Century-long coral evidence of climate and anthropogenic influences on tropical coastal phosphorus cycling in the northern South China Sea

Abstract

Phosphorus (P) is a critical and scarce nutrient in marine ecosystems, playing a vital role in sustaining primary productivity. Its dynamic variations are closely linked to coastal anthropogenic activities and climate change. However, due to the lack of long-term continuous observational data, our understanding of the evolution patterns of coastal nutrients at interannual to centennial scales remains limited. Coral skeletal phosphorus-to-calcium ratio (P/Ca) has been demonstrated to be a robust proxy for reconstructing long-term phosphate variations in seawater. This study utilized P/Ca ratios in Porites lutea coral from the Luhuitou fringing reef of Hainan Island to reconstruct the historical variations of dissolved inorganic phosphorus (DIP) in surface seawater since 1870. The results reveal that coral P/Ca ratios in the Sanya coastal waters are jointly regulated by terrestrial inputs and upwelling processes. Specifically, P fertilizer application in agricultural activities has significantly increased terrestrial P loading, which is subsequently transported to coastal waters via submarine groundwater discharge (SGD). Crucially, El Niño-Southern Oscillation (ENSO)-driven regional hydrothermal conditions play a pivotal regulatory role in this process. Precipitation provides the transport momentum, while the anomalous high temperatures during El Niño years significantly accelerate chemical weathering, thereby greatly enhancing the P transport flux. Furthermore, against the background of long-term climate warming, this temperature-dominated weathering and transport mechanism is intensifying. The ENSO-modulated upwelling transports P-enriched deep water to the surface, creating a cumulative effect with anthropogenic P sources that collectively enhances marine primary productivity. Notably, the decline in live coral coverage of the Luhuitou fringing reef occurred prior to the marked rise in the coral P/Ca ratio, indicating that elevated DIP concentrations in seawater are not the primary driver of ecological degradation in this reef. This study not only confirms the unique value of coral geochemical proxies in deciphering long-term nutrient dynamics, but also highlights that the ongoing intensification of soil P weathering-transport processes in tropical regions under anthropogenic climate warming may pose potential ecological risks. These findings provide critical scientific supports for integrated coastal zone management.