Preliminary Assessment of River Ecosystem Status in a Peri-Urban Area: A Case Study of the Lower Shitalakshya River, Bangladesh

The Shitalakshya River, a heavily stressed peri-urban system in Bangladesh, was assessed using high-resolution in situ sensors, phytoplankton analysis, and multiple pollution indicators. Ten stations within the lower Shitalakshya River were sampled during a single pre-monsoon campaign, comprising four upstream, two midstream, and four downstream stations within the surveyed reach. The assessment covered water quality, trophic status, metal contamination, microplastic abundance, and phytoplankton community characteristics. Dissolved oxygen declined from 6.13 to 5.19 mg L−1 downstream, while ammonia (1.33 mg L−1) and chlorophyll-a (6.4 μg L−1) peaked at SR-07. A phosphate-based modified TRIX ranged from 5.25 to 7.48 under a chemical-species mass assumption and 6.18–8.12 under an elemental N/P assumption, with higher downstream values under both conventions. Carlson’s chlorophyll-based scores ranged from 35.21 to 48.81, while estimated-transparency scores were interpreted separately as an optical diagnostic. Dissolved Cd reached 0.804 mg L−1 at SR-05, with elevated metal indices identifying SR-05–SR-06 as a potential contamination hotspot; however, these exceptionally high Cd concentrations require confirmation through replicate sampling and independent analytical verification. Mean microplastic abundance was highest at SR-09 (80 particles L−1). Shannon diversity was lowest at SR-06 (H′ = 1.55) and highest at SR-07 (H′ = 3.35). The phytoplankton quality index (PQI) and clustering suggested three station groups with contrasting chemical and biological profiles. These classifications remain exploratory because the PQI thresholds and station groupings lack independent validation. The combined assessment identifies priorities for targeted follow-up monitoring, but does not establish pollutant-specific biological causation, seasonal persistence, or whole-river ecological condition.