Three Distinct Views of Deep Pelagic Community Composition Based on Complementary Sampling Approaches

Abstract

Our perception of deep-sea communities has evolved as various sampling approaches have captured different components of deep-sea habitats. We sampled midwater zooplankton assemblages in Monterey Bay, California to quantify community composition (abundance and biomass) and biodiversity (at the Order level) across three depth ranges, and the effects of sampling methodology on community parameters. We collected zooplankton using two types of opening-closing trawls [Tucker Trawl and Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS)] and video recordings from a remotely operated vehicle (ROV). We quantified the relative contributions of microbes to community biomass using synoptic water-bottle casts and flow cytometry. Overall, the pelagic community was most similar between the Tucker trawl and ROV (dissimilarity = 52.4%) and least similar between the MOCNESS and ROV (dissimilarity = 65.8%). Dissimilarity between sampling methods was driven by the relative abundances of crustaceans and gelatinous taxa, where gelatinous animals (cnidarians, ctenophores, tunicates) were more abundant in ROV surveys (64.2%) and Tucker trawls (46.8%) compared to MOCNESS samples (14.5%). ROV surveys were the only method that sufficiently documented the most physically delicate taxa (e.g., physonect siphonophores, lobate ctenophores, and larvaceans). Biomass was also one order of magnitude lower in MOCNESS trawls compared to Tucker trawls. Due to these large differences, the relative contributions of microbes to total biomass were substantially lower in Tucker trawl samples (mean = 7.5%) compared to MOCNESS samples (mean = 51%). These results illustrate that our view of planktonic composition and community biomass is strongly dependent on sampling methodology.

Using low volume eDNA methods to sample pelagic marine animal assemblages

“…Animal communities are generally quantified using the abundances or biomass of each taxon (e.g., [ 4 ]). However, the methods we used do not share a comparable metric of abundance due to numerous issues with connecting ASV read counts to animal abundances across diverse taxa (e.g., variable gene copy numbers [ 46 ], differences in eDNA shedding rates [ 22 ], primer biases [ 23 , 37 ], and PCR biases [ 47 ]).…”

“…Monitoring marine animal communities over horizontal and vertical space thus aids in understanding variability in pelagic ecosystem structure and services. However, quantifying pelagic community composition often requires costly ship-based research operations and specialized oceanographic sampling gear [ 4 , 5 ]. Zooplankton and micronekton, comprising most of the animal biomass in pelagic systems [ 2 , 3 ], are commonly sampled with nets and quantifying their diversity requires life stage-specific taxonomic expertise across numerous phyla.…”

Inverse method applied to autonomous broadband hydroacoustic survey detects higher densities of zooplankton in near-surface aggregations than vessel-based net survey

“…These density estimates are most likely a more accurate representation of the sound scattering layers for the five dominant plankton taxonomic groups in the Norwegian Sea. Net and trawl sampling likely underestimated zooplankton densities within the SSL because of gear-specific biases when assessing species composition across size classes (Skjoldal et al, 2013;Hetherington et al, 2022).…”

Vertical trophic structure and niche partitioning of gelatinous predators in a pelagic food web: Insights from stable isotopes of siphonophores

“…Certain gelatinous zooplankton are particularly fragile and difficult to collect with common shipboard sampling techniques. Siphonophores (Cnidaria, Hydrozoa) are often disaggregated, destroyed, or undersampled with trawls (Remsen et al 2004; Hetherington et al 2022 a ), but they are ubiquitous in marine systems (Robison et al 1998; Condon et al 2012; Lucas et al 2014). Siphonophores are present across the water column, and their unique biology likely allows them to occupy numerous trophic niches in pelagic food webs (Damian‐Serrano et al 2021 a , b ).…”