Nitrogen Stable Isotope Biogeochemistry as a Tracer of Anthropogenic Nutrient Loading in the Lagos Lagoon System

Distinguishing the relative contributions of different anthropogenic nitrogen sources to coastal lagoon eutrophication is a fundamental challenge with direct management implications for water quality restoration. This research applies compound-specific nitrogen isotope analysis of amino acids and bulk delta nitrogen-15 measurements in multiple environmental compartments to fingerprint and quantify anthropogenic nitrogen sources in the Lagos Lagoon system. Sediment, particulate organic matter, macroalgae, bivalve tissue, and water samples were collected seasonally from 15 stations distributed across the lagoon system from 2022 to 2024. Source characterisation was conducted on end-member samples from municipal wastewater, agricultural drain water, industrial effluent, and atmospheric deposition. Isotope mixing models (MixSIAR) were applied to estimate fractional source contributions at each station. Compound-specific isotope analysis of glutamic acid and phenylalanine in particulate organic matter provided enhanced source resolution. Diagenetic correction was applied using paired bulk and compound-specific isotope relationships. Municipal wastewater was the dominant nitrogen source at inner lagoon stations, contributing 56 to 72 percent of total dissolved inorganic nitrogen. Compound-specific isotope analysis distinguished wastewater from agricultural sources at inner-lagoon stations where bulk delta nitrogen-15 values overlapped. Seasonal analysis revealed intensification of wastewater contribution during dry season low-flow conditions. Spatial integration of mixing model outputs showed significant nitrogen isotope enrichment gradient from the open coast to the innermost lagoon stations. The theoretical advance of this study lies in the validated compound-specific isotope source attribution protocol, which provides substantially higher resolution than bulk isotope methods. Keywords: nitrogen stable isotopes, compound-specific isotope analysis, Lagos Lagoon, eutrophication, nutrient source apportionment.