Drought-Squeezed Algerian Reservoir Reveals How Climate Stress Reshapes Tiny Ocean-Like Giants

Deep beneath the sun-scorched hills of northern Algeria, a drinking water reservoir has quietly become a natural laboratory for understanding how climate change rewires freshwater ecosystems. A two-year study of the Boukourdane Reservoir in Tipasa province has produced the first functional-group-based portrait of phytoplankton in this deep, sub-humid Mediterranean system, and the findings carry warnings that stretch far beyond one Algerian dam. The research, published in Environmental Monitoring and Assessment, tracked the microscopic plant life of the reservoir month by month from April 2023 through March 2025, a period that followed a severe hydrological crisis in 2021 that left the water body struggling under prolonged drought. What the scientists documented was a community of algae that is structurally rich but functionally locked into the grip of a single dominant species, a configuration that reflects the strong environmental filtering imposed by a warming, water-stressed Mediterranean climate.

Phytoplankton, the drifting microscopic organisms that form the base of aquatic food webs, are often described as sentinels of environmental change. Because they respond rapidly to shifts in temperature, light, and nutrient availability, their community composition can reveal the hidden fingerprints of hydroclimatic stress long before those fingerprints appear in water chemistry alone. In reservoirs that supply drinking water to growing populations, this sensitivity is both a gift and a threat: the same conditions that reshuffle phytoplankton communities can open the door to blooms of nuisance or toxic species, degrading water quality and driving up treatment costs. Yet North African reservoirs, despite their enormous socio-economic importance in a region where water scarcity is escalating, remain strikingly underrepresented in the functional ecology literature. The new study set out to close that gap by asking how a functionally organized algal community behaves under the kind of hydroclimatic variability that is becoming the Mediterranean norm.

The research team, led by Soumya Saidi of the Laboratory of Dynamics and Biodiversity at USTHB in Algiers, together with colleagues from the École Normale Supérieure in Kouba, sampled the reservoir monthly at four stations across two complete hydrological years. Their methodology combined classical taxonomic identification with the Reynolds functional group classification, a framework that sorts phytoplankton not by evolutionary lineage but by shared morphological and physiological traits that determine where and when particular species thrive. On top of this trait-based classification they layered the Phytoplankton Assemblage Index, known as the Q index, which scores ecological status based on the sensitivity of dominant assemblages to environmental conditions. Diversity metrics captured the breadth of the community, while redundancy analysis, a multivariate statistical technique, was used to link patterns in functional group composition to the physical and chemical conditions measured in the water column.

The sheer taxonomic wealth documented over the two years was remarkable. A total of 165 taxa, distributed across 76 genera and 10 algal classes, were identified in the reservoir. Species richness did not remain steady through the seasons; it swung between a low of 25 taxa in autumn 2023 and a high of 66 taxa in winter 2024, a fivefold oscillation that mirrors the seasonal pulse of the Mediterranean climate, where cool, wet winters give way to hot, desiccating summers. Biovolume, a measure of the total volume of living algal cells per liter of water, told an equally dramatic story. Phytoplankton biovolume peaked above 150 cubic millimeters per liter in spring 2023, then declined steadily to roughly 30 cubic millimeters per liter by spring 2025, an approximately fivefold drop across two years that the researchers attribute to interannual variability in nutrient availability and other physicochemical conditions within the reservoir.

Beneath that fluctuating surface of taxonomic richness, however, lay a community that was strikingly uniform in its functional architecture. Although 24 functional codons were recorded over the study period, the community was overwhelmingly dominated by a single codon, L0, whose flagship species is Ceratium hirundinella, a large armored dinoflagellate with distinctive horn-like projections. This one group consistently accounted for between 60 and 71 percent of total biovolume throughout the sampling campaign. The persistence of codon L0 at such dominance reveals something important about how the reservoir works: the environmental filters operating in Boukourdane, including thermal stratification during warm months and episodic mixing during cooler periods, appear to consistently favor large, mixotrophic dinoflagellates capable of exploiting both photosynthesis and the consumption of other microbes when nutrients run short. Ceratium hirundinella is a classic species of stable, stratified water columns, and its ability to regulate buoyancy and harvest resources across a range of conditions allows it to persist when smaller, faster-growing algae are starved out.

The Q index, which translates functional group identity into an ecological quality score, ranged from 2.18 to 2.89 across the study, values that correspond to a moderate ecological status throughout the two hydrological years. That plateau is significant for water resource managers because it suggests that, despite substantial swings in biovolume and species richness, the reservoir’s ecological condition remained stable rather than deteriorating or recovering during the study window. Moderate status in a drinking water reservoir is not a crisis, but neither is it comfort: it indicates that the system sits in a sensitive middle zone where shifts in nutrient loading or climate forcing could push it either toward improvement or toward the eutrophic conditions that fuel harmful algal blooms. Given that intense lake phytoplankton blooms have been increasing globally since the 1980s, and that Mediterranean water bodies face compounding pressures from warming, drought, and abstraction, the moderate status of Boukourdane should be read as a call for vigilance rather than reassurance.

The statistical heart of the study came from redundancy analysis, which disentangled the environmental variables most tightly coupled to the functional structure of the phytoplankton community. The analysis explained 62.5 percent of the constrained functional variance, a remarkably high figure in ecological research where stochastic processes and unmeasured variables often obscure clear relationships, and the result was statistically significant at p equal to 0.001. Three environmental filters emerged as the dominant orchestrators of community structure: thermal stratification, alkalinity, and dissolved inorganic nitrogen. Thermal stratification, the layering of warm surface water over cooler deep water that develops in summer, shapes which functional groups can persist by controlling access to light and nutrients. Alkalinity, which reflects the water’s buffering capacity and carbonate chemistry, constrains which species are physiologically comfortable in the reservoir. Dissolved inorganic nitrogen, the plant-available form of a key nutrient, governs how much biomass the community can build in any given season. Together, these three variables act as a nested set of gates through which only certain functional strategies can pass.

The implications of this environmental control extend well beyond the boundaries of the Boukourdane catchment. As hydroclimatic variability intensifies across the Mediterranean basin, prolonged drought cycles like the one that struck the reservoir in 2021 will increasingly concentrate nutrients, extend stratification periods, and alter the delivery of nitrogen and other elements from shrinking catchments. The study’s finding that interannual changes in biovolume reflect temporal variability in nutrient availability suggests that the algal community of the reservoir acts as an integrating recorder of upstream hydrological conditions. When drought reduces inflows and concentrates solutes, the phytoplankton respond; when wetter years flush the system with fresh nutrients, the community shifts again. Reading those responses through a functional lens, rather than a purely taxonomic one, gives water managers a diagnostic tool that is more robust than species counts alone, because functional groups remain interpretable even as species identities vary across regions and seasons.

What makes the study especially valuable is its demonstration that the functional group framework, combining Reynolds codons, the Q index, and multivariate ordination, can serve as an effective early warning architecture in a data-poor region. North African water managers often lack the long-term monitoring infrastructure available in Western Europe, and the ability to derive meaningful ecological intelligence from monthly sampling at a handful of stations is a practical advantage. The dominance of codon L0 and its Ceratium hirundinella flagship provides a clear baseline against which future change can be measured: if cyanobacterial codons begin to displace the dinoflagellate during warm, nutrient-enriched periods, that transition would signal a shift toward bloom-prone conditions demanding immediate management response. Conversely, continued stability in the L0 dominance pattern would suggest that the reservoir’s environmental filters remain within their historical envelope. The research team, which also included Siham Arab, Nour El Houda Malki, and Somia Hamil, has provided a template that can be replicated in other Mediterranean and North African reservoirs, transforming scattered monitoring efforts into a coherent, comparable network capable of tracking how the region’s most vital water infrastructure is coping with a climate that is increasingly unwilling to cooperate.

Subject of Research: Phytoplankton community structure and functional dynamics in a Mediterranean drinking water reservoir under hydroclimatic variability

Article Title: Phytoplankton community structure and functional dynamics in a Mediterranean reservoir under hydroclimatic variability

Article References: Saidi, S., Arab, S., Malki, N. E. H., & Hamil, S. (2026). Phytoplankton community structure and functional dynamics in a Mediterranean reservoir under hydroclimatic variability. Environmental Monitoring and Assessment, 198(10), Article 1066. https://doi.org/10.1007/s10661-026-15920-y

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15920-y

Keywords: phytoplankton, functional groups, Mediterranean reservoir, hydroclimatic variability, Ceratium hirundinella, Q Assemblage Index, redundancy analysis, thermal stratification, drought, Algeria, water quality, ecological monitoring