Changes in zooplankton communities from epipelagic to lower mesopelagic waters

Paris V Stefanoudis,Molly Rivers,Helen Ford,Igor M Yashayaev,Alex D Rogers,Lucy C Woodall

doi:10.1016/j.marenvres.2019.02.014

Abstract

Zooplankton form a trophic link between primary producers and higher trophic levels, and exert significant influence on the vertical transport of carbon through the water column (‘biological carbon pump’). Using a MultiNet we sampled and studied mesozooplankton communities (i.e. >0.2 mm) from six locations around Bermuda targeting four depth zones: ∼0–200 m, ∼200–400 m, ∼400–600 m (deep-scattering layer), and ∼600–800 m. Copepoda, our focal taxonomic group, consistently dominated samples (∼80% relative abundance). We report declines in zooplankton and copepod abundance with depth, concurrent with decreases in food availability. Taxonomic richness was lowest at depth and below the deep-scattering layer. In contrast, copepod diversity peaked at these depths, suggesting lower competitive displacement in these more food-limited waters. Finally, omnivory and carnivory, were the dominant trophic traits, each one affecting the biological carbon pump in a different way. This highlights the importance of incorporating data on zooplankton food web structure in future modelling of global ocean carbon cycling.

Highlights

The mesopelagic zone, known as dysphotic or twilight zone, is the part of the ocean where solar illumination is enough to enable visual predation but insufficient for photosynthesis (Sutton, 2013)
Following an adaptive sampling strategy we used the depth of the deep-scattering layers (DSLs) (∼350–600 m) as identified from the Acoustic Doppler Current Profiler (ADCP) (Teledyne Ocean Surveyor II, frequency 75 kHz, minimum particle size identification: > 10 mm) and targeted the following depths with a Multi Plankton Sampler MultiNet system (HYDRO-BIOS, mouth opening: 0.25 m2; net mesh size: 200 μm, winch speed: 0.5 m/s, towed vertically) corresponding to four depth zones: 10–100, 10–150 and 100–200 m; 150–250, 200–300, 200–400 and 300–400 m; 350–550 and 400–600 m (DSL); 550–750 and 600–800 m
Abundance was significantly higher in the epipelagic layer compared to the two deepest studied depths (Mann-Whitney's, DSL: p = 0.009 and 0.014 for zooplankton and copepods, respectively; below the DSL p = 0.001 and 0.001, respectively) (Fig. 2A–B)

Summary

Introduction

The mesopelagic zone, known as dysphotic or twilight zone, is the part of the ocean where solar illumination is enough to enable visual predation but insufficient for photosynthesis (Sutton, 2013). Considered to occur between 200 and 1000 m water depth (Proud et al, 2017; Sutton, 2013; Sutton et al, 2017), recent evidence suggests that the mesopelagic vertical range is not static but varies significantly by latitude and across ocean basins, and can be attributed to the quantity of available light, the mixed layer depth and the flux of particulate organic carbon (Reygondeau et al, 2018) This makes the mesopelagic biome one of the largest habitats in the marine realm comprising a third (31%) of the global ocean volume (Reygondeau et al, 2018). Enhancing our understanding of this oceanic zone is crucial

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Conclusion