Interdisciplinary Insights into the Island Mass Effect of the Cape Verde Archipelago

The Cape Verde Archipelago (CVA) stands out with exceptional biological productivity sustaining a highly diverse ecosystem. Higher primary productivity in the proximity of islands, known as the Island Mass Effect (IME), has been systematically observed on global scale but the details of the interplay between physical and biogeochemical processes remain not fully understood. Within this study, a comprehensive analysis of the IME within the CVA based on 20 years of physical, chemical, and biological observational data sets is presented for the first time. Three main physical processes are identified to be responsible for the IME of the CVA and are investigated in detail: I. The interactions of tidal flows/internal waves with topography: Internal wave breaking at critical slopes leads to elevated mixing rates of a factor 100 larger than at reference points. II. The generation of island wakes in lee of Santo Antãoand Fogo: wind interactions with the island’s topography cause local wind shear in lee, which can generate productive vortex patterns extending for several island diameters downstream. III. The interaction of remotely-generated mesoscale eddies with the CVA itself: After central collision of open ocean eddies with islands or seamounts or when the eddy passes by very closely, we observe submesoscale frontal dynamics driven by mesoscale flow brought out of geostrophic balance, thereby enabling vertical mixing hotspots. Our observations particularly show the interactions between eddies and the internal wave field (modified by vertical shear of the mesoscale eddy, tides, bathymetry, or a combination of all these factors).
All these mechanisms (I., II., and III.), albeit diverse, uniformly augment vertical exchange by turbulent motions with strongly enhanced diapycnal mixing rates and/or vertical velocities and thereby promote elevated nutrient flux into the euphotic layer. This flux ultimately results in significantly higher phytoplankton concentrations in areas where these physical processes occur, thus providing the basis of the local pelagic food web consisting of mesozooplankton and micronecton up to larger predators.