Zooplankton abundance and biovolume size-spectra in the western tropical Atlantic - From the shelf towards complex oceanic current systems

Mesozooplankton plays a pivotal role within marine food webs. However, there is a paucity of studies examining the size-spectra and trophic efficiency of these communities in tropical neritic and oceanic waters. Here, normalised biovolume (NBSS) and normalised numbers size-spectra (NNSS) were fitted on zooplankton data from the southwestern tropical Atlantic. The spectra were compared to assess the trophic efficiency slope (NBSS) across different regions, the Shelf, the western boundary current system (WBCS), and the South equatorial current system (SECS) off oceanic islands. Zooplankton was collected from September to October 2015, at 34 stations using oblique hauls with a 300 μm mesh bongo net from 200 m depth to the surface during either day or night. Samples were analysed using a ZooScan. A total of 30 zooplankton taxa were recorded. Mean abundance and biovolume were 62.6 ind m^-3 and 36.2 mm³ m^-3, respectively. Zooplankton community structure differed significantly between areas, in abundance and biovolume. Copepoda was the most abundant group, representing 66% and 57% of the abundance in coastal areas and oceanic islands, respectively. Fish larvae, gelatinous plankton (mostly Chaetognatha, Thaliacea, and Siphonophora), and Decapoda were the main contributors in terms of biovolume. Overall, smaller organisms were found at the shelf, while larger organisms were found in the SECS. Total abundance was significantly higher on the shelf and in the WBCS than in the SECS, while individual biovolume was higher in the SECS. The NNSS and NBSS slopes were significantly steeper on the shelf than in the other areas. This can be attributed to the higher contribution of small copepods on the shelf and the higher contribution of large-sized copepods and other large organisms in oceanic waters. Flatter NBSS and NNSS slopes offshore reveal a higher trophic efficiency, illustrating the importance of large zooplankton, particularly chaetognaths and decapods, in contributing to the ecosystem secondary productivity in oligotrophic tropical pelagic marine ecosystems. The combination of both NNSS and NBSS provided a more comprehensive view of ecosystem structure and fluxes.