of 26/28 (i.e., in 93% of samples). The highest relative abundances of this species were recorded in Guenniche wadi, where it reached 14.6% in the upstream station (G1) during summer 2023. In Haima and Ben Hessine wadis, the species had a comparatively lower contribution to the diatom assemblage, not exceeding 4.4%. It was even absent in B1 in summer 2023 and in H2 in autumn 2023. N. palea was found to be positively correlated with TP (r = 0.26, p < 0.05) and COD (r = 0.37, p < 0.01, Table 5). According to the ecological classification of Levkov and Mertens [47,50] (Table 4), this species is considered eutraphentic, indicating an affinity for nutrient-rich and organically polluted environments.

Surirella brebissonii (Figure 5D; Table 4) was also well represented within the investigated wadis, with an occurrence frequency of 21 out of 28 samples (i.e., 75% of samples). Ben Hessine wadi exhibited the highest occurrence frequency for this species in both upstream and downstream stations, along with the highest recorded relative abundance (7.9%) in B1 during winter 2023. This species also showed a low contribution in Haima wadi, with a maximal relative abundance of 3.5% recorded in the downstream station (H2) during winter 2024. In Guenniche wadi, S. brebissonii was mainly recorded in the upstream station (G1), where it reached a maximal relative abundance of 7.9% in autumn 2023. In contrast, this species was registered only once in the downstream station (G2) during autumn 2023, with a very low relative abundance (0.86%). No significant correlations were observed between this species and environmental variables. According to the ecological classification proposed by Levkov 2026 [47], S. brebissonii is considered a halophilic taxon (Table 4).

Tryblionella hungarica and Tryblionella apiculata (Figure 5A,B; Table 4) were also among the most frequently recorded taxa, occurring in 22 and 20 of the 28 samples (i.e., 71–78% of samples), respectively. Both species were predominantly associated with the upstream of Guenniche wadi, where they reached the highest relative abundances (17.6–18.8%) during winter 2023. During the other seasons, both species showed lower relative abundances, not exceeding 9%. T. apiculata was also relatively abundant at the upstream Haima wadi during several seasons. T. hungarica showed a significant positive correlation with TP (r = 0.40, p < 0.01; Table 5), whereas T. apiculata did not exhibit any significant correlation with the measured environmental variables. According to the ecological classification proposed by Levkov (2026) [47] (Table 4), T. hungarica is classified as eutraphentic, whereas T. apiculata is classified as halophilic.

3.3.2. Relatively Less Pollution-Tolerant Diatom Taxa

Several taxa indicative of low nutrient and organic pollution levels were recorded during the study, but with low occurrence frequencies and relative abundances. Encyonema minutum, an oligotraphentic species ([47]), was observed only three times in Ben Hessine wadi, reaching 3% relative abundance in the station B1 in winter 2024 and was negatively correlated with TP (r = −0.26, p < 0.05) and Chl a (r = −0.24, p < 0.05), but positively with temperature (r = −0.27, p < 0.05, Table 5). Similarly, Cymbella affinis and Gomphonema affine, known for their low tolerance to eutrophication, were found exclusively in Ben Hessine wadi during Winter 2024. While C. affinis remained rare (maximum relative abundance of 2%), G. affine reached 14.4% in the downstream station (B2). Both species showed negative relationships with pollution- and eutrophication-related variables, including TP, Chl a and BOD₅ (Table 4 and Table 5).

3.3.3. Halophilic and Marine-Related Diatom Taxa

Several diatom species were predominantly associated with Haima wadi, the site characterized by the highest conductivity. Among these taxa, Navicula simulata, Gyrosigma kuetzingii, Navicula salinarum and Halamphora coffeaeformis (Figure 6; Table 4) were the most frequent (18/28, 17/28, 12/28 and 7/28, respectively; i.e., in 34–64% of samples), reaching maximum relative abundances of 18.3%, 18.4%, 17.5% and 15.5%, respectively. Consistent with their Halophilic character (Levkov 2026 [47]), all species, except N. salinarum, showed positive and significant correlations with conductivity (r = 0.27–0.46, p < 0.01), while Navicula simulata was also significantly correlated with NO₃⁻ (r = 0.30, p < 0.01) and NH₄⁺ (r = 0.24, p < 0.05) (Table 4 and Table 5).

Marine taxa (Haslea ostrearia, Nitzschia nana, N. sigma, and N. acicularis; Figure 6; Table 4) were mainly recorded in the downstream stations of Haima and Ben Hessine wadis, but their low relative abundances (0–6.2%) and sporadic occurrence indicated limited ecological relevance within the benthic assemblages. Most of these species showed positive correlations with conductivity, notably H. ostrearia (r = 0.46, p < 0.001) and N. acicularis (r = 0.26, p < 0.05) (Table 5). Additionally, Entomoneis cf. alata and Conticribra weissflogii were also recorded during the study exclusively in Haima wadi, where they reached the maximum relative abundance (6.5% and 10.36%, respectively), particularly in summer and autumn 2023. These species also showed significant positive correlations with conductivity (r = 0.37–0.38, p < 0.001, Table 5), further supporting the influence of increasing salinity and marine-related conditions within this wadi.

3.4. Diatom-Based Ecological Indices

TDI values varied slightly among seasons or sampling, between 78.4 and 96.2. These consistently high values indicated hypertrophic conditions throughout the study area. Guenniche wadi exhibited a higher TDI value than the other wadis. Overall, downstream stations in all wadis displayed slightly elevated TDI values (82.1–96.2) compared to upstream stations (78.4–93.6), reflecting greater eutrophication pressure and increased nutrient enrichment in downstream locations (

Table 6. Trophic levels and ecological status across the investigated wadis according to diatom-based ecological indices (TDI, BDI and SPT).

			TDI		BDI/SPI		Ecological Status
			Values Trophic Level		Values	Water Quality Class
Ben Hessine	B1	Winter 2023	89.9	Hypertrophic	9.8/9.9	Class IV/Class IV	Poor/Poor
	B2	Winter 2023	92.8	Hypertrophic	8.2/6.0	Class IV/Class V	Poor/Bad
	B1	Spring 2023	82.5	Hypertrophic	13.9/13.0	Class III/Class III	Moderate/Moderate
	B2	Spring 2023	92.4	Hypertrophic	10.0/9.1	Class IV/Class IV	Poor/Poor
	B1	Summer 2023	83.9	Hypertrophic	14.3/12.5	Class III/Class III	Moderate/Moderate
	B2	Summer 2023	84.9	Hypertrophic	12.6/12.3	Class III/Class III	Moderate/Moderate
	B1	Autumn 2023	93.6	Hypertrophic	14.2/13.0	Class III/Class III	Moderate/Moderate
	B2	Autumn 2023	94.2	Hypertrophic	9.4/8.8	Class VI/Class IV	Poor/Poor
	B1	Winter 2024	87.7	Hypertrophic	12.2/12.1	Class III/Class III	Moderate/Moderate
	B2	Winter 2024	88.9	Hypertrophic	9.6/10.6	Class IV/Class IV	Poor/Poor
	G1	Winter 2023	84.7	Hypertrophic	4.2/4.6	Class V/Class V	Bad/Bad
Guenniche	G2	Winter 2023	82.1	Hypertrophic	5.1/3.1	Class V/Class V	Bad/Bad
	G1	Spring 2023	94.7	Hypertrophic	10.7/8.0	Class IV/Class V	Poor/Bad
	G2	Spring 2023	96.2	Hypertrophic	7.3/7.6	Class V/Class V	Bad/Bad
	G1	Summer 2023	92.6	Hypertrophic	7.9/5.3	Class V/Class V	Bad/Bad
	G2	Summer 2023	96.2	Hypertrophic	7.2/6.5	Class V/Class V	Bad/Bad
	G1	Autumn 2023	91.2	Hypertrophic	6.9/9.1	Class V/Class IV	Bad/Poor
	G2	Autumn 2023	87.1	Hypertrophic	7.6/9.9	Class V/Class IV	Bad/Poor
	G1	Winter 2024	82.0	Hypertrophic	8.7/9.6	Class IV/Class IV	Poor/Poor
	G2	Winter 2024	89.8	Hypertrophic	6.1/8.2	Class V/Class IV	Bad/Poor
	H1	Winter 2023	80.4	Hypertrophic	9.8/9.4	Class IV/Class IV	Poor/Poor
Haima	H2	Winter 2023	91.0	Hypertrophic	9.0/9.6	Class IV/Class IV	Poor/Poor
	H1	Spring 2023	78.40	Hypertrophic	13.2/12.6	Class III/Class III	Moderate/Moderate
	H2	Spring 2023	85.60	Hypertrophic	8.6/8.3	Class IV/class IV	Poor/Poor
	H1	Summer 2023	92.03	Hypertrophic	10.4/11.7	Class IV/Class IV	Poor/Poor
	H2	Summer 2023	92.80	Hypertrophic	10.7/10.8	Class IV/Class IV	Poor/Poor
	H2	Autumn 2023	90.56	Hypertrophic	8.3/7.4	Class IV/Class V	Poor/Bad
	H2	Winter 2024	93.00	Hypertrophic	9.5/9.6	Class IV/Class IV	Poor/Poor

).

BDI and SPI indices significantly varied among wadis and stations (ANOVA, p < 0.05), reflecting spatial and seasonal differences in water quality and diatom communities. The inter-site comparison indicated that both indices recorded high values in Ben Hessine wadi (BDI: 8.2–14.3; SPI: 6–13), followed by moderately lower ranges in Haima wadi (BDI: 8.3–10.7; SPI: 7.4–12.6) and the lowest in Guenniche wadi (BDI: 4.2–10.7; SPI: 3.1–9.9). This distribution pattern reflected an overall moderate to poor ecological quality in Ben Hessine wadi, with most observations corresponding to Class III and Class IV of water quality. The moderate status in this wadi was predominantly observed during summer. In contrast, Haima wadi was predominantly characterized by poor ecological status (Class IV quality), while Guenniche wadi experienced the most degraded conditions, with dominance of bad water quality (Class V quality) (Table 6).

In Ben Hessine wadi, the inter-station comparison revealed a clear upstream–downstream gradient in water quality. Station B1 had higher values of BDI (9.8–14.3) and SPI (9.9–13) across most seasons compared to the downstream station B2 (BDI: 8.2–12.6; SPI: 6–12.1). This pattern indicated better ecological conditions upstream, mostly in Class III quality, whereas the downstream environment was more degraded, which was mainly in Class IV quality. At station B1, the highest index values were observed in summer (BDI: 14.3; SPI: 12.5) and autumn 2023 (BDI: 14.2; SPI: 13), indicating that these seasons are most favorable for diatom development. Conversely, both indices decreased during winter 2023 (BDI: 9.8; SPI: 9.9), reflecting a deterioration in diatom community health, associated with poorer ecological condition.

No marked differences were observed between upstream and downstream stations in Haima and Guenniche wadis, as ecological quality remained mostly within Class IV and Class V throughout most of the seasons. This spatial homogeneity indicated a strong and persistent anthropogenic disturbance throughout the systems.

4. Discussion

The study provided valuable insights into the ecological status of freshwater systems in a poorly investigated region of Northern Africa, SW Mediterranean. By integrating, for the first time, a diatom-based assessment with physicochemical analyses, offered a comprehensive evaluation of water quality and ecological health.

4.1. Water Quality Assessment Based on Physical and Chemical Indicators

The study showed significant seasonal and spatial variability in key physicochemical indicators of ecological quality, particularly dissolved oxygen, BOD₅ and COD (Table 3), reflecting the combined influence of natural processes and anthropogenic activities on freshwater ecosystem functioning. Such variability is widely recognized as a major determinant of water quality and ecological status in lotic ecosystems [51,52]. Nevertheless, despite these variabilities, all indicators consistently highlighted a degraded ecological condition across the three wadis, with varying degrees of disturbance among sites, suggesting the existence of a pollution gradient. Seasonal mean concentrations of dissolved oxygen in Ben Hessine and Haima wadis ranged between 5.8 and 7.5 mg O₂ L⁻¹, indicating moderate ecological conditions according to Water Framework Directive-related classifications and the thresholds proposed by Best et al. (2007) [53]. Guenniche wadi exhibited substantially lower dissolved oxygen levels, particularly in the downstream station, where the seasonal mean concentration dropped to 2.8 mg O₂ L⁻¹, corresponding to critical ecological conditions and severe oxygen depletion. A previous study has also identified Guenniche wadi as the most oxygen-depleted system among several wadis in the region [26], highlighting that oxygen depletion is a recurrent feature of this system. In most wadis, the highest dissolved oxygen concentrations were observed during winter, likely due to lower water temperatures, which enhance oxygen solubility as well as increase flow conditions that promote water mixing. Similar seasonal patterns have been reported in the study wadis and in other Mediterranean watercourses, such as the Tiflet River in Morocco [54], confirming the influence of Mediterranean climatic conditions on oxygen dynamics within hydro-systems [55]. The reduced oxygen levels in study wadis are supported by the high seasonal mean of BOD₅ (12.6–26.4 mg O₂ L⁻¹) and COD (94.7–120.5 mg O₂ L⁻¹) measured throughout the study area. According to Water Framework Directive-related classifications, these values indicated poor to very poor water quality and reflected substantial organic pollution. This suggests significant inputs of biodegradable and oxidizable organic matter, primarily originating from agricultural runoff and urban discharges. Similar levels of organic pollution have been documented in heavily impacted North African wadis such as Mikkes and Mellah, where agricultural runoff and domestic inputs represent major sources of environmental degradation [16]. The highest seasonal mean values of BOD₅ and COD were consistently recorded in Guenniche wadi, suggesting a more advanced level of ecosystem degradation. This condition is likely driven by cumulative inputs from agricultural runoff, urban discharges, industrial effluents and wastewater treatment plant releases, all contributing to sustained organic loading and oxygen depletion.

The study also assessed the current eutrophication status by measuring the common trophic indicators, nutrient and Chl a concentrations [56,57]. Although these metrics showed significant seasonal and spatial variability (Figure 2, Figure 3 and Figure 4), they consistently revealed persistent eutrophication of the study wadis. Indeed, high concentrations of inorganic nitrogen (0.82–15.62 mg L⁻¹), TN (1.04–80.89 mg L⁻¹) and TP (0.05–2.91 mg L⁻¹) were measured during all seasons and in all sites, which was obviously associated with nutrient inputs from human activities [25,32]. Similar increased nutrient concentrations were reported in other North African wadis and Mediterranean lotic systems which are under strong anthropogenic pressures [58,59]. For most nutrients, including TP, concentrations increased in winter and remained elevated in spring. This seasonal pattern was primarily driven by increased surface runoff and watershed leaching during rainy periods. which carry phosphorus from agricultural and urban areas into the watercourse. Rainfall events can also stir up phosphorus-rich sediments from the streambed, adding to the TP levels observed during this period [60,61]. Seasonal precipitation events are known to promote nutrient mobilization toward aquatic ecosystems, leading to elevated nutrient loads [16,60,61,62]. Guenniche wadi displayed the highest nutrient concentrations, particularly TN (1.04–80.89 mg L⁻¹) and TP (2.19–2.91 mg L⁻¹) (Figure 2 and Figure 3), indicating strong nutrient enrichment from domestic wastewater discharges and industrial effluents [33]. In addition, intensive agricultural practices in the watershed of this wadi, particularly the extensive use of phosphate-based fertilizers and herbicides [29], likely contributed to the elevated TP levels observed in Guenniche wadi. With a few exceptions, the nutrient concentrations measured downstream of most wadis generally exceeded those registered upstream, demonstrating the accumulation of substances along the river flow and also the contribution of local pollution sources along the watercourse.

Consistent with the nutrient-enriched conditions of the wadis, Chl a, a proxy for microalgal biomass, exhibited moderate to very high concentrations (1–51 µg L⁻¹) (Figure 4). This reflects the key role of nutrient availability in regulating algal growth and primary productivity [20]. Chl a concentrations are comparable to those reported for other nutrient-rich freshwaters, such as the Nile River in Egypt, the Joumine Reservoir in Tunisia and Hammam Boughrara in Algeria [63,64]. The Chl a peak observed in Haim upstream during winter 2023 coincided with a pronounced microalgal bloom caused by the diatom Conticribra weissflogii. This event was likely explained by favorable conditions during this season (e.g., increased nutrient inputs from runoff, suitable light and temperature regimes, etc.), which together can promote rapid algal growth and biomass accumulation. For the other seasons, higher Chl a concentrations were registered in Guenniche wadi, particularly in the downstream station, which could be related to the substantial enrichment of this system by TN and TP. Indeed, Chl a concentrations were found to be significantly and positively correlated with TP and TN. These nutrients are widely recognized as major drivers of microalgal biomass and eutrophication in freshwater ecosystems, by stimulating algal growth under enriched conditions [65].

4.2. Response of Benthic Diatom Assemblages to Water Quality Degradation

The study presented an in-depth analysis of epilithic diatom assemblages to improve the evaluation of freshwater ecosystem quality in North Africa, where studies on diatom-based biomonitoring remain limited. Benthic diatoms are well known for their high sensitivity to environmental changes, which makes them reliable bioindicators of both ecological and trophic status in freshwater ecosystems [40,66].

The diatom assemblages recorded in the studied wadis exhibited relatively high taxonomic richness (134 taxa, 42 genera), exceeding that reported for five anthropogenically impacted wadis in the Greater Casablanca region (102 taxa, 44 genera) and for seven streams in the Drâa Basin, Morocco (86 taxa, 44 genera) [67,68]. This higher richness may reflect the greater environmental heterogeneity of the studied wadis, including their hydrological and physicochemical variability, as well as microhabitat differences among the sampled epilithic substrates (e.g., stone size, surface texture and exposure to current and light), which provide a range of ecological niches for diatom colonization.

Diatom assemblages were dominated, in terms of number of taxa, by Nitzschia (30 species) and Navicula (21 species), which are typical genera of disturbed aquatic ecosystems worldwide [16,69]. Furthermore, diatom assemblages clearly reflect spatial gradients of eutrophication and organic pollution among wadis, as taxa with varying degrees of pollution tolerance displayed distinct distribution patterns across sites.

Navicula veneta (Figure 5G) and Gomphonema parvulum (Figure 5C) were widely distributed across the investigated wadis, occurring in 89% and 75% of samples, respectively. Overall, they represented 0–28.21% of the total diatom assemblage. Both species are well known as pollution-tolerant taxa, frequently associated with eutrophic environments affected by organic pollution [47,70]. In our study, their preference for nutrient-enriched environments was supported by significant positive correlations between their relative abundances and concentrations of nitrogen and phosphorus nutrients. Both species showed negative correlations with dissolved oxygen. N. veneta and G. parvulum were further positively correlated with COD and Chl a, respectively (Table 5). Accordingly, these species displayed important presence in Guenniche wadi, with the highest relative abundances (24.88%) in the downstream station, the site most impacted by nutrient inputs and organic enrichment (Table 3, Figure 3). Both species also exhibited important relative abundances downstream of Ben Hessine wadi (28.21%), where inorganic nitrogen and TN reached quite high levels, further supporting their affinity for nutrient-rich environments. G. parvulum has been frequently reported in North African ecosystems, particularly along the Nile River in Egypt [71] and in Turkish ecosystems subjected to strong anthropogenic pressures [72]. Similarly, N. veneta was found to dominate across several freshwater and coastal ecosystems in Andalucía, Southern Spain, under strong anthropogenic pressure [65].

Nitzschia inconspicua (Figure 5F) was the most frequent species, occurring in 100% of samples and reflecting a broad ecological tolerance. The species formed significant fractions (33–36%) of diatom assemblages, particularly in downstream stations of Ben Hessine and Guenniche wadis. Its relative abundance was positively correlated with NO₂ concentrations (Table 5), consistent with previous observations in Mediterranean ecosystems [66]. The species is a small benthic taxon typically adapted to disturbance and closely associated with substrate growth [48]. It is recognized for its tolerance to nutrient enrichment [50] and its halophiliccharacter, which likely explain its occurrence in both coastal and nutrient-rich environments. Similar occurrences have also been reported in Algerian freshwater systems [49].

Nitzschia palea (Figure 5E) was recorded at nearly all sites and seasons (in 93% of samples), exhibiting important presence particularly in Guenniche wadi. Furthermore, its relative abundance was positively related to TP and COD (Table 5), indicating a preference for nutrient-enriched and organically polluted water [47,50]. Its broad ecological tolerance and affinity to degraded environments are previously documented [41]. The species has also been frequently reported in several South Mediterranean freshwater ecosystems, including Oued Mikkes and Oued Mellah in Morocco [16], Wadi Melliti in Algeria [68], and multiple sampling sites along the Nile stream, under relatively different ecological conditions [71]. Surirella brebissonii was frequently observed throughout the studied period, occurring in 75% of samples and exhibiting moderate relative abundances in Ben Hessine et Guenniche wadis (1.23–14.62%). Despite no clear relationship being found between this species and nutrients in our study, the species is considered tolerant to high nutrient loads [47]. It is also recognized for its halophiliccharacter ([41]), and is frequently reported from several coastal environments, including the coastal area around La Rochelle in France [73] and the coastal Kara-Bogaz-Gol Bay in Turkmenistan [74].

The widespread occurrence of Tryblionella hungarica (in 78% of samples) and Tryblionella apiculata (in 71% of samples) highlights the predominance of eutrophic conditions in the investigated wadis. Both species are recognized as tolerant to nutrient enrichment and are commonly associated with organically and phosphorus-rich environments. In particular, T. hungarica has been identified as an indicator of eutrophic to hyperthropic conditions in European freshwater ecosystems [75]. T. apiculata also showed similar ecological preferences, supporting its occurrence in nutrient-rich environments. This interpretation is further supported by the significant positive correlation observed between T. hungarica and TP (Table 5), suggesting that its frequent occurrence, particularly in Guenniche, may reflect phosphorus-enriched conditions.

Other pollution-sensitive diatom taxa, including Encyonema minutum, Cymbella affinis and Gomphonema affine, exhibited limited frequencies (10% of samples) and relative abundances (2–14%) and were exclusively detected in Ben Hessine wadi. E. minutum and C. affinis showed negative relationships to TP and Chl a concentrations, while G. affine was negatively associated with BOD₅ (Table 5), supporting their preferences for less impacted environments. Indeed, Encyonema species are generally associated with well-oxygenated waters with low nutrient inputs [47], and C. affinis and G. affine are commonly considered indicators of low nutrient enrichment [47]. The confinement of these sensitive taxa to Ben Hessine wadi suggests that this site may serve as a local refuge of diatom taxa associated with less degraded ecological quality. The decline of sensitive diatoms in response to nutrient enrichment and organic pollution, and their replacement by more tolerant taxa, is widely reported in Mediterranean freshwater ecosystems [47,48]. Consequently, the absence of E. minutum, C. affinis and G. affine from Guenniche and Haima wadis testified to their higher environmental degradation than Ben Hessine wadi.

4.3. Increasing Salinity as an Additional Driver of Diatom Community Composition

Although eutrophication was identified as the major ecological pressure affecting the studied wadis, increasing salinity associated with the decline in precipitation observed in recent years across the study area may also pose a significant threat to ecosystem quality. Such change could adversely affect aquatic communities, particularly benthic diatom assemblages.

In this context, several halophilictaxa were observed during our study, including Navicula simulata, Gyrosigma kuetzingii, Navicula salinarum and Halamphora coffeaeformis (Table 4; Figure 6). These taxa, indicative of saline influence, were frequently recorded throughout the study (34–64% of samples) and predominated in upstream and downstream stations of Haima wadi, which were characterized by the highest conductivity (Table 3). Their relative abundances were further positively correlated with conductivity (Table 5), confirming their affinity for saline conditions. N. simulate and H. coffeaeformis have also been observed in high-conductivity freshwater ecosystems [16,76,77]. N. salinarum has also been reported from coastal and marine ecosystems [41]. As for G. kuetzingii, it is a typical species of brackish and highly alkaline environments and is commonly associated with meso- to moderately eutrophic freshwater habitats, generally under conditions where organic pollution does not exceed the β-mesosaprobic level [41].

Several other taxa, including Entomoneis cf. alata, Haslea ostrearia, Nitzschia nana, Nitzschia sigma, Nitzschia acicularis and Conticribra weissflogii, which are typically marine or brackish species, were also recorded during the study period, although generally with relatively low abundances (0–10.36%). These taxa were mainly observed at the downstream station of Haima and Ben Hessine wadis, particularly during the warm seasons. This pattern may reflect the hydrological connectivity between the Bizerte Lagoon and the downstream sections of these wadis during dry periods, when reduced freshwater discharge and increased evaporation promote saline water intrusion and the upstream movement of brackish lagoon waters into the lower parts of the streams. Similar hydrological dynamics have been widely described in Mediterranean coastal systems and lagoons, where marine influence becomes more pronounced during summer and low-flow conditions due to density-driven saline intrusion and reduced hydraulic pressure from freshwater inflows. Most of these species also showed positive correlations with conductivity (Table 5), further supporting their affinity for saline environments. Their assemblages were relatively similar to those reported from coastal ecosystems such as the Northern Baltic Sea [77] and the Hartenbos Estuary in South Africa [78].

4.4. Ecological Relevance of Diatom-Based Indices

The study represents one of the first applications of diatom-based ecological indices for the quality assessment of aquatic ecosystems in Tunisian freshwater ecosystems.

These indices provided valuable information on the ecological conditions and environmental gradients characterizing the investigated wadis.

The TDI index (78.4–96.2, Table 6) indicated hypertrophic conditions in all investigated stations throughout the study period, reflecting widespread and persistent nutrient enrichment within the studied wadis. The highest TDI values were consistently recorded in both upstream and downstream stations of Guenniche wadi, where eutrophication-tolerant taxa, such as Navicula veneta and Gomphonema parvulum, were particularly abundant. Comparable findings have been reported from several Algerian rivers subjected to strong anthropogenic pressures and nutrient enrichment [49], as well as from various eutrophic Mediterranean streams and coastal rivers characterized by high nutrient loads and organic pollution [47]. These results are consistent with the ecological basis of the TDI, which has been widely recognized as a reliable indicator of nutrient enrichment, especially in relation to total phosphorus and nitrogen [9].

The BDI and SPI indices (Table 6), ranging from 4.2 to 14.3 and from 3.1 to 13, respectively, revealed ecological conditions ranging from moderate to bad across most investigated stations and seasons. Similar values of BDI were reported in several stations of the Sidi Chahed Reservoir in Morocco, which are under eutrophic and anthropogenically disturbed conditions [16]. In contrast to the TDI, which indicated similar hypertrophic conditions across wadis, BDI and SPI exhibited noticeable spatial variability, with a clear environmental gradient within the investigated sites.

Ben Hessine wadi exhibited a moderate ecological status (Class III quality) more frequently than the other wadis, particularly during the dry season (i.e., in summer). This seasonal improvement may be related to reduced surface runoff, which limits the transport of nutrients and organic matter from the surrounding catchments. Similar seasonal improvements in ecological quality during low-flow periods have been reported in Mediterranean freshwater ecosystems influenced by diffuse agricultural and urban pressures [78]. From a taxonomic perspective, the moderate ecological conditions in Ben Hessine wadi may also be explained by the occurrence of relatively less pollution-tolerant taxa, such as Encyonema minutum, Cymbella affinis and Gomphonema affine, which are generally associated with better ecological quality and lower organic pollution levels compared to strongly eutrophication-tolerant species [41]. The presence of these taxa, even with moderate abundances, likely contributed to the temporary increase in SPI and BDI values observed in Ben Hessine stations.

In contrast, most BDI and SPI values recorded in Haima wadi mostly indicated poor ecological status (Class IV quality) (Table 6), reflecting the significant anthropogenic pressures that affect the system, particularly agricultural activities [29]. The predominance of agricultural land use within the catchment likely enhances nutrient and organic matter inputs through runoff, thereby promoting eutrophication and altering benthic diatom assemblages. Similar impacts of agricultural pressures on nutrient enrichment and diatom-based ecological status have been widely reported in Mediterranean freshwater ecosystems [48,71]. BDI and SPI indices predominantly classified Guenniche wadi as bad (Class V quality) for most of the study period (Table 6). The particularly degraded ecological status observed in Guenniche, compared with the other investigated wadis, is likely the result of multiple pollution sources acting simultaneously rather than agricultural activities alone. Among these pressures, effluent discharge from the El Alia wastewater treatment plant represents a major driver of ecological degradation [33]. Such discharges are known to increase nutrient and organic matter loads, thereby promoting eutrophication and favoring pollution-tolerant diatom taxa (Navicula veneta and Gomphonema parvulum), which ultimately results in lower BDI and SPI values. Wastewater discharges are well known to promote nutrient enrichment, organic pollution, and severe alterations of benthic diatom assemblages, leading to the dominance of pollution-tolerant taxa and low IPS and IBD values. Similar degraded ecological conditions have been reported in the Ganhe River (China), subject to strong urban wastewater inputs, with very low IPS and IBD values [15]. Declines in diatom-based ecological indices in response to wastewater and urban pollution have been widely documented in eutrophic and heavily disturbed freshwater ecosystems, highlighting the sensitivity of these indices to organic and nutrient enrichment.

Currently, no diatom-based indices specifically developed for Tunisian freshwater are available. For this reason, the present study relied on indices originally established for Northern Mediterranean waters. However, the transferability of these indices remains limited, as eco-regional differences may lead to the occurrence of novel diatom species or to shifts in the ecological preferences of known taxa. In this context, the existing diatom-based indices have been recalibrated in several regions of the world, such as Brazil and Morocco, to better reflect local species composition and environmental gradients [16,79,80]. This highlights the need for an eco-regional adaptation of diatom indices to better reflect local environmental conditions. Therefore, more comprehensive investigations of benthic diatom assemblages across multiple wadis within the study region and ultimately throughout Tunisia are required to refine species ecological classification and develop a robust region-specific diatom index. Such effort would contribute to the refinement and calibration of diatom indices adapted to the regional context, thereby enhancing accuracy and ecological relevance of biomonitoring programs and supporting more effective water management in SW Mediterranean freshwater ecosystems.

5. Conclusions

The study advances knowledge on the ecological quality of freshwater ecosystems in the understudied Southwestern Mediterranean region by combining physical and chemical measurements with diatom-based bioassessment.

Physical and chemical analyses revealed persistent eutrophication and degraded ecological conditions throughout the investigated wadis, with the most pronounced impact observed in downstream stations and winter. These alterations were attributed to the combined influence of agricultural runoff, urban activities, industrial effluents, wastewater treatment plant releases, and salinization processes. However, the degree of disturbance varied among sites, suggesting the presence of a pollution gradient across the wadis. Guenniche wadi exhibited the most pronounced level of organic pollution and eutrophication, as evidenced by the highest seasonal mean concentrations of BOD₅, COD, TN, TP and Chl a, together with critically low dissolved oxygen levels. Haima wadi was, however, characterized by the highest seasonal mean conductivity values, highlighting the significant influence of salinization in this system. Ben Hessine wadi displayed comparatively lower physical and chemical alteration.

Benthic diatom assemblages responded clearly to the environmental gradient, confirming their value as reliable indicators of ecological disturbance. Pollution-tolerant and eutrophic taxa were predominantly recorded in Guenniche wadi, where eutrophication was most pronounced. Several halophilic and marine-related taxa were particularly abundant in Haima wadi, especially in the downstream station, indicating the saline influence. In contrast, pollution-sensitive taxa were exclusively recorded in Ben Hessine wadi, with low frequencies and relative abundances, indicating a relatively better ecological status of this system.

Diatom-based ecological indices confirmed the widespread occurrence of hypertrophic conditions across studied wadis throughout the sampling period, with the most severe impairment observed in Guenniche wadi. These indices effectively discriminated among sites, revealing a clear gradient of environmental degradation. Ben Hessine wadi frequently exhibited a moderate ecological status (Class III), while Haima wadi was mostly characterized by a poor ecological status (Class IV). In contrast, Guenniche wadi displayed an ecological quality predominantly classified as bad (Class V).

These findings highlight the high sensitivity of benthic diatoms to the main environmental pressures affecting SW Mediterranean wadis, including eutrophication and salinization. Consequently, they constitute a valuable tool for the development of ecological assessment frameworks and can provide a scientific basis for the sustainable management of Tunisian freshwaters. To the best of our knowledge, this study represents the first ecological assessment of Tunisian wadis based on benthic diatoms, providing a valuable baseline for future biomonitoring and ecological research in the region.

Nevertheless, further investigations are required to develop diatom indices better adapted to the specific environmental conditions in Tunisian aquatic ecosystems.