Human-produced nitrogen enters the adjacent coastal sea through the discharge of large amounts of untreated wastewater, a study by researchers from the Leibniz Centre for Tropical Marine Research (ZMT) shows. This not only leads to eutrophication in these waters, but also ends up in the food chain.
In many countries in Southeast Asia, the export of fish or shrimp from aquaculture is an important economic factor. Aquaculture ponds are lined up along the coasts for many miles. Their wastewater is usually discharged untreated into the sea.
The researchers studied the wastewater from the aquaculture facilities on Hainan, a tropical island in the South China Sea, analysed its organic waste and tracked its path into the adjacent coastal sea.
A major problem in aquaculture farming is that feeding is relatively untargeted. Excess feed accumulates in the pond water. Together with the fish excreta, the organic waste is discharged with the wastewater into the coastal sea. It is rich in nitrogen, which is a component of proteins.
High input of nitrogen leads to overfertilisation of coastal waters, which manifests itself in excessive algal blooms, according to the researchers. However, it is unclear whether and how this nitrogen affects other organisms in the food chain, they say.
In Hainan, the researchers detected high levels of the heavy nitrogen isotope N15 both in the water and in the organisms studied, including algae and seagrass, invertebrates such as mussels and crabs, and also various fish species. In the lagoon, the N15 isotope was present in all organisms in significantly greater quantities than in their conspecifics in the open sea.
Industrially produced pellets contain a relatively high proportion of the N15 isotope. As a component of proteins, nitrogen is essential for living organisms. However, the N15 isotope is utilised more slowly and therefore accumulates in animals and plants. Finding larger amounts of it in their tissues, indicates nitrogen produced by humans.
In China, aquaculture pond areas have now increased to cover more than 15,000 square kilometres. The ZMT researchers extrapolated the nitrogen input from aquaculture ponds measured off Hainan and came up with a resulting 510,000 tons per year for all of China. This corresponds to about three percent of the annual worldwide nitrogen input from rivers.
To date, however, little research has been done to determine the extent to which excess nitrogen has a negative impact on the organisms themselves. For the seagrasses of Hainan, ZMT PhD student Esther Thomsen found that the nitrogen concentration in the leaves increases. This, in turn, leads to reduced leaf stability and breaking leaf tips.
It is obvious, however, that the nitrogen-rich wastewater massively disturbs the species balance in coastal ecosystems through eutrophication. Off Hainan, the research team observed drastic seagrass and coral die-offs.
Herbeck, L.S., U. Krumme, I. Nordhaus, T.C. Jennerjahn (2021). Pond aquaculture effluents feed an anthropogenic nitrogen loop in a SE Asian estuary. Science of the Total Environment 756: 144083.