The review followed the PRISMA guidelines for systematic evidence synthesis, extracting data on pH, salinity, dissolved oxygen, turbidity, nutrients, and trace metals from both surface water and sediment. Of the 56 studies included, 44 examined surface water and only 12 examined sediment, a imbalance that itself reveals how much of the estuarine story remains untold. The authors then applied multivariate statistics, including principal component analysis and pooled regression, to detect patterns that no single field study could reveal on its own. What emerged was a portrait of ecosystems caught between two powerful forces: the relentless physical gradient from river to sea, and the growing fingerprint of industry, sewage, and agriculture along Nigeria’s 853-kilometre coastline.

The most striking physical pattern is the longitudinal salinity gradient. Salinity increased from the upper to the lower reaches of every estuary examined, but the strength of that gradient varied enormously. In the Bonny River Estuary during the wet season, salinity climbed from 2.50 parts per thousand in the upper reach to 27.43 parts per thousand near the mouth, while the Awoye Estuary showed a far gentler rise from 1.18 to 2.50 parts per thousand. During the dry season, when river discharge drops and evaporation intensifies, the marine influence deepens: the Bonny River reached 31.00 parts per thousand, approaching full seawater salinity. Using the Venice classification system, the review showed that individual estuarine sections shifted between oligohaline, mesohaline, polyhaline, and even euhaline regimes depending on the season, underscoring how dynamic these systems really are.

pH and dissolved oxygen followed their own spatial logic. Surface water became progressively more alkaline toward the sea in most estuaries, a pattern the authors attribute to seawater’s buffering capacity and the diminishing dominance of acidic freshwater inputs. Dissolved oxygen, by contrast, was generally highest in the upper reaches, particularly during the wet season, and declined downstream, likely reflecting greater organic matter decomposition and longer water residence times in the lower reaches. Yet the Awoye Estuary broke the rule, with oxygen increasing seaward during the wet season, a quirk previously linked to sewage discharges from nearby rural communities that stir up turbulence and promote short-term reaeration. In the lower sections of several estuaries, dissolved oxygen fell below the widely accepted ecological threshold of 5 milligrams per litre, signalling localized zones of oxygen stress even in systems that are, on the whole, well aerated.

Nutrients told a story of land-based pollution. Nitrate concentrations decreased from upstream to downstream in all five estuaries, consistent with agricultural runoff, domestic wastewater, and sewage entering the rivers far from the coast. In the Bonny River during the wet season, nitrate fell from 6.16 milligrams per litre in the upper reach to 2.50 milligrams per litre at the mouth, more than a twofold dilution. Phosphate behaved differently: while it declined downstream in Lagos Lagoon, the Bonny River, and the Imo River, it increased seaward in the Awoye Estuary, reaching 8.68 milligrams per litre in the wet season, a reversal the review attributes to a significant localized source such as wastewater discharge or agricultural runoff near the lower reach. Notably, phosphate concentrations exceeded the recommended guideline limit of 0.1 milligrams per litre in most sections of all the estuaries, pointing to persistent nutrient enrichment across the entire coastline.

Trace metals revealed two opposing spatial signatures. In Lagos Lagoon, metal concentrations in both water and sediment decreased from the industrialized, densely populated upper reaches toward the sea, a pattern of intense upstream effluent loading followed by natural filtration, dilution, and sedimentation. In the Awoye, Bonny, and Qua Iboe estuaries, the opposite occurred: metals accumulated toward the lower reaches, where petroleum-related activities, port operations, and domestic wastewater discharge cluster near the coast. Cadmium showed only moderate spatial variability, whereas lead, copper, nickel, zinc, and iron differed significantly among estuaries. The Bonny River emerged as a multi-metal hotspot, recording the highest concentrations of copper and zinc, with zinc peaking at 6.51 milligrams per litre in the dry season. In the Imo River, lead reached 1.54 milligrams per litre during the dry season, and nickel in Lagos Lagoon peaked at 9.00 milligrams per litre during the wet season, possibly reflecting episodic pulses of contaminated runoff.

The statistical backbone of the review came from principal component analysis, which extracted five components explaining 72.16 percent of the total variance in water quality. The first component, dominated by dissolved oxygen, turbidity, and nickel, captured particle-associated metal transport and oxygen dynamics. The second, defined by pH and salinity with a negative contribution from nitrate, represented the freshwater-marine mixing gradient. The third and fourth components, driven by zinc-cadmium and copper-lead pairings respectively, pointed to spatially structured metal enrichment from localized human sources. Pooled regression analysis added a crucial insight: spatial variability generally exerted a greater influence on estuarine conditions than seasonality, although the balance shifted depending on the process in question. Season significantly influenced the particle-metal-oxygen gradient, while the copper-lead component was tied to estuarine section alone, consistent with fixed point sources such as ports and industrial outfalls.

Correlation analysis reinforced the picture of interconnected pollution pathways. Nitrate correlated strongly with copper (r = 0.97), iron (r = 0.77), and zinc (r = 0.75), while nickel tracked closely with zinc (r = 0.86) and iron (r = 0.74). These strong metal-nutrient associations, with correlation coefficients at or above 0.7, suggest shared anthropogenic origins in wastewater discharges, industrial effluents, and agricultural runoff. Salinity, meanwhile, was strongly negatively correlated with turbidity, phosphate, and lead, supporting the interpretation that land-based pollution travels with freshwater and is progressively diluted or transformed as marine influence strengthens downstream. The review concludes that Nigerian estuaries function as biogeochemical filters, temporarily attenuating contaminant signals while simultaneously accumulating them in sediments, particularly in the lower reaches where fine particles and hydrodynamic trapping favour deposition.

The ecological implications are sobering. In Lagos Lagoon, concentrations of nickel, copper, and cadmium in the upper reach exceeded the Threshold Effects Levels defined by the United States National Oceanic and Atmospheric Administration for marine sediments, and cadmium in the lower reaches of the Awoye, Bonny, and Qua Iboe estuaries also surpassed its threshold. The authors caution that these benchmarks were developed for temperate systems and should be read as indicators of relative enrichment rather than proof of ecological harm, but the pattern is difficult to ignore. Acidic to near-neutral conditions in upstream reaches during the wet season can increase metal mobility and bioavailability, exposing benthic organisms such as crabs, shrimps, and periwinkles, as well as fish larvae, to higher dissolved metal fractions. Meanwhile, sustained nutrient loading raises the risk of eutrophication, harmful algal blooms, and subsequent hypoxia, which can in turn release sediment-bound metals back into the water column.

Perhaps the most alarming finding is one of absence: only 8.9 percent of the reviewed studies assessed entire estuarine systems, and just five investigated all three estuarine sections. Research attention has concentrated on the lower, more accessible reaches, leaving the middle and upper sections, where distinct hydrological processes and localized inputs may dominate, largely unexamined. The review also notes that few studies accounted for tidal phase, that vertical stratification is rarely measured, and that the lack of harmonized datasets prevents the application of standard assessment tools such as the geo-accumulation index, enrichment factor, pollution load index, and water quality index. The authors call for integrated catchment-to-coast monitoring, tide-resolved sampling protocols, region-specific sediment quality guidelines, and strengthened pollution control measures. As Nigeria pursues its blue economy ambitions and the Sustainable Development Goals on clean water and ocean health, the review makes clear that protecting these estuaries will require seeing them whole, from the headwaters to the open sea.

Subject of Research: Physicochemical characteristics and trace metal contamination of Nigerian estuarine waters and sediments

Article Title: Physicochemical characteristics of Nigerian estuaries and their patterns, drivers, and implications for environmental management

Article References: Ogunsuyi, M. T., Dada, O. A., Ogundare, M. O., & Asiwaju-Bello, Y. A. (2026). Physicochemical characteristics of Nigerian estuaries and their patterns, drivers, and implications for environmental management. Discover Oceans, 3(1), Article 66. https://doi.org/10.1007/s44289-026-00182-w

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00182-w

Keywords: Nigerian estuaries, water quality, trace metals, salinity gradient, nutrient enrichment, Lagos Lagoon, Bonny River, systematic review, anthropogenic pollution, sediment contamination, dissolved oxygen, environmental management