Macrozooplankton rather than Calanus are responsible for autumn DVM in Arctic fjords and pack ice: Backscatter contribution by net samples compared with acoustics

• Main Authors: Berge, J., Cottier, F., Varpe, Øystein, Renaud, P, Falk-Petersen, S., Aubert, Anais, Bjærke, Oda, Hovinen, Johanna, Jung-Madsen, Signe, Tveit, Martha
• Format: Conference Object
• Language: English
• Published: 2011
• Subjects: Arctic; Svalbard; Themisto; Themisto libellula; Zooplankton
• Online Access: https://pure.au.dk/portal/da/publications/macrozooplankton-rather-than-calanus-are-responsible-for-autumn-dvm-in-arctic-fjords-and-pack-ice-backscatter-contribution-by-net-samples-compared-with-acoustics(10f38e3a-5a75-433d-9a84-c2a0c0ff237a).html; http://www.pices.int/publications/book_of_abstracts/2011-5th-zoopl-symp-book-of-abstracts.pdf

Diel vertical migration (DVM) of arctic marine zooplankton has previously been shown to be a general characteristic of Polar waters throughout the year. Despite the fact that the DVM patterns are well documented, very little knowledge exists as to which species are performing the DVM at different times of the year. Herein we report on an autumn sampling campaign at four different locations around Svalbard using a combination of net sampling and 36 hr mooring deployments with 300kHz ADCPs. DVM patterns were strong but differed among the locations in terms of both backscatter strength and migration characteristics such as speed, number of scattering layers and depths to which the organisms migrated during day. Based on the net sampling, the most important contributors to the observed DVM signals at the three fjord locations were chaetognaths, Thysanoessa spp. and Themisto libellula. At the station in the pack ice, zooplankton abundance was generally low, but Metridia longa together with chaetognaths and Themisto libellula were most dominant. In order to relate net samples to the acoustics, a theoretical backscatter and target strength for each of dominant zooplankton taxon were calculated. Previous studies have implicated the later developmental stages of Calanus spp as responsible for the early autumn migrations. However, calculations of relative target strength showed that even when Calanus were found to be aggregating at depth with abundances up to 300-600 ind. m-3, their contribution to the overall backscatter was many orders of magnitude lower than that of e.g. Thysanoessa spp. Hence, previous reports of acoustically detected DVM during autumn were probably also due to macrozooplankton migrations.