The end of resilience: Surpassed nitrogen thresholds in coastal waters led to severe seagrass loss after decades of exposure to aquaculture effluents

Although eutrophication is considered a major driver for global seagrass loss with aquaculture effluents being a main factor, little is known about the effect on seagrass meadows in eastern Asia and their resilience to long-term nutrient impact. Seagrass meadows impacted by land-based aquaculture since the 1990s, were visited in 2008/2009 and revisited after another 9 years of effluent exposure. During that period seagrass aboveground biomass declined by 87%. Species diversity decreased with increasing effluent exposure. A δ15N of 9.0‰ of seagrass leaves and additional biogeochemical and biological indicators identify pond effluents as the driver of the observed eutrophication. When continuously exposed to dissolved inorganic nitrogen (DIN) concentrations exceeding a calculated threshold of 8 μM DIN seagrass meadows will disappear. Chronic nutrient pollution from aquaculture effluents can lead to a reduction of biodiversity and ultimately to a complete loss of seagrasses along the aquaculture-dominated coasts in E and SE Asia. [Display omitted] •Eutrophication through aquaculture effluents causes biodiversity and seagrass loss.•Seagrass aboveground biomass declined by 87% in one decade.•δ15N values in seagrass leaves were among the highest measured worldwide.•Thalassia hemprichii appears to be the most resistant species.•We found DIN concentration thresholds for seagrass occurrence and abundance.