Analysis of Zooplankton Size Distribution in the Tropical Atlantic

Abstract

Marine zooplankton play a fundamental role in driving the energetic transfer of marine production between the first and the third trophic level, and in controlling several biogeochemical processes. Size structure is often key in describing the zooplankton community and their biogeochemical and ecological processes in the ocean as physiological contributions to biogeochemistry are size-dependent. This work aims at computing zooplankton size spectra from datasets generated by a low-cost generic scanner to explore the correlations or links between the zooplankton Normalized Biovolume Size Spectrum (NBSS) slopes and intercepts and environmental factors such as oxygen concentration, temperature, salinity, chlorophyll-a, and day/night differences in the tropical Atlantic. The investigation reveals latitudinal and vertical variations in the NBSS slope and intercept, indicating a high abundance in productive areas such as the equatorial and Guinea Dome upwelling zones. The slope positively correlates with depth, suggesting the presence of larger organisms at greater depths, while the intercept shows a negative correlation, indicating reduced overall abundance at depth. Compared to latitude, depth seems to shape zooplankton size structure to a greater extent, as depth-induced variability is more pronounced than latitudinal trends. Diel vertical migration influences zooplankton size distribution, with greater daytime abundance found in deep waters and higher nighttime abundance found at the surface. The relationship between NBSS parameters and chlorophyll-a concentration was not significant, indicating a complex link between bulk productivity estimates and zooplankton size structure. Salinity and temperature exhibit positive correlations with NBSS parameters at greater depths, indicating increased abundance as the temperature and salinity levels increase. Unexpectedly, negative correlations are observed between NBSS parameters and oxygen levels, indicating higher abundance under low oxygen, possibly due to predator avoidance under low-oxygen, low light conditions, or zooplankton migration for nutrient availability. In summary, this study underscores the vital role of zooplankton size spectra in revealing intricate links between marine biogeochemistry and ecology, shedding light on their sensitivity to environmental factors, their potential to influence energy transfer and biogeochemical cycling in the tropical Atlantic, and the unexpected correlation between zooplankton abundance and low-oxygen conditions, offering valuable insights with broader implications for marine ecosystem management and conservation in a dynamic environment.