Plankton Dynamics and Trophic Interactions During Seasonal Upwelling in the Nigerian Shelf

Seasonal coastal upwelling in the Gulf of Guinea drives episodic nutrient enrichment of the Nigerian continental shelf, supporting elevated biological productivity and the aggregation of commercially important pelagic fish species. This study investigates the response of phytoplankton and zooplankton communities to upwelling forcing on the Nigerian shelf, with emphasis on trophic linkage efficiency between primary producers, mesozooplankton, and fish larvae. Five research cruises are conducted between April and November 2024 to capture the pre-upwelling, early upwelling, peak upwelling, and relaxation phases. At 20 stations per cruise, vertical profiles of temperature, salinity, and dissolved oxygen are measured using a CTD rosette. Nutrient samples are analysed for nitrate, phosphate, and silicate by colorimetric flow injection analysis. Phytoplankton biomass is estimated from chlorophyll-a fluorescence, size-fractionated chlorophyll is measured by sequential filtration, and photosynthesis-irradiance curves are determined from 14C uptake incubations. Mesozooplankton are collected using bongo nets and enumerated by taxa. Fish larval assemblages from ichthyoplankton tows are identified to species. Structural equation modelling is used to test hypothesised trophic cascade pathways linking upwelling intensity to fish larval density via phytoplankton and zooplankton biomass. Results are expected to show maximum biological production during peak upwelling in July to August, with significant variability in trophic transfer efficiency driven by size structure of the phytoplankton assemblage. Keywords: upwelling, plankton dynamics, trophic interactions, Nigerian shelf, pelagic ecosystem