The research team sampled 16 sites distributed across the basin’s main watercourses, from near sea level up to 1,062 metres in the Rif Mountains. The sites were grouped into four categories reflecting altitude and land use: natural and semi-natural upstream reaches, semi-natural and agricultural midstream sites, agricultural lower reaches, and heavily urbanised downstream stations. Sampling covered four campaigns between April 2017 and February 2018, spanning spring, summer, autumn, and winter, and thus capturing both the wet Mediterranean season, when rain falls mainly between October and April, and the long dry period from May to September.

In total, the surveys yielded 1,268 individual beetles representing 50 species from 10 families and 27 genera. Diversity was strikingly uneven. The richest site, Tkaraa, lies inside Bouhachem Natural Park and hosted 22 species, while the most abundant site, Zarka, produced 194 individuals. At the opposite extreme, urbanised downstream sites such as Taboula and Roumana, close to Tétouan, yielded only a single species each. Gyrinidae, the whirligig beetles that skim the water surface in swirling groups, dominated numerically with 54 percent of all individuals, yet the diving beetles of the family Dytiscidae proved the most diverse, contributing 17 species, with the genus Agabus the richest in the basin.

Seasonality left a clear signature on the assemblages. Species richness peaked in spring 2017 with 31 species and 377 individuals, whereas the post-flood winter of 2018 produced only 208 individuals from 19 species. The two great beetle suborders responded in opposite ways. Adephaga, which include the whirligigs and diving beetles, dominated the dry period, averaging 20.3 occurrences per station in spring and favouring warm, standing pools. Polyphaga, including the riffle beetles of Elmidae and the minute moss beetles of Hydraenidae, were more frequent under wetter, flowing conditions, reaching an average of 9.38 occurrences per station in autumn. A factorial correspondence analysis separated the fauna cleanly by season, with rheophilic species tied to wet-season flows, thermophilic species concentrated in summer pools, and eurythermic generalists associated with spring’s relatively stable conditions.

Functional traits reinforced the pattern. Predators made up 73 percent of individuals and swimmers 72 percent, although the authors caution that this signal is heavily skewed by the sheer abundance of surface-dwelling whirligig beetles, which count as both. When Gyrinidae were excluded, the picture changed: shredders preferred winter, clingers replaced swimmers as the dominant group in wetter months, and the assemblage shifted toward benthic, substrate-associated forms. The physiological explanation is elegant. Many Adephaga breathe atmospheric air stored beneath their elytra, allowing them to endure warm, oxygen-poor isolated pools as drought advances. Riffle beetles and their relatives, by contrast, depend on plastron or cuticular respiration and require well-oxygenated running water, so they suffer when flow ceases and dissolved oxygen collapses.

The environmental gradient along the river was stark. Mean water temperature rose from 15.5°C at upstream sites to 20.8°C downstream, and electrical conductivity climbed from 269 to 936 microsiemens per centimetre. Dissolved oxygen fell from 7.84 to 2.49 milligrams per litre, while biochemical oxygen demand increased from 10.7 to 32.3 milligrams per litre and chemical oxygen demand from 24.8 to 74.6. Beetle abundance mirrored this degradation, plunging from a mean of 29.6 individuals per upstream site to just 1.19 at the most impacted stations, with species richness dropping from 5.5 taxa to 0.81. A co-inertia analysis, statistically confirmed by Monte Carlo permutation tests, captured this structure: the first two axes explained over 94 percent of the joint variation and opposed well-oxygenated upland refuges dominated by sensitive rheophilic species against lowland, organically enriched reaches occupied by tolerant generalists such as Laccophilus minutus and Helophorus atlantis.

Perhaps the most consequential finding concerns where the compositional break occurs. A permutational multivariate analysis of variance showed that habitat type explained 33.7 percent of the variation in community composition, but pairwise comparisons revealed that urban sites differed significantly from every other category, while upstream, midstream, and agricultural sites did not differ significantly from one another. In other words, moderate agricultural pressure leaves beetle assemblages broadly continuous along the river, but urbanisation produces a threshold-like rupture. Beta-diversity partitioning added a second insight: total dissimilarity across sites was high, at 0.701, and species turnover accounted for 73.7 percent of it, meaning that communities along the Martil are structured mainly by the replacement of specialist species by generalists, not simply by the progressive loss of species.

The study also carries a warning for endemic species. Helophorus atlantis, a Moroccan endemic classified as highly vulnerable, was recorded only in the polluted downstream zone. The authors note that in intermittent rivers, floods can act as mass dispersal events that sweep upstream individuals into lowland reaches, so such records may reflect transient, trapped populations rather than genuine ecological tolerance. Whether this beetle truly survives under urban pollution or was merely drifting through remains an open question, and one that matters for assessing how North Africa’s endemic aquatic insects cope with disturbance. Meanwhile, sites within Bouhachem Natural Park and permanent midstream watercourses showed high endemism, particularly among Polyphaga, confirming their role as critical refuges.

These results resonate across the wider Mediterranean, a global freshwater biodiversity hotspot where climate-driven aridification is lengthening dry phases and human pressure is fragmenting the remaining permanent waters. As riffles disappear and pools isolate, desiccation-tolerant, air-breathing, actively dispersing species gain ground while oxygen-dependent specialists retreat upstream, a dynamic that risks biotic homogenisation outside protected areas. In the Rif Mountains, one of Morocco’s principal aquatic endemism hotspots, the stakes are especially high, yet national knowledge of aquatic beetle distributions and the ecological requirements of endemic taxa remains incomplete, and IUCN criteria designed primarily for vertebrates often fail to capture the needs of invertebrates.

The authors’ central recommendation is practical: conservation planning should extend beyond headwater parks to include the permanent mid-reaches where sensitive and endemic species persist, and pollution and hydromorphological degradation in lowland reaches must be reduced to restore longitudinal connectivity. Because water beetles respond so sharply to flow permanence, oxygen levels, and organic pollution, they can serve as effective bioindicators for identifying priority sites and guiding the expansion of protected area networks through complementarity-based approaches. With more than half of the Martil basin already protected but its lowland waters still deteriorating, the message from these 1,268 beetles is unambiguous: the river’s future diversity will be decided not only in its mountain strongholds, but in the connecting reaches that urban Morocco has so far allowed to degrade.

Subject of Research: Effects of seasonal river drying and human pollution on aquatic beetle diversity in a Mediterranean river basin in northern Morocco

Article Title: Effects of hydrological intermittency and anthropogenic pressure on aquatic beetle assemblages in the Martil River (north-western Morocco)

Article References: Guellaf, A., Bennas, N., & Kettani, K. (2026). Effects of hydrological intermittency and anthropogenic pressure on aquatic beetle assemblages in the Martil River (north-western Morocco). Biodiversity Data Journal, 14, Article e192023. https://doi.org/10.3897/bdj.14.e192023

Image Credits: AI Generated

DOI: 10.3897/BDJ.14.e192023

Keywords: aquatic beetles, hydrological intermittency, Mediterranean freshwater, Morocco, Rif Mountains, urban pollution, biodiversity indicators, species turnover, water quality, conservation planning, endemism, macroinvertebrates