ABSTRACT: Despite the enormous growth in aquaculture in recent years, little is known of the impact of effluents from large-scale pond agglomerations on tropical coastal ecosystems. The aim of the present study was to evaluate the dispersal and ecological impact of effluents from fish and shrimp ponds along 3 seagrass-covered back-reef areas on Hainan Island, tropical China, with pond areas in their hinterland differing in extent (Yelin: 0.04 km²; Qingge: 2.4 km²; Changqi: 8.7 km²). Concentrations of dissolved and particulate nitrogen, chlorophyll a (chl a) and stable nitrogen isotopes of ammonium (δ¹⁵N-NH₄⁺), nitrate (δ¹⁵N-NO₃^-) and suspended matter (δ¹⁵N-TSM) were used to trace pond effluents along transects perpendicular to the shore. Additionally, δ¹⁵N-TSM and chl a samples were taken from a phytoplankton bioassay experiment, during which offshore surface water was incubated in dialysis bags at stations along these transects. High nutrient concentrations, particularly ammonium, in combination with high chl a (~10 µg l^-1) and elevated chl a levels in the bioassays after incubation indicate eutrophication of the effluent-exposed back-reef areas Qingge and Changqi, with decreasing intensity in the offshore direction. We report the first δ¹⁵N-NH₄⁺ of pond effluents which were as high as 17‰. Consequently, elevated δ¹⁵N values in TSM (5 to 12‰) and a δ¹⁵N increase from <7 up to 14‰ over time in the phytoplankton bioassays specified pond effluents as the predominant nutrient source. The effluents affect the entire back-reef areas over a distance of at least 2.5 km from shore. Our results show that analysis of δ¹⁵N in phytoplankton bioassays is a powerful bioindicator for tracing pond-derived nutrient dispersal and eutrophication effects.