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Aim: In this study, we investigated whether environmental factors can effectively control the spatial distribution of various benthic algae and examined the critical ecological impacts of algae on corals across the South China Sea (SCS). Relationships between benthic algae and environmental factors were assessed, and potential ecological impacts of algae on coral communities were evaluated across spatial scales. Location: A total of 104 sites at 12 coral reefs in four coral reef regions (CRRs) of the SCS (latitude range of 9–22° N). Taxa: Turf algae, macroalgae (including fleshy macroalgae and Halimeda), crustose coralline algae (CCA), coral and juvenile coral. Methods: Using diver‐based surveys (2015–2018), we investigated four CRRs with different reef distance to mainland (RDM) across the SCS. We obtained field data on benthic algal composition and cover, coral cover and diversity, and juvenile coral density. We also measured and collected the environmental factors (including seawater environmental parameters and reef fish diversity). Results: The cover of turf algae and CCA gradually decreased and increased, respectively, with an increase in RDM, which had a strong relationship with the gradually changing environmental factors. Random forest models suggested that nutrients, reef fish diversity, seawater transparency and temperature were the most important factors for predicting turf algae and CCA cover. Linear regression analyses showed a significant relationship between the turf algae, macroalgae, and CCA covers, and juvenile coral density. Main conclusions: Our results showed that gradually changing environmental factors were correlated with the spatial distributions of turf algae and CCA. However, the natural biophysical relationships between macroalgae and environmental factors may be disrupted by turf algal overgrowth and nutrient subsidies. Increases in all benthic algal groups significantly impacted coral recruitment, highlighting the critical role of benthic algae in determining the recovery trajectory of the Indo‐Pacific reefs that are threatened by human activities. [ABSTRACT FROM AUTHOR]

The decadal variability of ocean primary productivity (PP) has mainly been studied using relatively short term satellite data, limiting its in‐depth analysis. Here, by removing the influence of the 13C Suess effect, we extract the PP signal on decadal timescales using a record of coral δ13C values from the northern South China Sea (NSCS), where the physical‐biogeochemical conditions are influenced largely by the winter monsoon. Our results show that the PP in the NSCS increased from 1851 to the 1920s and decreased from the 1920s to 2007, and large superimposed decadal‐scale changes were observed. The in‐phase relationship observed between the PP reconstruction and winter monsoon records suggests that the changes in the PP in the NSCS are linked to decadal‐scale changes in the winter monsoon. Considering that the winter monsoon intensity may weaken in the future under global warming, the PP in the NSCS might decrease in the coming decades. Plain Language Summary: Marine phytoplankton contributes roughly half of the global primary production and thus constitutes a vital component of the marine ecosystem that serves as the foundation of the marine food web. Due to the relatively short length of observation records, evaluations of the long‐term changes in ocean primary productivity (PP) have suffered from large uncertainties, which have hampered further discussion of the decadal variability of PP in a long‐term context. To discuss the decadal variability of the PP beyond the range of satellite observations, we present a coral‐based PP reconstruction since 1851 CE from the northern South China Sea (NSCS), where the physical‐biogeochemical conditions are influenced largely by the winter monsoon. Our results show considerable decadal variability of the PP in the NSCS superimposed on a pre‐1920s increase and a post‐1920s decrease. We also found an in‐phase relationship between the decadal variability of the reconstructed PP and the winter monsoon records over the past 150 years, indicating that the PP in the NSCS is linked to changes in the winter monsoon forcing on decadal timescales. Considering that the winter monsoon intensity may weaken in the future under global warming scenarios, the PP in the NSCS might decrease in the coming decades. Key Points: The decadal variability of primary productivity was extracted from a coral δ13C record by removing the influence of the 13C Suess effectThe coral‐based primary productivity reconstruction for the past 150 years showed a pre‐1920s increase and a post‐1920s decreaseThe primary productivity of the northern South China Sea varied in phase with the intensity of the winter monsoon on decadal timescales [ABSTRACT FROM AUTHOR]

To date, the decadal variability of the Global Monsoon (GM) has been mainly studied using instrumental data collected over the last 50 years, and further discussion has been hampered by the limited data length. Here, we present a coral δ18O record from the northern South China Sea, where the hydrology is related to the seasonal Intertropical Convergence Zone (ITCZ) migration, and we use this record to reconstruct the decadal migration of the ITCZ since 1851 A.D. Combining our record with a synthesis of monsoon records reveals an anti‐phase inter‐hemispheric variability of the GM over the last 150 years, indicating that the decadal‐scale hydrologic variability in the tropics is modulated by the meridional migration of the ITCZ. The plausible relationship observed between the decadal variability of the GM and the Atlantic Multidecadal Oscillation suggests that the decadal variability of tropical hydrological changes is likely linked to the climate perturbations in the North Atlantic. Plain Language Summary: The prediction of monsoon variability in the coming decades will be of great importance for policy‐making and socio‐economic development. The Global Monsoon (GM) is the global‐scale seasonal atmospheric overturning associated with the migration of the Intertropical Convergence Zone (ITCZ). The variability and dynamics of the GM on decadal timescales are still unclear due to the limited length of the instrumental data (< 50 years). Here, we integrated a synthesis of the monsoon and the ITCZ records, including published records and our new coral record, and found that the GM varied in an anti‐phase inter‐hemispheric mode on decadal timescales associated with the ITCZ migration over the past 150 years, suggesting that the simultaneous intensification of the monsoons in both hemispheres since the ~1980s was unusual in a long‐term context. The linkage between the decadal variability of GM and the Atlantic Multidecadal Oscillation/Variability indicated that the North Atlantic has been driving the anti‐phase inter‐hemispheric variability of the GM on decadal timescales. Key Points: The coral δ18O record from the northern South China Sea captures the migration of the regional ITCZ on decadal timescalesThe Global Monsoon varied in an anti‐phase inter‐hemispheric mode on decadal timescales associated with the ITCZ migration since 1851 A.DThe decadal variability of the Global Monsoon was found to be linked to the Atlantic Multidecadal Oscillation [ABSTRACT FROM AUTHOR]