Distribution and biomass of gelatinous zooplankton in relation to an oxygen minimum zone and a shallow seamount in the Eastern Tropical North Atlantic Ocean.

Physical and topographic characteristics can structure pelagic habitats and affect the plankton community composition. For example, oxygen minimum zones (OMZs) are expected to lead to a habitat compression for species with a high oxygen demand, while upwelling of nutrient-rich deep water at seamounts can locally increase productivity, especially in oligotrophic oceanic waters. Here we investigate the response of the gelatinous zooplankton (GZ) assemblage and biomass to differing oxygen conditions and to a seamount in the Eastern Tropical North Atlantic (ETNA) around the Cape Verde archipelago. A total of 16 GZ taxa (>1100 specimens) were found in the upper 1000 m with distinct species-specific differences, such as the absence of deep-living species Atolla wyvillei and Periphylla periphylla above the shallow seamount summit. Statistical analyses considering the most prominent groups, present at all stations, namely Beroe spp., hydromedusae (including Zygocanna vagans, Halicreas minimum, Colobonema sericeum, Solmissus spp.) and total GZ, showed a strong positive correlation of abundance with temperature for all groups, whereas oxygen had a weak negative correlation only with abundances of Beroe spp. and hydromedusae. To account for size differences between species, we established length-weight regressions and investigated total GZ biomass changes in relation to physical (OMZ) and topographic characteristics. The highest GZ biomass was observed at depths of lowest oxygen concentrations and deepest depth strata at the southeastern flank of the seamount and at two stations south of the Cape Verde archipelago. Our data suggest that, irrespective of their patchy distribution, GZ organisms are ubiquitous food web members of the ETNA, and their habitat includes waters of low oxygen content.

Distribution, associations and role in the biological carbon pump of Pyrosoma atlanticum (Tunicata, Thaliacea) off Cabo Verde, NE Atlantic.

Gelatinous zooplankton are increasingly acknowledged to contribute significantly to the carbon cycle worldwide, yet many taxa within this diverse group remain poorly studied. Here, we investigate the pelagic tunicate Pyrosoma atlanticum in the waters surrounding the Cabo Verde Archipelago. By using a combination of pelagic and benthic in situ observations, sampling, and molecular genetic analyses (barcoding, eDNA), we reveal that: P. atlanticum abundance is most likely driven by local island-induced productivity, that it substantially contributes to the organic carbon export flux and is part of a diverse range of biological interactions. Downward migrating pyrosomes actively transported an estimated 13% of their fecal pellets below the mixed layer, equaling a carbon flux of 1.96-64.55 mg C m-2 day-1. We show that analysis of eDNA can detect pyrosome material beyond their migration range, suggesting that pyrosomes have ecological impacts below the upper water column. Moribund P. atlanticum colonies contributed an average of 15.09 ± 17.89 (s.d.) mg C m-2 to the carbon flux reaching the island benthic slopes. Our pelagic in situ observations further show that P. atlanticum formed an abundant substrate in the water column (reaching up to 0.28 m2 substrate area per m2), with animals using pyrosomes for settlement, as a shelter and/or a food source. In total, twelve taxa from four phyla were observed to interact with pyrosomes in the midwater and on the benthos.

In situ observations lead to the discovery of the large ctenophore Kiyohimea usagi (Lobata: Eurhamphaeidae) in the eastern tropical Atlantic.

We report on the first records of Kiyohimea usagi Matsumoto Robison 1992 (Ctenophora; Eurhamphaeidae) in the Atlantic Ocean. This large, fragile ctenophore cannot be captured by nets, and can only be studied in its natural habitat, the pelagic ocean. In the eastern Atlantic, in the Cape Verde region, in situ observations were obtained using the manned submersible JAGO and a towed pelagic observation system. Between 2015 and 2018 we documented 10 individuals which were encountered between 47-590 m depth. A description of the gastrovascular canal system is provided and potential feeding behavior is discussed. Our study confirms how in situ observations in the poorly explored pelagic realm will lead to the discovery of relatively large and previously undocumented fauna.