Analysis of the 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 (Kiko et al., 2020; Lombard et al., 2019), and in controlling several biogeochemical processes, as they are known to partly regulate the transformation and export of organic carbon, nutrients, and phosphorus to the deep ocean as part of the biological pump (Ducklow et al., 2001). They do so by feeding and repackaging phytoplankton production into heavy fecal pellets that sink relatively fast. Additionally, their oxygen consumption might contribute to the formation of Oxygen Minimum Zones (OMZs), like the one found in the Eastern Tropical North and South Atlantic (Maas et al., 2021), as they migrate up in the water column at night to feed on surface phytoplankton, and migrate down in the water column at day time to hide from visual predation. They then respire the organic carbon acquired at the surface during nighttime at mesopelagic depth during daytime. Their controls on major biogeochemical and ecological processes in the ocean depend on their abundance and biomass. However, biomass alone does not adequately describe the zooplankton community and is not enough to estimate the biogeochemical pathways in general since physiological contributions to biogeochemistry are size-dependent (Maas et al., 2021). Also, most trophic dynamics are generally related to organismal size, hence distinct communities may present the same biomass while having diverse fingerprints on marine biogeochemical cycles. The size distribution of marine plankton can therefore reveal processes by which biogenic matter is transferred to high trophic levels like fishes, transformed, and removed, eventually driving important climatic and societal changes (Clements et al., 2022).