Underestimated nutrient from aquaculture ponds to Lake Eutrophication: A case study on Taihu Lake Basin

Nitrogen and phosphorus discharges from freshwater pond aquaculture have become an important source of eutrophication in lakes. Based on multi-source remote sensing and Google Earth Engine, we extracted aquaculture ponds nationwide and calculated their quantities and area density distributions. Experimental results reveal that regions with densely distributed aquaculture ponds are also hotspots for severe lake cyanobacterial blooms, especially in the middle and lower reaches of the Yangtze River. Presently, freshwater aquaculture production in this basin accounts for 1/3 of the total in the world. With the increasing pond area, Total Nitrogen (TN) and Total Phosphorus (TP) discharged into the Yangtze River Basin rises year by year, leading to decrease in the water quality of the basin. Downstream of this basin, Taihu Lake, the number and area of ponds have increased fivefold since 1984, reaching 575 km2 in 2016. Using the unit area production and the nitrogen-phosphorus discharge per kilogram of aquatic product (27.7 g/kg of TN, 4.43 g/kg of TP), we calculated that the TN and TP discharge in 2016 was as high as 7,185 tons and 1,150 tons. These discharges essentially enter Lake Taihu through rivers, they would account for 21.7 % and 64.4 % of Taihu Lake's net nitrogen and phosphorus input. From 2000 to 2020, both the scale of aquaculture ponds and floating cyanobacteria inversion results showed an increasing trend, and the failure of cyanobacteria control in Taihu Lake is linked to the increase in TN and TP excretion from these ponds. From 2020 to 2022, the strict zero-Covid policy significantly reduced social gatherings and, consequently, led to a substantial decrease in the demand for aquatic products. However, with the relaxation of control measures, the cyanobacteria in Taihu Lake resurged and cyanobacteria harvesting enters the norm in 2023. The underestimated discharge of nitrogen and phosphorus from aquaculture ponds will affect public health and the sustainability of development.