Plankton and seston size spectra estimated by the LOPC and ZooScan in the Abrolhos Bank ecosystem (SE Atlantic)

Abstract

The biomass size spectrum provides valuable information about the functioning of plankton systems. We evaluated hydrographic and bathymetric influences on biomass size spectra and on vertical distributions of plankton and seston above the Abrolhos Bank and in adjacent oceanic areas off Eastern Brazil. We used both in situ Laser Optical Particle Counter (LOPC) and preserved plankton samples analyzed with a ZooScan system to determine seston and plankton abundances, size distributions, and biomasses. Shelf stations, including those on the Abrolhos Bank, had higher particle concentrations and mesozooplankton biomasses than the vertically stratified oceanic stations. The latter were influenced by cold, nutrient-rich South Atlantic Central Water (SACW) below the mixed layer, particularly toward the south of the study area. Small particles (<1mm) were more abundant above and within the pycnocline, whereas large particles (>1mm) had a more heterogeneous vertical distribution, but were more abundant above the pycnocline, especially at the oceanic stations. Calanoid copepods usually dominated the mesozooplankton biomass spectra, but were accompanied by cyclopoids, appendicularians, and ostracods, the latter being particularly abundant during nighttime stations on the Abrolhos Bank. Both LOPC and ZooScan data showed significant differences in NBSS slopes and intercepts between shelf and oceanic stations. The higher intercepts and steeper slopes over the shelf are characteristic of higher productivity. The shallower slopes and presence of more biomass in larger particles indicate a more important contribution of large organisms and higher energy transfer efficiencies at the open ocean stations. Our results highlight the importance of the Abrolhos Bank for pelagic production in an otherwise oligotrophic ocean.