Isotopic Tracing of Carbon and Nitrogen Sources in the Foodwebs of Nigerian Freshwater-Marine Transition Zones

The freshwater-marine transition zones of Nigerian estuaries and deltaic systems represent zones of intense biogeochemical mixing and ecological complexity where the sources and pathways of organic matter supporting aquatic food webs remain poorly understood. This dissertation employs compound-specific stable isotope analysis of carbon and nitrogen, combined with Bayesian isotope mixing modelling, to trace the relative contributions of terrestrial organic matter, mangrove-derived carbon, autochthonous primary production, and marine-derived organic matter to aquatic food webs across a salinity gradient in four contrasting Nigerian transition zone ecosystems: the Niger Delta mangrove estuaries (Rivers and Bayelsa States), the Cross River Estuary (Cross River State), the Volta-influenced Badagry Creek (Lagos State), and the seasonally closed Lekki Lagoon. At each site, bulk stable isotopes (d13C, d15N, d34S) and compound-specific isotopes (d13C of essential amino acids and fatty acids) are measured for 25 to 30 species representing all trophic levels from primary producers to top predators. Source end-member isotopic signatures are characterised for C3 terrestrial plants, mangrove leaf litter, estuarine phytoplankton, benthic microalgae, and marine phytoplankton at each site. MixSIAR Bayesian isotope mixing models with informative priors estimate proportional dietary contributions and propagate uncertainty through the analysis. The dissertation advances isotope food web ecology methodologically by demonstrating that compound-specific amino acid isotope analysis substantially reduces the uncertainty in source apportionment relative to bulk isotope mixing models in systems with overlapping source signatures, an advance of particular relevance to turbid tropical estuaries. Keywords: stable isotopes, food web, freshwater-marine transition, estuarine biogeochemistry, MixSIAR