Monthly and seasonal variations in ocean near-surface gradients around Santo Antão Island: A 20-year MODIS-aqua study

• Chl a , Kd 490 , and POC co-varied, indicating oligotrophic waters in Santo Antão. • Light is not the primary limiting factor for productivity in this region. • PIC patterns were distinct from all other bio-optical parameters. • Surface gradients shaped by ocean–atmosphere and large to regional scales dynamics. Understanding the baseline environmental conditions of the Santo Antão marine ecosystems is essential for effective conservation. This study characterised the long-term spatial variability of near-surface biophysical properties of the waters surrounding Santo Antão by utilizing 20 years of MODIS-Aqua Level-2 imagery with 1 km spatial resolution for Ocean Colour and Thermal Infrared parameters, supplemented by altimetry data. Sea Surface Temperature (SST) results revealed a persistent thermal gradient, with warmer waters in the NW-W-SW quadrant and cooler waters in the NE-E-SE quadrant, indicating the influence of regional oceanic currents and interactions between subtropical and tropical gyres. Chlorophyll a (Chl a ), Diffuse Attenuation Coefficient (Kd 490 ), and Particulate Organic Carbon (POC) exhibited similar spatial gradients and were highly correlated, reflecting the oligotrophic nature of Case-1 waters. Notably, the correlation between Kd 490 and Chl a was strong (r ≈ 0.92), while POC showed an even higher correlation (r ≈ 0.95), reinforcing their roles as significant indicators of phytoplankton biomass. In contrast, Particulate Inorganic Carbon (PIC) concentrations displayed distinct spatial patterns and demonstrated a weak negative correlation with Chl a (r ≈ −0.27), indicating that PIC operates independently of the bulk phytoplankton community represented by Chl a . This independence emphasises the need to interpret PIC values with caution, as they may not accurately reflect biological productivity in these waters. Photosynthetically Available Radiation (PAR) displayed limited seasonal variation and a weak negative correlation with Chl a , suggesting that light availability is not a primary limiting factor for phytoplankton productivity in the region. The observed spatial patterns and gradients in biophysical parameters are interpreted in the context of various ocean–atmosphere interactions, including the prevailing northeast trade winds, Sea Surface Height (SSH), and surface currents, which are modulated by larger-scale circulation patterns. These mechanisms should influence nutrient availability and ultimately affect productivity distribution around the island. Overall, this study highlights the complex interplay of monthly and seasonal ocean–atmosphere spatial dynamics, along with regional and large-scale oceanographic conditions, in shaping the near-surface gradients around Santo Antão. By integrating these diverse datasets, this study provides new insights into the region’s ocean dynamics, emphasising its significance for conservation efforts in this area.