Polychaete Dominance and A Localised Nephtys Aggregation in The Macrobenthos of a Tidal Peri-Urban River in The Niger Delta: The Egbelu Reach of The New Calabar River, Nigeria

Benthic macroinvertebrates live in close contact with sediments, cannot easily escape disturbance and reflect conditions over weeks to months, which makes them good indicators of river condition. We described the macrobenthos of the Egbelu reach of the New Calabar River, a tidal oligohaline river in the Niger Delta, at three stations sampled monthly from May to July 2026, and measured eleven water quality variables together with sediment texture and organic matter. The water was well oxygenated (6.50–6.80 mg L−1) and BOD5 was low (2.50–2.80 mg L−1), but salinity, conductivity and total dissolved solids differed among stations. Twenty-seven grabs yielded 768 individuals in nine taxa, a mean density of 1,264 ind. m−2. Polychaetes made up 78.4% of the fauna and decapod crustaceans 21.6%. Nereididae (31.6%) and Capitellidae (27.9%) were the leading families, and opportunistic capitellids (Capitella capitata complex and Notomastus) occurred at every station. Station 2 had the highest density (1,595 ind. m−2), and Nephtys sp. alone made up a third of its fauna, giving it the lowest diversity (H′ = 2.011) and evenness (J′ = 0.915). Stations 1 and 3 were more even (J′ = 0.960–0.990) and most similar to each other (Bray–Curtis similarity 81.8%). Sediments were sandy clay loam to sandy clay with 3.5–4.0% organic matter. In a descriptive canonical correspondence analysis, Nephtys sp. lined up with the finest, most organic sediment at Station 2. The assemblage suggests a moderately stressed, enrichment-tolerant soft-bottom community, and the Nephtys patch at Station 2 is a priority for follow-up monitoring.