Effect of Eddies on Plankton Productivity in the Cabo Verde region

Abstract

The aim of this study was to assess and quantify the plankton distribution in the Cabo Verde region and determine how eddies do affect its productivity. This was done using acoustic measurements conducted at sea utilising a scientific echosounder operating with a 200 kHz transducer, because based on the backscattering characteristics of “fluid like” organisms such as zooplankton, they yield a stronger echo at this frequency. We also used the integrated backscatter (NASC) as a proxy for plankton abundance. Data were collected during the ATL2MED mission, from October to December 2019. Environmental conditions (Salinity, Temperature and Photosynthetically Active Radiation - PAR) were analysed in order to determine their effects on plankton biomass distribution. Chlorophyll a was analysed as a proxy for phytoplankton distribution. Salinity varied from 35.6 to 37.2, with an average of 36.4, temperature ranged from 20.5 to 27.3 °C, with an average of 24.1 °C, PAR ranged between 0 and 1921 and chlorophyll ranged from 0.06 to 4.1 mg/m3, with highest values within eddy 1. Two productivity hotspots were visible, one in the region impacted by eddy 1 and the other close to the southern tip of Sal. Within and outside the eddy, NASC was higher during the night compared to the daytime hours. NASC was plotted against distance to the eddy core, to the coast of Sal and against to Senghor seamount. The correlation between NASC and surface environmental conditions was also done to check the effect of them on plankton distribution.