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Check-dams as monitoring windows for ecological collapse and microplastic trapping: diatom bioassessment and sediment-core evidence from the Cooum River basin
Summary
Scientists studying a river in India found that small dams built to control water flow also act as pollution traps, collecting harmful bacteria-friendly conditions, chemical waste, and plastic particles from urban areas upstream. By studying the tiny algae living there and testing sediment layers, researchers could see a clear timeline of how city pollution and microplastics build up over time. This matters because these check-dams could become simple, affordable checkpoints for catching plastic pollution before it spreads further into water systems that eventually connect to the water and food we consume.
Urban rivers in rapidly growing cities face compounding pressures from nutrient enrichment, organic loading, and plastic debris, yet cost-effective tools for diagnosing degradation hotspots remain limited, particularly in data-scarce tropical systems. We hypothesised that small check-dams along a rural-to-urban river gradient act as convergence points for multiple pollution indicators such as biological, chemical, and polymeric, and that their sediments provide a time-integrated record of upstream stressor inputs. To test this, we developed an integrated diagnostic framework combining benthic diatom bioassessment, sediment-core stratigraphy, and Fourier transform infrared (FTIR) spectroscopy along a peri-urban tributary of the Cooum River (Tamil Nadu, India). Twelve sites spanning rural headwaters to urbanised reaches were surveyed, and undisturbed sediment cores were collected from two check-dams. Reference segments were dominated by attached, low-profile diatoms with higher richness and evenness, whereas impounded and urban reaches shifted toward motile, pollution-tolerant taxa (e.g., Nitzschia palea complex, Gomphonema parvulum), consistent with chronic nutrient enrichment and fine-sediment loading. Core profiles from the downstream check-dam showed surface enrichment in organic matter, phosphorus, and chloride, and principal component analysis separated organic- and salt-rich surface layers from deeper, less-impacted strata. FTIR spectra indicated episodic polyolefin deposition in the rural core but persistent aliphatic polymer signals throughout the urban reservoir core, evidencing sustained plastic trapping. These findings confirm that small check-dams function as practical monitoring windows where biotic, chemical, and polymer indicators co-occur. This framework offers a replicable, low-cost approach for targeted pollution interception and sediment management in data-limited tropical river basins, with direct applicability to ongoing urban river restoration programmes across South Asia.