Differential sensitivity of diatom and macrophyte indices to road-construction-related physical disturbance in two Mediterranean intermittent rivers (Sicily, Italy)
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Ecological status assessment in Mediterranean intermittent rivers is routinely challenged by natural hydrological variability, and the coherence between diatom and macrophyte responses to transient anthropogenic pressures such as road construction remains poorly resolved. This study compared the sensitivity of the Intercalibration Common Metric index (ICMi, diatoms) and the Macrophyte Biological Index for Rivers (IBMR, macrophytes) at four permanently marked stations, paired upstream/downstream of an active road-building site ("Lotto 4"), on two intermittent watercourses of the Hyblaean Plateau (San Leonardo River and Margi Stream, SE Sicily, Italy). Sampling was conducted across three hydrological phases (winter recharge, spring transition, and summer recession; December 2024–September 2025). Turbidity and total suspended solids (TSS) were measured alongside standard physicochemical LIMeco supporting elements. The ICMi declined sharply at the Margi downstream station in spring (ICMi = 0.239, "Bad"; reference site ICMi = 0.363) coinciding with elevated turbidity (38.4±4.1 NTU downstream vs. 9.2±1.3 NTU upstream) and TSS (64.7±6.0 vs. 14.1±1.8 mg L?¹) under stable, "Good" nutrient conditions (LIMeco = 0.530), while the IBMR remained comparatively stable across stations (range 0.645-0.730). Diatom assemblages downstream shifted from adnate/pedunculate taxa dominated by Achnanthidium minutissimum (28%) towards motile, silt-tolerant taxa dominated by Nitzschia palea (38%) and Navicula gregaria (27%), while macrophyte cover and composition changed only modestly. The results support a hypothesis that diatoms provide a faster, higher-resolution signal of localized physical-mechanical disturbance than macrophytes in these systems. Given the constraints of a four-station, three-season design without a pre-construction baseline, we present this as a hypothesis-generating case study rather than as a definitive general rule. Implications for environmental impact monitoring of linear infrastructure projects in Mediterranean intermittent rivers are discussed.
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