Pond aquaculture effluents feed an anthropogenic nitrogen loop in a SE Asian estuary

Coastal aquaculture expansion resulted in mangrove area loss and ecosystem degradation in the past decades, mainly in tropical Asia. Despite increasing environmental concerns regarding nutrient and organic matter-rich effluents, little is known on the effects on adjacent estuarine and coastal food webs. To assess the impact and fate of anthropogenic nitrogen released from aquaculture facilities, we studied water quality and nitrogen (N) flow across an estuarine food web in an estuary in Hainan, China, using nitrogen stable isotopes (δ¹⁵N). We found higher δ¹⁵N values of ammonium, nitrate and suspended matter in the pond-covered inner estuary than further upstream, suggesting a strong influence of untreated pond effluents, which had a high δ¹⁵N (ammonium: ~16‰, nitrate: ~7‰, suspended matter: ~8‰). Fish and benthic invertebrates of the inner estuary had a higher δ¹⁵N than consumers further upstream and in similar aquaculture-free estuaries elsewhere, most likely due to direct or indirect uptake of ¹⁵N-enriched aquaculture effluents by phytoplankton and benthic algae. A major part of the artisanal catches from the estuary consists of small-size fish which is used as feed in the local aquaculture. Thus, estuarine fish incorporating aquaculture-effluent based food web signals are harvested and recycled as feed in aquaculture facilities, whose effluents sustain this local food web. The δ¹⁵N being at the high end of the global range on all trophic levels indicates an anthropogenic nitrogen loop in which some portion of the reactive nitrogen initially introduced into aquaculture ponds is continuously recycled and affects the estuarine food web. This recycling also indicates a shortcut in the otherwise inefficient nitrogen sink function of estuaries. Therefore, in areas with large-scale coastal aquaculture like in China and SE Asia the effect of reactive nitrogen from aquaculture sources on the performance of coastal ecosystems may be larger than previously thought.