Plankton or benthos: where krill belongs in Spitsbergen fjords? (Svalbard Archipelago, Arctic)

• Published in: Polar biology 2019-08, Vol.42 (8), p.1415-1430
• Main Authors: Deja, Kajetan, Ormańczyk, Mateusz, Dragańska-Deja, Katarzyna
• Format: Article
• Language: English
• Subjects: Aquatic crustaceans; Archipelagoes; Bays; Benthos; Benthos collecting devices; Biomass; Biomedical and Life Sciences; Brackishwater environment; Cameras; Ecological aggregations; Ecology; Ecosystems; Environmental factors; Estuaries; Estuarine dynamics; Estuarine environments; Euphausiacea; Fjords; Glaciers; Krill; Life Sciences; Marine crustaceans; Microbiology; Nose; Ocean bottom; Ocean floor; Oceanography; Original Paper; Plankton; Plant Sciences; Sedimentation; Sediments (Geology); Trawling; Trawlnets; Water circulation; Water column; Zoology; Zooplankton
• ISSN: 0722-4060; 1432-2056
• DOI: 10.1007/s00300-019-02524-1

This study couples observations of krill ( Thysanoessa inermis , Thysanoessa longicaudata , Thysanoessa rashii ) from Tucker trawl nets and cameras, trying to test hypothesis that in the glaciated fjords of Svalbard most of the euphausiids biomass is located in near-bottom habitat and explains why in this region there are a substantial part of the krill population near the sea floor. Photographic material from the summers of 2013–2017 shows large numbers of near-bottom euphausiids (39% of the total krill biomass in Hornsund and 41% in Kongsfjorden), which reached a maximum density of 751 ± 224 indiv. m −3 in Kongsfjorden, 731 ± 198 indiv. m −3 in Hornsund, and 426 ± 124 indiv. m −3 in Adventfjorden. Regional distribution of near-bottom aggregations of krill seem to be associated with close proximity to the glacier front rather than with depth. The highest densities were located in the glacial bays. Where and why these aggregations occur is probably complicated and dependent on many environmental factors acting together. However, the dominant factors seem to be sedimentation and estuarine circulation. No krill aggregations were found during the winter cruise. The dominating species was T. inermis which made up 90% of the community. Other krill species— T. rashii and T. longicaudata , made up 6% and 4%, respectively. In the summer, aggregations of other macrozooplankton were also observed: amphipods of the genus Themisto and chaetognaths of the genera Eukrohnia and Parasagitta . Euphausiid densities in the water column (from Tucker trawl hauls) were an order of magnitude lower (0.33 indiv. m −3 for Kongsfjorden and 0.61 indiv. m −3 for Hornsund) than those of the near-bottom aggregations observed on cameras system. At most stations, the krill exhibited a behaviour, known as “nose diving” in the sediment, which is likely related to feeding. Observation of this phenomenon may indicate that krill (mostly T. inermis ), found near the bottom of Spitsbergen fjords, is looking for food there. Near-bottom aggregations of zooplankton, mainly krill, are common in glacial bays and can be important in the function of the fjord ecosystem. Our research proves that the zooplankton biomass can be highly underestimated if only Tucker trawl sampling is done, due to neglecting the near-bottom layer in this type of method.