Abstract
Observations of the diversity, distribution and abundance of pelagic fauna are absent for many ocean regions in the Atlantic, but baseline data are required to detect changes in communities as a result of climate change. Gelatinous fauna are increasingly recognized as vital players in oceanic food webs, but sampling these delicate organisms in nets is challenging. Underwater (in situ) observations have provided unprecedented insights into mesopelagic communities in particular for abundance and distribution of gelatinous fauna. In September 2018, we performed horizontal video transects (50–1200 m) using the pelagic in situ observation system during a research cruise in the southern Norwegian Sea. Annotation of the video recordings resulted in 12 abundant and 7 rare taxa. Chaetognaths, the trachymedusa Aglantha digitale and appendicularians were the three most abundant taxa. The high numbers of fishes and crustaceans in the upper 100 m was likely the result of vertical migration. Gelatinous zooplankton included ctenophores (lobate ctenophores, Beroe spp., Euplokamis sp., and an undescribed cydippid) as well as calycophoran and physonect siphonophores. We discuss the distributions of these fauna, some of which represent the first record for the Norwegian Sea.
Highlights
The pelagic ocean is the largest habitat on the planet and houses a high diversity, abundance and biomass of organisms
Pelagic tunicates are filter feeders (Holland 2016), while medusae, siphonophores and ctenophores are predators feeding on a wide spectrum of zooplankton including gelatinous organisms and micronekton
The major faunal components consisting of ray-finned fishes, euphausiids, shrimps, hydrozoans, ctenophores, chaetognaths and appendicularians, as well as the overall pelagic diversity and abundance, are in line with previous observations in the North Atlantic, where optical sampling was performed using remotely operated vehicles (ROVs) and UVP (Vinogradov 2005; Stemmann et al 2008)
Summary
The pelagic ocean is the largest habitat on the planet and houses a high diversity, abundance and biomass of organisms. Up to 25% of the pelagic biomass can consist of gelatinous zooplankton (Robison 2004). Gelatinous organisms mainly consisting of the pelagic Cnidaria, Ctenophora and Tunicata have a variety of different body morphologies and feeding strategies, allowing them to occupy different niches in the oceanic foodweb (Haddock 2004). Pelagic tunicates (thaliaceans and appendicularians) are filter feeders (Holland 2016), while medusae, siphonophores and ctenophores are predators feeding on a wide spectrum of zooplankton including gelatinous organisms and micronekton
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