Geo-Statistical Mapping of Chlorophyll-a Concentration in the Nigerian Continental Shelf

Spatial mapping of chlorophyll-a concentration on the Nigerian continental shelf provides critical information on the distribution of phytoplankton biomass, areas of elevated primary productivity, and the seasonal dynamics of biological oceanographic conditions that underpin commercial fisheries. This study applies geo-statistical methods (ordinary kriging, indicator kriging) to in-situ chlorophyll-a measurements collected during six research cruises on the Nigerian shelf between 2022 and 2024, combined with satellite-derived chlorophyll-a products from MODIS Aqua and Sentinel-3 OLCI to produce spatially continuous maps of chlorophyll-a concentration from the coast to the 200-metre isobath. Cross-validation procedures compare kriging interpolation accuracy against satellite products and assess the influence of sampling network design on mapping uncertainty. Temporal analysis employs empirical orthogonal function decomposition of the monthly MODIS chlorophyll-a time series from 2003 to 2024 to identify dominant modes of spatial and temporal variability associated with upwelling seasonality, river plume dynamics, and interannual variability. Correlation analysis relates the principal component time scores to the West African monsoon index, Atlantic Nino SST index, and Niger River discharge anomalies. Geo-statistical maps reveal persistent chlorophyll-a hotspots in the eastern shelf sector near the Niger Delta plume and the central shelf upwelling zone off Ondo and Ogun States. The study provides the highest-resolution empirical chlorophyll-a climatology for the Nigerian shelf to date, supporting identification of potential marine protected area boundaries based on biological productivity criteria. Keywords: chlorophyll-a mapping, geo-statistics, Nigerian continental shelf, kriging, primary productivity