Abstract. Cultural eutrophication of coastal aquatic systems is a major undesired phenomenon of today, which is mainly ascribed to the application of inorganic fertilizers in agriculture. Consequently, dissolved inorganic nitrogen (DIN) is considered the major problem and widely studied. However, human interventions also strongly influence the riverine dissolved organic nitrogen (DON) concentrations and fluxes. Studies of nutrient inputs from tropical river catchments are biased towards DIN, even though they account for only a portion of the total dissolved nitrogen (TDN) pool, whereas the rest is comprised of DON and has been largely ignored. The tropical Pamba River was studied because of its manifold human activities in the catchment and was sampled during the south west monsoon (SWM), north east monsoon (NEM) and the pre monsoon (PM) months during 2010 to 2013. The largest pilgrim center on earth, the Sabarimala temple, located in the upstream forest is a unique feature of the catchment. Fertilizer application, livestock farming and inadequate sewage treatment are the prevailing land use practices. The goals of this study were to (i) define cause-effect relationships by assessing the effect of various human interventions such as the pilgrims, agriculture and sewage disposal in combination with the seasonal variations in hydrology on the DON concentrations and fluxes and to (ii) quantify the inputs from respective land use segments.

The global maximum DON concentration (29 302 μM) was measured for the Pamba River. Pilgrim activities, high population density, agricultural and livestock farming as well as the lack of infrastructure for sanitation facilities were the cause for extremely high DON concentrations and fluxes in the plantation and settlement with mixed tree crop (SMT) segments. A DON yield of 745 kg ha⁻¹ yr⁻¹ was calculated for the Pamba catchment. The total DON inputs from all quantifiable sources amounted to 514 kg ha⁻¹ yr⁻¹ comprising of 69% of the total Pamba DON yield. In the Pamba River, sewage is the major source of DON and the unique Sabarimala pilgrim event accounts for most of it. Nevertheless, sewage input from the rest of the densely-populated catchment is high, which is a common feature of developing countries that lack adequate sanitation and water technology, i.e. in South and Southeast Asia and tropical Africa. Our study shows that DON makes up a significant portion of anthropogenic nitrogen in rivers, in particular in those regions, which are, however, scarce in respective data. It underscores the need for more quantitative studies from densely-populated tropical river catchments in order to improve global nitrogen budgets and the assessment of the consequences of anthropogenic nitrogen inputs into coastal aquatic systems.