A comprehensive assessment of MODIS and GLASS gross primary productivity products across China's coastal zones during 2001-2020

Coastal ecosystems are critical components of the global carbon cycle, yet the reliability of satellite-derived gross primary productivity (GPP) products in these regions remains poorly understood. Compared with inland ecosystems, coastal ecosystems are exposed to distinctive environmental stresses, including tidal inundation and salinity stress, which challenge the applicability of existing GPP models primarily parameterized for terrestrial systems. Here, we conducted a comprehensive assessment of MODIS and GLASS GPP products across China’s coastal zones during 2001–2020. Product performance was evaluated from multiple perspectives, including spatiotemporal continuity analysis, inter-product comparison, consistency with vegetation indices, and validation against eddy covariance observations. Particular attention was given to variations across offshore distance gradients and vegetation types. Results revealed that GLASS outperforms MODIS in data completeness and spatiotemporal continuity. The two products exhibited generally moderate to high agreement (R = 0.60–0.95), varying substantially with season, offshore distance, and vegetation type. The weakest agreement occurred in summer and in evergreen forests. Correlations between GPP and three vegetation indices (NDVI, EVI and NIRv) generally strengthened with offshore distance, and GLASS exhibited stronger correlations with the three vegetation indices than MODIS. Validation against EC observations showed that GLASS generally achieved higher correlations at larger pixel windows and consistently lower RMSE and bias. MODIS and GLASS GPP showed widespread significant increasing trends, accounting for 43.9% and 50.8% of valid pixels, respectively, broadly consistent with the greening patterns indicated by NDVI, EVI, and NIRv, which showed significant increases across 59.1%–60.4% of valid pixels. Our findings highlight substantial and spatially heterogeneous uncertainties in satellite-derived GPP products in coastal ecosystems and underscore the need for coastal-specific model parameterization and improved representation of environmental stressors.