Zooplankton biomass around marine protected islands in the tropical Atlantic Ocean

Abstract

Zooplankton biomass (as wet weight) was studied around marine protected islands in the tropical Atlantic Ocean. The study was based on 96 zooplankton samples collected during a 3-year period; specifically, 2010 was considered a year of thermal stress, and 2012 and 2014 were considered years without thermal stress. The analysis showed that zooplankton biomass varied significantly among protected areas, where the smallest and most isolated archipelago among the tropical islands, Saint Peter and Saint Paul Archipelago (SPSP), had approximately twice the biomass of Fernando de Noronha Archipelago (FN) and three times that of Rocas Atoll (RA). The position of SPSP near the equatorial divergence zone, the seasonal occurrences of phytoplankton blooms east of the Equator, the contribution of the productive waters that SPSP receives from the African coast under the influence of the South Equatorial Current (SEC) system and the local upwelling effect induced by the presence of the island were considered to be the factors responsible for the high pelagic productivity in this remote archipelago. Differences between day and night were also recorded. The high nocturnal biomass was considered an effect of the capture of larger-sized animals, which are considered to be strong migrators. The lowest zooplankton biomass was recorded in 2010 and was considered an apparent effect of the high sea surface temperature observed in that year. However, the interaction between spatial and interannual factors showed that, in FN and SPSP, the zooplankton biomass was lower in the year under thermal stress (2010). In contrast, RA presented a higher biomass value in this period. We suggest that this increase in zooplankton biomass is the result of the contribution of autochthonous sources (e.g., as a consequence of local physical events, such as current wakes, recorded during this period at RA and responsible for the increase in local planktonic productivity) and allochthonous sources (e.g., organisms supplied by FN via the zonal current).