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

Abstract

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.

New strategies for the new environment. Stowaway feather star on crab Hyas sp. - Spitsbergen (Arctic)

“…First of all there is massive rain of marine plankton killed at the surface by the osmotic shock (Węsławski and Legeżyńska 1998;Zajaczkowski and Legezyńska 2001). This large and predictable food source sustains rich community of mobile carrion feeders and occasionally also krill that is able to collect phytoplankton food from the sediment (Hirche et al 2016;Deja et al 2019). This may be a convenient niche for crinoids, but also for the broadly de ned megafauna throughout the fjord, also because that large benthic predators such as Atlantic cod at avoid such environments (Szczucka et al 2017).…”

“…From the captured material, a time-lapse movie (15 fps) was made showing the course of the entire experiment. Drop camera photos were taken according to the methodology described by Deja et al 2019 in the summer season between 2015-2020 from the research vessel RV Oceania. In total, material was collected from 202 locations.…”

Polish Polar Research

“…Krill concentrations may be caused by different behavioural reactions (Mauchline and Fisher 1969), yet schools near the glaciers are most likely the result of hydraulic forcing (Urbański et al 2017). The increase in concentration might be caused by the krill becoming entrapped below the sill while trying to avoid the brackish water surface (Węsławski and Legeżynska 1998) or by the presence of feeding aggregations near the seabed (Deja et al 2019). Do glacier types matter?…”

“…This is associated with the meltwater supply, drainage characteristics and internal structure of the glacier. On the other hand, the initial concentration of food organisms (krill and other macroplankton) occurs either as a result of the presence of a trophic trap behind the sill (Węsławski et al 2000) or feeding aggregations near the seabed (Deja et al 2019). The depth of the glacier bay seems to be a predictor of food concentrations, as krill enter the fjord with the inflow of the Atlantic shelf waters, which requires a certain depth (Deja et al 2019).…”

“…In glacial bays with a discharge of suspended matter (called the "brown zone"), large amounts of sea zooplankton, stunned or killed by osmotic shock, can be found (Węsławski et al 2000;Urbański et al 2017). Recent findings suggest that krill (Deja et al 2019) and small fish (mainly polar cod) (Szczucka et al 2017) tend to be abundant near the glacier front, and massive meltwater discharges create uplift that brings fish and krill to the sea surface, opening a window of opportunity for surface-feeding birds (mainly gulls, fulmars and terns). It is not a local upwelling, as was suggested in earlier studies (Hartley and Fisher 1936), nor estuarine circulation (Lydersen et al 2014), but rather a jet-like outflow that may occur at various locations along the glacier front (Urbański et al 2017).…”