Decreasing Chlorophyll-a Trend in the Arabian Gulf and Associated Climate Variables

Chlorophyll-a (Chl-a), a proxy for phytoplankton biomass and marine primary productivity, is a sensitive biological indicator of ecosystem response to environmental and climate variables. The Arabian Gulf (Gulf) is a dynamic marine ecosystem and one of the warmest and most saline semi-enclosed seas in the world, where phytoplankton productivity is strongly influenced by environmental and climatic factors. The spatial and temporal variability of Chl-a and its driving environmental factors is critical for assessing primary productivity and ecosystem dynamics of the Gulf. This study examined the long-term spatio-temporal variability in Chl-a and its causative dominant environmental factors in the Gulf using 21 years of merged Ocean Colour-Climate Change Initiative (OC-CCI) Chl-a and other environmental data for the period 2003-2023. The methodology includes a combination of climatology, time series, long-term trend, multivariate statistical techniques such as correlogram and principal component analysis (PCA) to evaluate the distribution of Chl-a in the Gulf. Seasonal climatology revealed a pronounced winter bloom associated with convective mixing and enhanced nutrient supply, followed by a decline in summer driven by strong water-column stratification. The box averaged time series with long-term trend analysis for the selected regions in the Gulf showed a general declining trend with distinct geographical differences. While the western and southern coasts exhibited relatively moderate/weak decline in Chl-a with summer peaks, the eastern and northern coasts indicated significant decrease and winter dominance. Correlogram and principal component analysis revealed distinct regional patterns, with the northern and central Gulf exhibiting higher variability linked to wind-driven mixing and freshwater influx, while the western Gulf displayed a more stable Chl-a distribution influenced by thermal stratification. Sea surface temperature, sea surface salinity, photosynthetically available radiation and wind speed were attributed as primary drivers of Chl-a variability. The overall findings highlight the complex interaction between physical and biogeochemical processes, emphasizing the crucial roles of regional hydrography, climate variability, and anthropogenic influences in shaping Chl-a distribution. These findings establish an important baseline for future monitoring efforts and the development of predictive ecosystem models for the Gulf under evolving climate conditions.