Differences in the trophic ecology of micronekton driven by diel vertical migration

Jeffrey C. Drazen,C. Anela Choy,Brian N. Popp,Cecelia C. S. Hannides,Sonia Romero‐Romero

doi:10.1002/lno.11128

Abstract

Many species of micronekton perform diel vertical migrations (DVMs), which ultimately contributes to carbon export to the deep sea. However, not all micronekton species perform DVM, and the nonmigrators, which are often understudied, have different energetic requirements that might be reflected in their trophic ecology. We analyze bulk tissue and whole animal stable nitrogen isotopic compositions (δ 15N values) of micronekton species collected seasonally between 0 and 1250 m depth to explore differences in the trophic ecology of vertically migrating and nonmigrating micronekton in the central North Pacific. Nonmigrating species exhibit depth‐related increases in δ 15N values mirroring their main prey, zooplankton. Higher variance in δ 15N values of bathypelagic species points to the increasing reliance of deeper dwelling micronekton on microbially reworked, very small suspended particles. Migrators have higher δ 15N values than nonmigrators inhabiting the epipelagic zone, suggesting the consumption of material during the day at depth, not only at night when they migrate closer to the surface. Migrating species also appear to eat larger prey and exhibit a higher range of variation in δ 15N values seasonally than nonmigrators, likely because of their higher energy needs. The dependence on material at depth enriched in 15N relative to surface particles is higher in migratory fish that ascend only to the lower epipelagic zone. Our results confirm that stark differences in the food habits and dietary sources of micronekton species are driven by vertical migrations.

Highlights

15% and 40% of the total particle export flux to the mesopelagic zone
The δ15N values of both day- and night-collected zooplankton showed the same increase with depth as the slopes of the δ15N values vs. depth were not significantly different (ANCOVA p > 0.1)
Differences in the source of organic matter between some species of micronekton has been described previously (Valls et al 2014; Choy et al 2015; McClain-Counts et al 2017; Gloeckler et al 2018), but this is the first time that a comprehensive dataset was used to resolve depth-related, size-based, and seasonal differences in the δ15N values between species of micronekton driven by diel vertical migrations (DVMs)

Summary

Introduction

15% and 40% of the total particle export flux to the mesopelagic zone. This active transport of carbon by DVM contributes to the functioning of deep-sea ecosystems (Burd et al 2010), and it is needed to meet the overall energetic demands of mesopelagic ecosystems (Steinberg et al 2008; Robinson et al 2010). Past studies of stomach contents have revealed epipelagic zooplankton as the dominant prey of micronekton (Hopkins et al 1997), as well as higher stomach fullness indices (Clarke 1977; Pearcy et al 1979) or higher proportions of fresh food (Pearcy et al 1979) in stomachs of individuals captured at shallower depths during the night. These findings have led to the general assumption that vertically migrating micronekton feed at night near the surface and not during the day at depth (Robison 2003), following the movement patterns of zooplankton, their main prey. Despite the potential of stable isotopic compositions to assess vertical migrations, this approach has been rarely used (Hannides et al [2013] and Harris et al [2014] in zooplankton and McClain-Counts et al [2017] in micronekton), and there is a lack of studies addressing differences in isotopic composition between migrators and nonmigrators along a depth gradient

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