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Seasonal hypoxia (≤2 mg dissolved oxygen L−1) can have detrimental effects on marine food webs. Recent studies indicate that some jellyfish can tolerate low oxygen and may have a competitive advantage over other zooplankton and fishes in those environments. We assessed community structure and distributions of cnidarian and ctenophore jellyfish in seasonally hypoxic Hood Canal, WA, USA, at four stations that differed in oxygen conditions. Jellyfish were collected in June through October 2012 and 2013 using full-water-column and discrete-depth net tows, concurrent with CTD casts to measure dissolved oxygen (DO). Overall, southern, more hypoxic, regions of Hood Canal had higher abundances and higher diversity than the northern regions, particularly during the warmer and more hypoxic year of 2013. Of fifteen species identified, the most abundant—the siphonophore Muggiaea atlantica and hydrozoan Aglantha digitale—reached peak densities > 1800 Ind m−3 and 38 Ind m−3, respectively. M. atlantica were much more abundant at the hypoxic stations, whereas A. digitale were also common in the north. Vertical distributions explored during hypoxia showed that jellyfish were mostly in the upper 10 m regardless of the oxycline depth. Moderate hypoxia seemed to have no detrimental effect on jellyfish in Hood Canal, and may have resulted in high population densities, which could influence essential fisheries and trophic energy flow.
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Abundance, composition and distribution of carnivorous gelatinous zooplankton in the Northern Gulf of Alaska
Abstract Abundance, biomass, size and distribution of macro-jellyfish were measured in the Northern Gulf of Alaska (NGA). Nearly 1000 kg dispersed among ~13 800 jellyfish were collected using a 5-m2 Methot net. We present length-weight regressions for seven most-common taxa. Catches were dominated by the hydrozoan Aequorea victoria and the scyphozoan Chrysaora melanaster. During 2018, epipelagic macro-jellyfish biomass averaged 1.46 ± 0.36 g WW m−3 for July and 1.14 ± 0.23 g WW m−3 for September, while during 2019 they averaged 0.86 ± 0.19 g WW m−3 for July and 0.72 ± 0.21 g WW m−3 by September. Despite similar biomass among seasons within a year, July abundances were fivefold greater than abundances in September, with July catches dominated by smaller-sized jellyfish over the inner shelf, while during September larger jellyfish were more prominent and most predominant at offshore stations. Comparison to 20 years of data from standard towed nets allowed determination of the relative magnitude of the dominant carnivorous zooplankton components: scyphozoans, hydrozoans and chaetognaths in the NGA. The biomass of these smaller epipelagic predators (5.4 mg WW m−3 for hydrozoans and 10.5 mg WW m−3 for chaetognaths) is a low percentage of the macro-jellyfish, despite their much higher numerical abundance.
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- Award ID(s):
- 1656070
- PAR ID:
- 10430021
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Journal of Plankton Research
- Volume:
- 45
- Issue:
- 4
- ISSN:
- 0142-7873
- Format(s):
- Medium: X Size: p. 693-708
- Size(s):
- p. 693-708
- Sponsoring Org:
- National Science Foundation
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