Four decades of Landsat data reveal the evolution of nitrogen and phosphorus components driven by human activities in the northern South China Sea.

The Northern South China Sea (NSCS) supports the livelihoods of hundreds of millions of people as one of the most important sea areas in China. However, research on the long-term evolution of nitrogen (N) and phosphorus (P) components in the surface water of NSCS and their ecological impacts and driving factors from a remote sensing perspective remains to be studied. To fill this research gap, we reconstructed the concentrations of dissolved inorganic nitrogen (DIN) and total phosphorus (TP), as well as their components nitrate nitrogen (NO3-) and reactive phosphate (PO43-) in surface water from 1987 to 2023. This was achieved by combining 4968 in-situ measurements from 623 stations with Landsat satellite data on the Google Earth Engine (GEE) platform using a Random Forest (RF) machine learning model. The model exhibited the idea simulation accuracy of four water quality parameters (WQPs) on the validation set (R2 > 0.82). The Pearl River Estuary (PRE) and the Huangmao Estuary (HM) had the highest degree of eutrophication. The satellite successfully captured the continuous increase in the mean concentration of N in the PRE and HM areas since 1987, with the mean concentration of N reached the highest in 2012 (about 1.19 mg/L) in PRE, and P showed a significant downward trend since 1999. N and P concentrations exhibited opposite seasonal patterns. The area of excellent water quality continued to increase in NSCS, but the area of excellent DIN water quality in PRE decreased from 22.5% to 9.5%. NO3- and PO43- contributed at least 50% and 17% of DIN and TP, respectively. The possible impact of the West Guangdong Coastal Current (WGCC) on the specific water quality concentration areas in the western NSCS was quantified for the first time, with an influence area of at least 3000 km2 and an expansion of the offshore up to 28 km. Additionally, the high N:P ratio may indirectly limited the growth of target marine fish. Moreover, agricultural production and fertilizer application dominated WQPs concentration in PRE in past two decades. The concentration and proportion of NO3- indicated a significant positive correlation with the area of algal bloom in NSCS. This research not only systematically reveals the spatio-temporal evolution of N and P and their components over a 40-year time scale for the first time, but also quantifies the possible impact of WGCC on the western area of the NSCS. This study offers scientific guidance for ecological environment assessment and provides a scientific basis for targeted nutrient management strategies.