Seagrass meadows in Hainan are disappearing

Small algae, which are fed by nitrogen-rich wastewater from aquaculture ponds, grow on the seagrass off the Chinese island of Hainan. | Photo: Esther Thomsen, ZMT

Seagrass meadows are natural filters for nutrients and organic matter (OM). However, excessive enrichment is detrimental for seagrasses. Our ECOLOC subproject6 ‘Response of seagrasses to aquaculture effluents and the filtering capacity of seagrass meadows for anthropogenic nutrients and organic matter’ has recently published results in Marine Environmental Research, biogeochemist Tim Jennerjahn from the Leibniz Centre for Tropical Marine Research (ZMT) and his team for the first time identify the critical point at which this habitat, which is so important for the environment, is irretrievably lost.

In the study, the researchers compared seagrass density in 2009 and 2017 at three sites which were exposed to different magnitudes of aquaculture effluents since the 1990s. It has found that both seagrass density and species number were significantly lower, compared to 2009. The dissolved inorganic nitrogen (DIN) concentration in the water column qualified as a proxy for the intensity of pollution. Threshold for seagrass presence was identified: at concentrations > 8 µM DIN seagrasses will disappear.

“For the first time, we succeeded in determining a threshold value for nitrogen pollution above which the seagrass meadow habitat can no longer recover from this environmental stress and dies”, says Jennerjahn. “It lies at a concentration of 112 micrograms of dissolved inorganic nitrogen per litre of water, which is effective for at least ten years.”

Effluent treatment is crucially needed to ensure a healthy seagrass ecosystem.

More details are published on ZMT webnews:


Thomsen, E., Herbeck, L.S., Jennerjahn, T.C. (2020). The end of resilience: Surpassed nitrogen thresholds in coastal waters led to severe seagrass loss after decades of exposure to aquaculture effluents: Long-term aquaculture effluents exposure causes seagrass loss. Marine Environmental Research 160, 104986.

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