Cascading influence of land use on riverine carbon and nutrients in a tropical coastal and inland headwaters streams.

Anthropogenic pressures such as agricultural activities and urban development increasingly alter biogeochemical processes in headwaters catchments by modifying hydrological pathways, sediment and nutrient inputs, and carbon cycling. These impacts are intensified in tropical regions where high temperatures and intense episodic rainfall promote enhanced mobilisation of particulate matter and solutes from the landscape and rapid in-stream processing. Despite this, integrated studies linking land-use gradients to changes in carbon isotopes, nutrient dynamics and respiration pathways remain limited in tropical streams. Here we investigate how agricultural activities and human development (as reflected in land use cover) influence nutrients (PO₄^3-, NO₃^-, NO₂^-,NH₄⁺), dissolved inorganic and organic carbon concentrations and  isotopes (δ¹³C-DIC, δ^¹³C-DOC), particulate organic carbon isotopes (δ¹³C-POC, δ¹⁵N-PN), sediment δ¹³C and δ¹⁵N, and carbon-processing pathways (pelagic and sediment respiration) across inland (Chania, Sagana, Thiba) and coastal (Ramisi, Mkurumudzi) catchments in Kenya. Seasonal sampling during both wet and dry periods captured contrasting biogeochemical conditions. We observed pronounced spatial contrasts in carbon sources and processing. Inland catchments exhibited progressively lower δ¹³C-DIC values (-3.1‰ to -12.6‰) with increasing agricultural cover, whereas coastal catchments showed an increase of δ¹³C–DIC (−16.9‰ to −2.1‰) along similar land use gradients. Across all catchments, δ¹³C-DOC, sediment δ¹³C, and sediment δ¹⁵N increased significantly with the fraction of agricultural land use . Nutrient responses were spatially and seasonally variable, with urbanised sections, particularly in the Thiba catchment showing strong relationships between built-up land use and NO₂⁻ concentrations, as well as between built-up land within 2 km–200 m buffers and N₂O. Both PO₄³⁻ and NO₃⁻ increased with agricultural land cover, though correlation strength varied among Catchments and season. By integrating hydrology, land use, isotopes, nutrients and respiration metrics, this study demonstrates that tropical catchments exhibit distinct inland to coastal controls on carbon sources and nutrient enrichment. These findings underscore the need for region-specific understanding of how land-use change is reshaping carbon and nutrient dynamics in the tropical  river systems, in order to guide adequate management strategies to improve water quality.