Trophic Structure of Neuston Across Tropical and Subtropical Oceanic Provinces Assessed With Stable Isotopes

Rui Albuquerque,Antonio Bode,Juan Ignacio González-Gordillo,Carlos M Duarte,Henrique Queiroga

doi:10.3389/fmars.2020.606088

Abstract

The marine neuston, organisms living in the vicinity of the ocean surface, is one of the least studied zooplankton groups. Neuston occupies a restricted ecological niche and is affected by a wide range of endogenous and exogenous processes while also being a food source to zooplankton fish migrating from the deep layers and seabirds. In this study, the neustonic communities were characterized along the Malaspina global expedition sampling tropical and subtropical oceanic provinces using stable carbon and nitrogen isotopes to explore their trophic structure and relationships with environmental variables. The differences in stable isotopes mirrored the patterns in environmental characteristics of each province. High δ13C values were associated with atmospheric carbon inputs, while the presence of dinoflagellates, coccolithophorids, and upwelling influence is related to low δ13C values. Similarly, provinces presenting high δ15N values were associated with denitrification and nitrate diffusive fluxes, whereas the presence of low δ15N is attributable to nitrogen supplied through N2 fixation by diazotrophs. Neuston showed a large overlap among the isotopic niches of four functional groups, with chaetognaths and detritivores generally exhibiting a smaller degree of overlap compared to carnivores and omnivores/herbivores. These results support the hypothesis of a common trophic structure in the neuston community across the ocean. However, the size of the niche, small in coastal areas and those influenced by upwelling and large in oligotrophic regions, and their overlap, low in more productive provinces and high in oligotrophic provinces, may be associated with food availability. Small trophic niches are associated with a dominance of specialized over-opportunistic feeding in productive environments.

Highlights

The neuston, one of the less described and known aquatic ecological groups, is paradoxically the closest to our sampling platforms as it inhabits the upper centimeters of the ocean
Our results strongly suggest that the low δ15N values measured for Oncaeidae reflect the role of N2 fixation by diazotrophs in supplying N in some provinces, while the high δ15N values measured in other provinces indicate inputs of nitrogen derived from denitrification processes (e.g., Pacific equatorial counter current (PNEC), North Pacific tropical gyre (NPTG), and South subtropical convergence (SSTC))
Uptake of atmospheric carbon were found to be associated with high δ13C values, whereas upwelling influenced regions or the presence of dinoflagellates and coccolithophores appear to be related with low δ13C

Summary

Introduction

The neuston, one of the less described and known aquatic ecological groups, is paradoxically the closest to our sampling platforms as it inhabits the upper centimeters of the ocean. The neuston provides a food source to the zooplankton migrating from deeper layers to the surface (Hempel and Weikert, 1972), as well as to seabirds roaming over the oceans (Cheng et al, 2010). For these reasons, the neustonic community is believed to play a critical role on the structure and function of marine food webs. Neuston complexity is still poorly understood as studies on the community structure and the taxonomical composition of organisms inhabiting this ecological niche remain few (Rezai et al, 2019), and global scale analyses are yet lacking

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Conclusion