Fingerprints of upstream catchment land use in suspended particulate organic matter (SPOM) at the river discharge sites in Lake Victoria (Kenya): insights from element, stable isotope and lipid biomarker analysis

Riverine transport and discharge are the primary pathways for terrigenous organic matter (OM) input to lacustrine environment. Understanding the influence of catchment land use on the source, composition and fate of terrigenous particulate OM in a lake is therefore vital. We used C/N, δ13C, δ15N and lipid biomarkers in the suspended particulate OM (SPOM) at the seven river discharge sites in Lake Victoria, in an effort to evaluate the potential of upstream catchment land use on the biogeochemistry of SPOM. The total fatty acids (FAs), alcohols and sterols in SPOM ranged from 4.61 to 7.42 μg/g, 1.52 to 3.32 μg/g and 3.01 to 5.81 μg/g respectively. At sites receiving discharge from industrial effluent the FAs were dominated by short chain FAs (SCFAs), polyunsaturated FAs (PUFAs), monounsaturated FA (MUFAs) and branched FAs (brFAs); the alcohols were dominated by short chain alcohols (SCOHs) and phytol; sterols were dominated by 27Δ5, 27Δ0, 28Δ5,22, 28Δ5, which are associated either with phytoplankton or zooplankton. Sites receiving discharge from substantial agricultural activity in the catchment were dominated by long chain FAs (LCFAs), long chain alcohols (LCOHs) and 29Δ5,22 and 29Δ5 sterols, reflecting vascular plant terrigenous input. There was also evidence of OM from mixed autochthonous and allochthonous sources at the sites regardless of the upstream catchment land use activities. The findings suggest that human land use in upstream watersheds affect the source, composition and distribution of SPOM discharged by rivers, which may not only impact nutrient cycling in the lacustrine environment, but may also change the quality of SPOM.