Inter-comparison of Ocean Color chlorophyll products for assessing the marine productivity of the Arabian Gulf

The Arabian Gulf (Gulf) is a dynamic marine ecosystem and optically complex marginal sea, where the retrieval of chlorophyll-a (Chl-a) from ocean color observations is challenging due to intense atmospheric dust, high suspended sediments, and shallow bathymetry. Nevertheless, satellite-derived Chl-a, widely used as a proxy for marine primary productivity, is one of the key essential climate variables for monitoring long-term marine ecosystem variability and climate change. Given the absence of a single satellite mission spanning the full historical record of ocean color data, the construction of continuous, multi-decadal Chl-a time series relies on the integration of multiple sensors, highlighting the need for careful regional evaluation of available products. This study assessed the performance and consistency of widely used single-sensor and multi-sensor merged Chl-a products in the Arabian Gulf, including MODIS-Aqua, the Ocean Colour Climate Change Initiative (OC-CCI), and GlobColour datasets. The assessment was based on statistical comparisons with available in situ Chl-a measurements and analyses of spatiotemporal variability. Temporal consistency was also examined during both common operational periods and continuous overlapping lifespans of the datasets, with analyses covering time period from September 1997 to September 2025.

The results showed strong and stable correlations among the products, indicating robust temporal coherence in the Gulf. All products demonstrated valuable insights into Chl-a variability; however, notable differences were observed in spatial pattern and long-term trends, particularly over coastal and shelf regions. OC-CCI provided better spatial and temporal coverage with greater overall stability, making it particularly suitable for long-term environmental monitoring in the Gulf. Trend analysis revealed contrasting spatial patterns among the products, with OC-CCI indicated an overall declining Chl-a trend with localized coastal increases, while GlobColour exhibited increasing trends along the northern and southern coasts and a declining trend along the eastern coast. These differences are likely linked to known quality limitations in the early phases of merged datasets, especially during periods dominated by single-sensor contributions. Consistent with this, the OC-CCI record during 1997-2002 also shows regional discrepancies and less coherent trend behavior, which become substantially reduced after 2003, coinciding with increased sensor overlap and improved temporal consistency. This study highlights the necessity of regional validation of global ocean-color products in optically complex environments and identifies the most reliable dataset for developing long-term Chl-a records in the Gulf, supporting improved assessments of marine productivity and climate change impacts.