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DNA metabarcoding and morphological taxonomic (microscopic) analysis of the gut contents was used to examine diet diversity of seven species of fishes collected from mesopelagic depths (200-1000 m) in the NW Atlantic Ocean Slope Water during Summer 2018 and 2019. Metabarcoding used two gene regions: V9 hypervariable region of nuclear 18S rRNA and mitochondrial cytochrome oxidase I (COI). V9 sequences were classified into 14 invertebrate prey groups, excluding fish due to predator swamping. Ecological network analysis was used to evaluate relative strengths of predator-prey linkages. Multivariate statistical analysis revealed consistently distinct diets of four fish species in 2018 and/or 2019:Argyropelecus aculeatus, Chauliodus sloani, Hygophum hygomii, andSigmops elongatus. Three other species analyzed (Malacosteus niger, Nemichthys scolopaceus, andScopelogadus beanii) showed more variability between sampling years. COI sequences were classified into eight invertebrate prey groups, within which prey species were detected and identified. Considering all predator species together, a total of 77 prey species were detected with a minimum of 1,000 COI sequences, including 22 copepods, 18 euphausiids, and 7 amphipods. Morphological prey counts were classified into seven taxonomic groups, including a gelatinous group comprised of soft-bodied organisms. The ocean twilight zone or is home to exceptional diversity and biomass of marine fish, which are key players in deep sea food webs. This study used integrative morphological-molecular analysis to provide new insights into trophic relationships and sources of productivity for mesopelagic fishes, including identification of key prey species, recognition of the importance of gelatinous prey, and characterization of differences in diet among fish predators in the NW Atlantic Slope Water. 

The impacts of climate change are increasingly apparent in the physical oceanographic environment of the global ocean, with cascading effects through individual species to entire food webs. Despite their importance, these ecosystem effects can be challenging to quantify and track. One angle from which to analyse ecosystem linkages is via compound-specific stable isotope analysis of carbon and nitrogen focused on individual amino acids. These analyses can provide individual-level information (e.g., dietary sources, trophic position) as well as ecosystem-level information (e.g., variability in biogeochemical cycling at the base of the food web, nutrient regimes, food web structure). In this study, we analyzed C and N stable isotopes in archived scales of haddock (Melanogrammus aeglefinus) collected over almost a century from Georges Bank (northeast US) to investigate changes in the diet and trophic status of the haddock population driven by climate change. Specifically, we used nitrogen isotopes to identify secular changes in the input of warm slope waters to the Gulf of Maine over the time series. In contrast, carbon isotopes in essential amino acids suggest that there have been relatively small changes in the source of carbon fueling haddock biomass over the past 100 years and nitrogen isotopes indicate negligible changes in haddock trophic position despite major oceanographic and climatic changes over this time period. Overall, we demonstrate the application of cutting edge molecular isotope tools to a historical archive to examine food web architecture over time in a changing oceanographic environment. 

Abstract The ocean's twilight zone (TZ) is a vast, globe-spanning region of the ocean. Home to myriad fishes and invertebrates, mid-water fishes alone may constitute 10 times more biomass than all current ocean wild-caught fisheries combined. Life in the TZ supports ocean food webs and plays a critical role in carbon capture and sequestration. Yet the ecological roles that mesopelagic animals play in the ocean remain enigmatic. This knowledge gap has stymied efforts to determine the effects that extraction of mesopelagic biomass by industrial fisheries, or alterations due to climate shifts, may have on ecosystem services provided by the open ocean. We propose to develop a scalable, distributed observation network to provide sustained interrogation of the TZ in the northwest Atlantic. The network will leverage a “tool-chest” of emerging and enabling technologies including autonomous, unmanned surface and underwater vehicles and swarms of low-cost “smart” floats. Connectivity among in-water assets will allow rapid assimilation of data streams to inform adaptive sampling efforts. The TZ observation network will demonstrate a bold new step towards the goal of continuously observing vast regions of the deep ocean, significantly improving TZ biomass estimates and understanding of the TZ's role in supporting ocean food webs and sequestering carbon.